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Zika virus: History, epidemiology, transmission, and clinical presentation

  • Byung-Hak Song
    Affiliations
    Department of Animal, Dairy, and Veterinary Sciences, College of Agriculture and Applied Sciences, Utah State University, Logan, UT 84322, USA
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  • Sang-Im Yun
    Affiliations
    Department of Animal, Dairy, and Veterinary Sciences, College of Agriculture and Applied Sciences, Utah State University, Logan, UT 84322, USA
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  • Michael Woolley
    Affiliations
    Department of Animal, Dairy, and Veterinary Sciences, College of Agriculture and Applied Sciences, Utah State University, Logan, UT 84322, USA
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  • Young-Min Lee
    Correspondence
    Corresponding author at: Department of Animal, Dairy, and Veterinary Sciences, College of Agriculture and Applied Sciences, Utah State University, 9830 Old Main Hill, Logan, UT 84322, USA.
    Affiliations
    Department of Animal, Dairy, and Veterinary Sciences, College of Agriculture and Applied Sciences, Utah State University, Logan, UT 84322, USA
    Search for articles by this author
Open AccessPublished:March 03, 2017DOI:https://doi.org/10.1016/j.jneuroim.2017.03.001

      Highlights

      • This review summarizes the history and epidemiology of ZIKV.
      • This review outlines the transmission of ZIKV.
      • This review highlights the clinical presentation of ZIKV.

      Abstract

      Zika virus (ZIKV), a mosquito-borne positive-stranded RNA virus of the family Flaviviridae (genus Flavivirus), is now causing an unprecedented large-scale outbreak in the Americas. Historically, ZIKV spread eastward from equatorial Africa and Asia to the Pacific Islands during the late 2000s to early 2010s, invaded the Caribbean and Central and South America in 2015, and reached North America in 2016. Although ZIKV infection generally causes no symptoms or only a mild self-limiting illness, it has recently been linked to a rising number of severe neurological diseases, including microcephaly and Guillain-Barré syndrome. Because of the continuous geographic expansion of both the virus and its mosquito vectors, ZIKV poses a serious threat to public health around the globe. However, there are no vaccines or antiviral therapies available against this pathogen. This review summarizes a fast-growing body of literature on the history, epidemiology, transmission, and clinical presentation of ZIKV and highlights the urgent need for the development of efficient control strategies for this emerging pathogen.

      Keywords

      1. Classification and nomenclature

      Zika virus (ZIKV) belongs to the genus Flavivirus in the family Flaviviridae (
      • Lindenbach B.D.
      • Thiel H.J.
      • Rice C.M.
      Flaviviridae: the viruses and their replication.
      ). Currently, the genus comprises 53 virus species (
      • Simmonds P.
      • Becher P.
      • Collett M.S.
      • Gould E.A.
      • Heinz F.X.
      • Meyers G.
      • Monath T.
      • Pletnev A.
      • Rice C.M.
      • Stiasny K.
      • Thiel H.J.
      • Weiner A.
      • Bukh J.
      Family Flaviviridae.
      ), which are transmitted by the bite of mosquitoes (27 species), ticks (12 species), or no known arthropod vector (14 species). Within the Flavivirus genus, ZIKV is a mosquito-borne virus that is phylogenetically closely related to other medically important mosquito-borne flaviviruses of global public health significance (Fig. 1), such as Japanese encephalitis (JEV), West Nile (WNV), dengue (DENV), and yellow fever (YFV) viruses (
      • Gubler D.J.
      • Kuno G.
      • Markoff L.
      Flaviviruses.
      ). These mosquito-borne flaviviruses can be divided into two major classes based on their clinical presentation in humans (
      • Gaunt M.W.
      • Sall A.A.
      • de Lamballerie X.
      • Falconar A.K.
      • Dzhivanian T.I.
      • Gould E.A.
      Phylogenetic relationships of flaviviruses correlate with their epidemiology, disease association and biogeography.
      ,
      • Kramer L.D.
      • Ebel G.D.
      Dynamics of flavivirus infection in mosquitoes.
      ): (1) encephalitic flaviviruses (represented by JEV and WNV), which cause invasive neurological diseases, with birds serving as their natural vertebrate hosts and Culex species mosquitoes as their principal vectors (
      • Brinton M.A.
      Replication cycle and molecular biology of the West Nile virus.
      ,
      • Yun S.I.
      • Lee Y.M.
      Japanese encephalitis: the virus and vaccines.
      ); and (2) non-encephalitic or viscerotropic flaviviruses (exemplified by DENV and YFV), which cause lethal hemorrhagic fever, with non-human primates acting as their vertebrate hosts and Aedes species mosquitoes as their primary vectors (
      • Monath T.P.
      • Vasconcelos P.F.
      Yellow fever.
      ,
      • Weaver S.C.
      • Barrett A.D.
      Transmission cycles, host range, evolution and emergence of arboviral disease.
      ). Of note, DENV has fully adapted to humans and no longer needs animal hosts for viral transmission (
      • Clyde K.
      • Kyle J.L.
      • Harris E.
      Recent advances in deciphering viral and host determinants of dengue virus replication and pathogenesis.
      ,
      • Mackenzie J.S.
      • Gubler D.J.
      • Petersen L.R.
      Emerging flaviviruses: the spread and resurgence of Japanese encephalitis, West Nile and dengue viruses.
      ).
      Fig. 1
      Fig. 1A rooted phylogenetic tree based on the nucleotide sequence of complete or near-complete genomes of all 46 available flaviviruses (as of November 2016). Multiple sequence alignments were produced by ClustalX (
      • Thompson J.D.
      • Gibson T.J.
      • Plewniak F.
      • Jeanmougin F.
      • Higgins D.G.
      The CLUSTAL_X windows interface: flexible strategies for multiple sequence alignment aided by quality analysis tools.
      ), and the rooted phylogenetic tree was generated by the neighbor-joining method (
      • Saitou N.
      • Nei M.
      The neighbor-joining method: a new method for reconstructing phylogenetic trees.
      ). The scale bar represents the genetic distance in nucleotide substitutions per site. Bootstrap values (1000 replications) are shown at each node. The complete genome sequence of the bovine viral diarrhea virus, a member of the genus Pestivirus in the family Flaviviridae, was used as an outgroup. The virus abbreviations assigned by the International Committee on Taxonomy of Viruses are listed, and the strain names and their GenBank accession numbers are provided in parenthesis. The known or probable vector (mosquito or tick) is indicated on the right side of the tree.

      2. History and epidemiology

      Discovered in Uganda in 1947, ZIKV was confined for the first 60 years to an equatorial zone across Africa and Asia. Outside this zone, however, it first emerged in Yap Island in 2007, spread eastward to French Polynesia and other Pacific Islands in 2013–2014, reached Latin America in 2015, and disseminated further to North America in 2016. Now, ZIKV is circulating in the Americas, Southeast Asia, and the Pacific Islands.

      2.1 Virus discovery and seroprevalence in Africa and Asia prior to the year 2000

      ZIKV was first isolated in 1947 from the serum of a sentinel rhesus macaque monkey that was placed in the Zika forest on the Entebbe peninsula, Uganda, during the course of surveillance for YFV (
      • Dick G.W.
      • Kitchen S.F.
      • Haddow A.J.
      Zika virus. I. Isolations and serological specificity.
      ). This isolate, named MR-766, is the African prototype strain of ZIKV. Shortly thereafter, it was also recovered on multiple occasions from A. africanus mosquitoes caught in the same area (
      • Dick G.W.
      Zika virus. II. Pathogenicity and physical properties.
      ,
      • Dick G.W.
      • Kitchen S.F.
      • Haddow A.J.
      Zika virus. I. Isolations and serological specificity.
      ,
      • Haddow A.J.
      • Williams M.C.
      • Woodall J.P.
      • Simpson D.I.
      • Goma L.K.
      Twelve isolations of Zika virus from Aedes (Stegomyia) Africanus (Theobald) taken in and above a Uganda forest.
      ,
      • Weinbren M.P.
      • Williams M.C.
      Zika virus: further isolations in the Zika area, and some studies on the strains isolated.
      ). Although there was no indication that ZIKV caused disease in the residents of Uganda, the prevalence of antibodies against the virus in their serum was approximately 10–20% (
      • Dick G.W.
      Epidemiological notes on some viruses isolated in Uganda; yellow fever, Rift Valley fever, Bwamba fever, West Nile, Mengo, Semliki forest, Bunyamwera, Ntaya, Uganda S and Zika viruses.
      ,
      • Dick G.W.
      • Kitchen S.F.
      • Haddow A.J.
      Zika virus. I. Isolations and serological specificity.
      ). Despite the need for caution because of antibody cross-reactivity with other flaviviruses, a large number of serological studies in the half century since the discovery of ZIKV have revealed a broad but confined geographic distribution of human infection with the virus, across a relatively narrow equatorial belt running from Africa to Asia: Senegal (
      • Monlun E.
      • Zeller H.
      • Le Guenno B.
      • Traore-Lamizana M.
      • Hervy J.P.
      • Adam F.
      • Ferrara L.
      • Fontenille D.
      • Sylla R.
      • Mondo M.
      Surveillance of the circulation of arbovirus of medical interest in the region of eastern Senegal.
      ), Sierra Leone (
      • Robin Y.
      • Mouchet J.
      Serological and entomological study on yellow fever in Sierra Leone.
      ), Nigeria (
      • Adekolu-John E.O.
      • Fagbami A.H.
      Arthropod-borne virus antibodies in sera of residents of Kainji Lake Basin, Nigeria 1980.
      ,
      • Fagbami A.
      Epidemiological investigations on arbovirus infections at Igbo-Ora, Nigeria.
      ,
      • Macnamara F.N.
      Zika virus: a report on three cases of human infection during an epidemic of jaundice in Nigeria.
      ), Gabon (
      • Jan C.
      • Languillat G.
      • Renaudet J.
      • Robin Y.
      A serological survey of arboviruses in Gabon.
      ,
      • Saluzzo J.F.
      • Ivanoff B.
      • Languillat G.
      • Georges A.J.
      Serological survey for arbovirus antibodies in the human and simian populations of the South-East of Gabon.
      ), Central African Republic (
      • Saluzzo J.F.
      • Gonzalez J.P.
      • Herve J.P.
      • Georges A.J.
      Serological survey for the prevalence of certain arboviruses in the human population of the south-east area of Central African Republic.
      ), Egypt (
      • Smithburn K.C.
      • Taylor R.M.
      • Rizk F.
      • Kader A.
      Immunity to certain arthropod-borne viruses among indigenous residents of Egypt.
      ), Uganda (
      • Smithburn K.C.
      Neutralizing antibodies against certain recently isolated viruses in the sera of human beings residing in East Africa.
      ), Tanzania (
      • Smithburn K.C.
      Neutralizing antibodies against certain recently isolated viruses in the sera of human beings residing in East Africa.
      ), Kenya (
      • Geser A.
      • Henderson B.E.
      • Christensen S.
      A multipurpose serological survey in Kenya. 2. Results of arbovirus serological tests.
      ), Pakistan (
      • Darwish M.A.
      • Hoogstraal H.
      • Roberts T.J.
      • Ahmed I.P.
      • Omar F.
      A sero-epidemiological survey for certain arboviruses (Togaviridae) in Pakistan.
      ), India (
      • Smithburn K.C.
      • Kerr J.A.
      • Gatne P.B.
      Neutralizing antibodies against certain viruses in the sera of residents of India.
      ), Thailand (
      • Pond W.L.
      Arthropod-borne virus antibodies in sera from residents of South-East Asia.
      ), Malaysia (
      • Pond W.L.
      Arthropod-borne virus antibodies in sera from residents of South-East Asia.
      ,
      • Smithburn K.C.
      Neutralizing antibodies against arthropod-borne viruses in the sera of long-time residents of Malaya and Borneo.
      ), Vietnam (
      • Pond W.L.
      Arthropod-borne virus antibodies in sera from residents of South-East Asia.
      ), Indonesia (
      • Olson J.G.
      • Ksiazek T.G.
      • Gubler D.J.
      • Lubis S.I.
      • Simanjuntak G.
      • Lee V.H.
      • Nalim S.
      • Juslis K.
      • See R.
      A survey for arboviral antibodies in sera of humans and animals in Lombok, Republic of Indonesia.
      ), and the Philippines (
      • Hammon W.M.
      • Schrack Jr., W.D.
      • Sather G.E.
      Serological survey for a arthropod-borne virus infections in the Philippines.
      ). In 1966, the first non-African ZIKV strain, designated P6-740, was isolated from a pool of A. aegypti mosquitoes collected in Malaysia (
      • Marchette N.J.
      • Garcia R.
      • Rudnick A.
      Isolation of Zika virus from Aedes aegypti mosquitoes in Malaysia.
      ).

      2.2 The Yap outbreak and sporadic cases in Southeast Asia during the late 2000s to mid-2010s

      Human illness caused by ZIKV infection was first reported in 1954 during an outbreak of jaundice in Nigeria, when infection was confirmed in three patients by isolation of the virus or a rise in serum antibody titer, with a correlation observed between the development of ZIKV neutralizing antibodies and jaundice (
      • Macnamara F.N.
      Zika virus: a report on three cases of human infection during an epidemic of jaundice in Nigeria.
      ). From that time until the early 2000s, only about a dozen cases of benign human ZIKV-associated illness were documented in countries in Africa and Asia (
      • Hayes E.B.
      Zika virus outside Africa.
      ), such as Uganda (
      • Simpson D.I.
      Zika virus infection in man.
      ), Nigeria (
      • Fagbami A.H.
      Zika virus infections in Nigeria: virological and seroepidemiological investigations in Oyo State.
      ,
      • Moore D.L.
      • Causey O.R.
      • Carey D.E.
      • Reddy S.
      • Cooke A.R.
      • Akinkugbe F.M.
      • David-West T.S.
      • Kemp G.E.
      Arthropod-borne viral infections of man in Nigeria, 1964–1970.
      ), and Indonesia (
      • Olson J.G.
      • Ksiazek T.G.
      • Suhandiman Triwibowo
      Zika virus, a cause of fever in Central Java, Indonesia.
      ). In 2007, however, ZIKV caused the first large outbreak outside of Africa and Asia on Yap Island, a part of the Federated States of Micronesia (Fig. 2) in the northwestern Pacific Ocean, with a relatively mild disease characterized by fever, rash, arthralgia, and conjunctivitis (
      • Duffy M.R.
      • Chen T.H.
      • Hancock W.T.
      • Powers A.M.
      • Kool J.L.
      • Lanciotti R.S.
      • Pretrick M.
      • Marfel M.
      • Holzbauer S.
      • Dubray C.
      • Guillaumot L.
      • Griggs A.
      • Bel M.
      • Lambert A.J.
      • Laven J.
      • Kosoy O.
      • Panella A.
      • Biggerstaff B.J.
      • Fischer M.
      • Hayes E.B.
      Zika virus outbreak on Yap Island, Federated States of Micronesia.
      ,
      • Lanciotti R.S.
      • Kosoy O.L.
      • Laven J.J.
      • Velez J.O.
      • Lambert A.J.
      • Johnson A.J.
      • Stanfield S.M.
      • Duffy M.R.
      Genetic and serologic properties of Zika virus associated with an epidemic, Yap State, Micronesia, 2007.
      ). During this outbreak, ~73% of the 6892 Yap residents aged ≥3 years were estimated to be infected with ZIKV, and ~18% of the infected people had a clinical illness that was probably attributable to ZIKV infection (
      • Duffy M.R.
      • Chen T.H.
      • Hancock W.T.
      • Powers A.M.
      • Kool J.L.
      • Lanciotti R.S.
      • Pretrick M.
      • Marfel M.
      • Holzbauer S.
      • Dubray C.
      • Guillaumot L.
      • Griggs A.
      • Bel M.
      • Lambert A.J.
      • Laven J.
      • Kosoy O.
      • Panella A.
      • Biggerstaff B.J.
      • Fischer M.
      • Hayes E.B.
      Zika virus outbreak on Yap Island, Federated States of Micronesia.
      ). Sequence analysis suggested that ZIKV was introduced to Yap Island from Southeast Asia (
      • Duffy M.R.
      • Chen T.H.
      • Hancock W.T.
      • Powers A.M.
      • Kool J.L.
      • Lanciotti R.S.
      • Pretrick M.
      • Marfel M.
      • Holzbauer S.
      • Dubray C.
      • Guillaumot L.
      • Griggs A.
      • Bel M.
      • Lambert A.J.
      • Laven J.
      • Kosoy O.
      • Panella A.
      • Biggerstaff B.J.
      • Fischer M.
      • Hayes E.B.
      Zika virus outbreak on Yap Island, Federated States of Micronesia.
      ,
      • Haddow A.D.
      • Schuh A.J.
      • Yasuda C.Y.
      • Kasper M.R.
      • Heang V.
      • Huy R.
      • Guzman H.
      • Tesh R.B.
      • Weaver S.C.
      Genetic characterization of Zika virus strains: geographic expansion of the Asian lineage.
      ,
      • Lanciotti R.S.
      • Kosoy O.L.
      • Laven J.J.
      • Velez J.O.
      • Lambert A.J.
      • Johnson A.J.
      • Stanfield S.M.
      • Duffy M.R.
      Genetic and serologic properties of Zika virus associated with an epidemic, Yap State, Micronesia, 2007.
      ). In the early to mid-2010s, a handful of sporadic cases of ZIKV infection were also reported in Southeast Asian countries, such as Thailand (
      • Buathong R.
      • Hermann L.
      • Thaisomboonsuk B.
      • Rutvisuttinunt W.
      • Klungthong C.
      • Chinnawirotpisan P.
      • Manasatienkij W.
      • Nisalak A.
      • Fernandez S.
      • Yoon I.K.
      • Akrasewi P.
      • Plipat T.
      Detection of Zika virus infection in Thailand, 2012–2014.
      ,
      • Tappe D.
      • Rissland J.
      • Gabriel M.
      • Emmerich P.
      • Gunther S.
      • Held G.
      • Smola S.
      • Schmidt-Chanasit J.
      First case of laboratory-confirmed Zika virus infection imported into Europe, November 2013.
      ), Cambodia (
      • Heang V.
      • Yasuda C.Y.
      • Sovann L.
      • Haddow A.D.
      • Travassos da Rosa A.P.
      • Tesh R.B.
      • Kasper M.R.
      Zika virus infection, Cambodia, 2010.
      ), Malaysia (
      • Tappe D.
      • Nachtigall S.
      • Kapaun A.
      • Schnitzler P.
      • Gunther S.
      • Schmidt-Chanasit J.
      Acute Zika virus infection after travel to Malaysian Borneo, September 2014.
      ), Indonesia (
      • Kwong J.C.
      • Druce J.D.
      • Leder K.
      Zika virus infection acquired during brief travel to Indonesia.
      ,
      • Perkasa A.
      • Yudhaputri F.
      • Haryanto S.
      • Hayati R.F.
      • Ma'roef C.N.
      • Antonjaya U.
      • Yohan B.
      • Myint K.S.
      • Ledermann J.P.
      • Rosenberg R.
      • Powers A.M.
      • Sasmono R.T.
      Isolation of Zika virus from febrile patient, Indonesia.
      ), and the Philippines (
      • Alera M.T.
      • Hermann L.
      • Tac-An I.A.
      • Klungthong C.
      • Rutvisuttinunt W.
      • Manasatienkij W.
      • Villa D.
      • Thaisomboonsuk B.
      • Velasco J.M.
      • Chinnawirotpisan P.
      • Lago C.B.
      • Roque Jr., V.G.
      • Macareo L.R.
      • Srikiatkhachorn A.
      • Fernandez S.
      • Yoon I.K.
      Zika virus infection, Philippines, 2012.
      ).
      Fig. 2
      Fig. 2Countries and territories that have reported autochthonous mosquito-borne transmission of ZIKV in the past 3 months. The term “widespread transmission” (red) indicates one of the following three situations: (1) more than 10 locally transmitted ZIKV cases reported in a single area, (2) at least two separate areas reporting locally transmitted ZIKV cases, and (3) ZIKV transmission ongoing in an area for more than 3 months. The term “sporadic transmission” (orange) indicates that no more than 10 locally transmitted ZIKV cases have been reported in a single area in the past 3 months. The term “past transmission” (blue) indicates that local ZIKV transmission has been reported since 2007, but not in the past 3 months.
      Source: European Centre for Disease Prevention and Control, current status of ZIKV transmission in the world as of November 23, 2016 (http://ecdc.europa.eu/en/healthtopics/zika_virus_infection/zika-outbreak/pages/zika-countries-with-transmission.aspx).

      2.3 The French Polynesia outbreak and spread in the Pacific Islands in the early 2010s

      In 2013–2014, a major epidemic of ZIKV occurred in French Polynesia (Fig. 2), a French overseas territory located in the middle of the southern Pacific Ocean, with its ~270,000 people living on 67 islands distributed among five archipelagoes (
      • Cao-Lormeau V.M.
      • Roche C.
      • Teissier A.
      • Robin E.
      • Berry A.L.
      • Mallet H.P.
      • Sall A.A.
      • Musso D.
      Zika virus, French Polynesia, South Pacific, 2013.
      ). During this outbreak, ~11% of the total population was estimated to have sought medical treatment for suspected ZIKV infection (
      • ECDC
      Rapid Risk Assessment: Zika Virus Infection Outbreak, French Polynesia.
      ,
      • Mallet H.P.
      • Vial A.L.
      • Musso D.
      Bilan de l’épidémie à virus Zika en Polynésie française, 2013–2014. Bulletin d'information sanitaires, épidémiologiques et statistiques (BISES).
      ,
      • Musso D.
      • Nilles E.J.
      • Cao-Lormeau V.M.
      Rapid spread of emerging Zika virus in the Pacific area.
      ). The magnitude of the outbreak was presumably the result of a combination of the low level of pre-existing immunity to ZIKV and the high density of competent mosquito vectors in that area (
      • Aubry M.
      • Finke J.
      • Teissier A.
      • Roche C.
      • Broult J.
      • Paulous S.
      • Despres P.
      • Cao-Lormeau V.M.
      • Musso D.
      Seroprevalence of arboviruses among blood donors in French Polynesia, 2011–2013.
      ). Although a vast majority of the clinical cases seen in this outbreak were similar to those observed in the 2007 Yap outbreak, a small fraction of severe cases were associated with neurological complications, such as Guillain-Barré syndrome, in the context of co-circulating DENV and chikungunya (
      • ECDC
      Rapid Risk Assessment: Zika Virus Infection Outbreak, French Polynesia.
      ,
      • Oehler E.
      • Watrin L.
      • Larre P.
      • Leparc-Goffart I.
      • Lastere S.
      • Valour F.
      • Baudouin L.
      • Mallet H.
      • Musso D.
      • Ghawche F.
      Zika virus infection complicated by Guillain-Barre syndrome — case report, French Polynesia, December 2013.
      ). Retrospectively, the incidence of Guillain-Barré syndrome was estimated to be increased by ~20-fold in French Polynesia (
      • Oehler E.
      • Watrin L.
      • Larre P.
      • Leparc-Goffart I.
      • Lastere S.
      • Valour F.
      • Baudouin L.
      • Mallet H.
      • Musso D.
      • Ghawche F.
      Zika virus infection complicated by Guillain-Barre syndrome — case report, French Polynesia, December 2013.
      ). Although the origin of the ZIKV in French Polynesia remains unknown, it is genetically related to the strains isolated from Yap Island in 2007 and from Cambodia in 2010 (
      • Cao-Lormeau V.M.
      • Roche C.
      • Teissier A.
      • Robin E.
      • Berry A.L.
      • Mallet H.P.
      • Sall A.A.
      • Musso D.
      Zika virus, French Polynesia, South Pacific, 2013.
      ). During or shortly after the French Polynesia outbreak, ZIKV spread further to other neighboring islands in the South Pacific Ocean (
      • Cao-Lormeau V.M.
      • Musso D.
      Emerging arboviruses in the Pacific.
      ,
      • Musso D.
      • Cao-Lormeau V.M.
      • Gubler D.J.
      Zika virus: following the path of dengue and chikungunya?.
      ,
      • Musso D.
      • Nilles E.J.
      • Cao-Lormeau V.M.
      Rapid spread of emerging Zika virus in the Pacific area.
      ), including New Caledonia (
      • Dupont-Rouzeyrol M.
      • O'Connor O.
      • Calvez E.
      • Daures M.
      • John M.
      • Grangeon J.P.
      • Gourinat A.C.
      Co-infection with Zika and dengue viruses in 2 patients, New Caledonia, 2014.
      ), the Cook Islands (
      • Roth A.
      • Mercier A.
      • Lepers C.
      • Hoy D.
      • Duituturaga S.
      • Benyon E.
      • Guillaumot L.
      • Souares Y.
      Concurrent outbreaks of dengue, chikungunya and Zika virus infections — an unprecedented epidemic wave of mosquito-borne viruses in the Pacific 2012–2014.
      ), and Easter Island (
      • Tognarelli J.
      • Ulloa S.
      • Villagra E.
      • Lagos J.
      • Aguayo C.
      • Fasce R.
      • Parra B.
      • Mora J.
      • Becerra N.
      • Lagos N.
      • Vera L.
      • Olivares B.
      • Vilches M.
      • Fernandez J.
      A report on the outbreak of Zika virus on Easter Island, South Pacific, 2014.
      ). Also, it was imported to other faraway countries such as Australia (
      • Pyke A.T.
      • Daly M.T.
      • Cameron J.N.
      • Moore P.R.
      • Taylor C.T.
      • Hewitson G.R.
      • Humphreys J.L.
      • Gair R.
      Imported Zika virus infection from the Cook Islands into Australia, 2014.
      ), Italy (
      • Zammarchi L.
      • Stella G.
      • Mantella A.
      • Bartolozzi D.
      • Tappe D.
      • Gunther S.
      • Oestereich L.
      • Cadar D.
      • Munoz-Fontela C.
      • Bartoloni A.
      • Schmidt-Chanasit J.
      Zika virus infections imported to Italy: clinical, immunological and virological findings, and public health implications.
      ), Japan (
      • Kutsuna S.
      • Kato Y.
      • Takasaki T.
      • Moi M.
      • Kotaki A.
      • Uemura H.
      • Matono T.
      • Fujiya Y.
      • Mawatari M.
      • Takeshita N.
      • Hayakawa K.
      • Kanagawa S.
      • Ohmagari N.
      Two cases of Zika fever imported from French Polynesia to Japan, December 2013 to January 2014.
      ), and Norway (
      • Waehre T.
      • Maagard A.
      • Tappe D.
      • Cadar D.
      • Schmidt-Chanasit J.
      Zika virus infection after travel to Tahiti, December 2013.
      ).

      2.4 The Brazil outbreak and spread in the Americas in 2015–2016

      At the beginning of 2015, the first autochthonous transmission of ZIKV was detected in the northeastern part of Brazil (
      • Campos G.S.
      • Bandeira A.C.
      • Sardi S.I.
      Zika virus outbreak, Bahia, Brazil.
      ,
      • Zammarchi L.
      • Tappe D.
      • Fortuna C.
      • Remoli M.E.
      • Gunther S.
      • Venturi G.
      • Bartoloni A.
      • Schmidt-Chanasit J.
      Zika virus infection in a traveller returning to Europe from Brazil, March 2015.
      ,
      • Zanluca C.
      • Melo V.C.
      • Mosimann A.L.
      • Santos G.I.
      • Santos C.N.
      • Luz K.
      First report of autochthonous transmission of Zika virus in Brazil.
      ), in association with an outbreak of an acute exanthematous illness (
      • Cardoso C.W.
      • Paploski I.A.
      • Kikuti M.
      • Rodrigues M.S.
      • Silva M.M.
      • Campos G.S.
      • Sardi S.I.
      • Kitron U.
      • Reis M.G.
      • Ribeiro G.S.
      Outbreak of exanthematous illness associated with Zika, chikungunya, and dengue viruses, Salvador, Brazil.
      ). Toward the end of 2015, ZIKV activity expanded into at least 14 Brazilian states (
      • WHO
      Zika virus outbreaks in the Americas.
      ), with an estimated 440,000–1,300,000 suspected cases (
      • Hennessey M.
      • Fischer M.
      • Staples J.E.
      Zika virus spreads to new areas — region of the Americas, May 2015–January 2016.
      ). To our surprise, it was noted in Brazil that the number of newborn infants with microcephaly had increased in the ZIKV-affected areas by September 2015 (
      • Schuler-Faccini L.
      • Ribeiro E.M.
      • Feitosa I.M.
      • Horovitz D.D.
      • Cavalcanti D.P.
      • Pessoa A.
      • Doriqui M.J.
      • Neri J.I.
      • Neto J.M.
      • Wanderley H.Y.
      • Cernach M.
      • El-Husny A.S.
      • Pone M.V.
      • Serao C.L.
      • Sanseverino M.T.
      • Brazilian Medical Genetics Society–Zika Embryopathy Task Force
      Possible association between Zika virus infection and microcephaly — Brazil, 2015.
      ), and >4000 cases of suspected microcephaly were reported by February 2016, although these cases may have been misdiagnosed in some cases or over-reported (
      • Victora C.G.
      • Schuler-Faccini L.
      • Matijasevich A.
      • Ribeiro E.
      • Pessoa A.
      • Barros F.C.
      Microcephaly in Brazil: how to interpret reported numbers?.
      ). In agreement with this finding, retrospective studies in French Polynesia indicated an increased number of instances of microcephaly and other fetal abnormalities after the 2013–2014 ZIKV outbreak (
      • Cauchemez S.
      • Besnard M.
      • Bompard P.
      • Dub T.
      • Guillemette-Artur P.
      • Eyrolle-Guignot D.
      • Salje H.
      • Van Kerkhove M.D.
      • Abadie V.
      • Garel C.
      • Fontanet A.
      • Mallet H.P.
      Association between Zika virus and microcephaly in French Polynesia, 2013–15: a retrospective study.
      ,
      • Jouannic J.M.
      • Friszer S.
      • Leparc-Goffart I.
      • Garel C.
      • Eyrolle-Guignot D.
      Zika virus infection in French Polynesia.
      ). In October 2015, Colombia reported the local transmission of ZIKV infection outside Brazil (
      • WHO
      Zika virus outbreaks in the Americas.
      ), and by March 2016, a total of 51,473 suspected ZIKV infections were recorded in that country, with 2090 laboratory-confirmed cases (
      • WHO
      Situation Report: Zika Virus Microcephaly and Guillain–Barré Syndrome.
      ). Since its emergence in Brazil, ZIKV has spread at an alarming rate throughout much of Central and South America and the Caribbean, and the possibility that microcephaly is linked to ZIKV has increased, prompting the World Health Organization to declare a “public health emergency of international concern” from February to November 2016 (
      • WHO
      WHO Statement on the First Meeting of the International Health Regulations 2005 (IHR 2005) Emergency Committee on Zika Virus and Observed Increase in Neurological Disorders and Neonatal Malformations.
      ,
      • WHO
      WHO Statement: Fifth Meeting of the Emergency Committee under the International Health Regulations (2005) Regarding Microcephaly, Other Neurological Disorders and Zika Virus.
      ). ZIKV is still causing an unprecedented ongoing epidemic in Latin America and threatening North America and potentially the rest of the world (
      • Fauci A.S.
      • Morens D.M.
      Zika virus in the Americas — yet another arbovirus threat.
      ,
      • Lessler J.
      • Chaisson L.H.
      • Kucirka L.M.
      • Bi Q.
      • Grantz K.
      • Salje H.
      • Carcelen A.C.
      • Ott C.T.
      • Sheffield J.S.
      • Ferguson N.M.
      • Cummings D.A.
      • Metcalf C.J.
      • Rodriguez-Barraquer I.
      Assessing the global threat from Zika virus.
      ).
      As of November 17, 2016, 48 countries and territories in the Americas had reported the autochthonous mosquito-borne transmission of ZIKV (
      • PAHO/WHO
      Zika — Epidemiological Update.
      ), with an accumulated number of 171,553 confirmed cases (
      • PAHO/WHO
      Zika Suspected and Confirmed Cases Reported by Countries and Territories in the Americas (Cumulative Cases), 2015–2016.
      ): Anguilla; Antigua and Barbuda; Argentina; Aruba; the Bahamas; Barbados; Belize; Bolivia; Bonaire, Sint Eustatius and Saba; Brazil; the British Virgin Islands; Cayman Islands; Colombia; Costa Rica; Cuba; Curaçao; Dominica; the Dominican Republic; Ecuador; El Salvador; French Guiana; Grenada; Guadeloupe; Guatemala; Guyana; Haiti; Honduras; Jamaica; Martinique; Mexico; Montserrat; Nicaragua; Panama; Paraguay; Peru; Puerto Rico; Saint Barthélemy; Saint Kitts and Nevis; Saint Lucia; Saint Martin; Saint Vincent and the Grenadines; Sint Maarten; Suriname; Trinidad and Tobago; Turks and Caicos Islands; the United States of America (US); the US Virgin Islands; and Venezuela (Fig. 2). Sexually transmitted cases have also been reported in Argentina, Canada, Chile, Peru, and the US (
      • PAHO/WHO
      Zika — Epidemiological Update.
      ). In November 2016, a decreasing trend in ZIKV cases had been noted in all the ZIKV-affected countries and territories in the Americas, except for Mexico, Panama, and the islands of Turks and Caicos (
      • PAHO/WHO
      Zika — Epidemiological Update.
      ). To date, a total of 20 countries and territories in the Americas have documented 2311 confirmed cases of ZIKV-associated congenital syndrome (
      • PAHO/WHO
      Zika — Epidemiological Update.
      ,
      • PAHO/WHO
      Zika Suspected and Confirmed Cases Reported by Countries and Territories in the Americas (Cumulative Cases), 2015–2016.
      ). In the US states and federal districts, there were 4444 confirmed ZIKV cases reported to ArboNET as of November 23, 2016; of these, 36 were sexually transmitted cases throughout the country, and 182 were locally acquired mosquito-borne cases in Florida, where autochthonous transmission was first reported in July 2016 and is currently ongoing in the area of Miami Beach and in the county of Miami-Dade (
      • CDC
      Case Counts in the US.
      ,
      • CDC
      Zika Cases Reported in the United States.
      ). By November 17, 2016, a total of 33 newborn infants and pregnancy losses with birth defects had been reported to the US Zika Pregnancy Registry (
      • CDC
      Outcomes of Pregnancies With Laboratory Evidence of Possible Zika Virus Infection in the United States, 2016.
      ).

      2.5 Current status and future prospects

      As illustrated in Fig. 2, ZIKV is a potential pandemic threat, currently circulating not only in the Americas but also in the Pacific Islands (American Samoa, Federated States of Micronesia, Fiji, Marshall Islands, New Caledonia, Palau, Papua New Guinea, Samoa, and Tonga), Southeast Asia (Singapore, Thailand, the Philippines, and Vietnam), and the islands of Cape Verde off the coast of West Africa (
      • CDC
      All Countries and Territories With Active Zika Virus Transmission.
      ,
      • ECDC
      Current Zika Transmission.
      ,
      • WHO
      WHO Confirms Zika Virus Strain Imported From the Americas to Cabo Verde.
      ). In addition, since early 2015, there have been an increasing number of travel-related imported ZIKV cases in non-endemic countries: Australia (
      • AGDH
      Zika Virus — Notifications of Zika Virus Infection (Zika).
      ,
      • Pyke A.T.
      • Moore P.R.
      • Hall-Mendelin S.
      • McMahon J.L.
      • Harrower B.J.
      • Constantino T.R.
      • van den Hurk A.F.
      Isolation of Zika virus imported from Tonga into Australia.
      ), Belgium (
      • De Smet B.
      • Van den Bossche D.
      • van de Werve C.
      • Mairesse J.
      • Schmidt-Chanasit J.
      • Michiels J.
      • Arien K.K.
      • Van Esbroeck M.
      • Cnops L.
      Confirmed Zika virus infection in a Belgian traveler returning from Guatemala, and the diagnostic challenges of imported cases into Europe.
      ), Canada (), China (
      • Yin Y.
      • Xu Y.
      • Su L.
      • Zhu X.
      • Chen M.
      • Zhu W.
      • Xia H.
      • Huang X.
      • Gong S.
      Epidemiologic investigation of a family cluster of imported ZIKV cases in Guangdong, China: probable human-to-human transmission.
      ,
      • Zhang J.
      • Jin X.
      • Zhu Z.
      • Huang L.
      • Liang S.
      • Xu Y.
      • Liao R.
      • Zhou L.
      • Zhang Y.
      • Wilder-Smith A.
      Early detection of Zika virus infection among travellers from areas of ongoing transmission in China.
      ), France (
      • Maria A.T.
      • Maquart M.
      • Makinson A.
      • Flusin O.
      • Segondy M.
      • Leparc-Goffart I.
      • Le Moing V.
      • Foulongne V.
      Zika virus infections in three travellers returning from South America and the Caribbean respectively, to Montpellier, France, December 2015 to January 2016.
      ), the US (Hawaii) (
      • CDC
      Case Counts in the US.
      ), Italy (
      • Nicastri E.
      • Pisapia R.
      • Corpolongo A.
      • Fusco F.M.
      • Cicalini S.
      • Scognamiglio P.
      • Castilletti C.
      • Bordi L.
      • Di Caro A.
      • Capobianchi M.R.
      • Puro V.
      • Ippolito G.
      Three cases of Zika virus imported in Italy: need for a clinical awareness and evidence-based knowledge.
      ), Portugal (
      • Ze-Ze L.
      • Prata M.B.
      • Teixeira T.
      • Marques N.
      • Mondragao A.
      • Fernandes R.
      • Saraiva da Cunha J.
      • Alves M.J.
      Zika virus infections imported from Brazil to Portugal, 2015.
      ), Spain (
      • Diaz-Menendez M.
      • de la Calle-Prieto F.
      • Montero D.
      • Antolin E.
      • Vazquez A.
      • Arsuaga M.
      • Trigo E.
      • Garcia-Bujalance S.
      • de la Calle M.
      • Sanchez Seco P.
      • de Ory F.
      • Arribas J.R.
      Grupo de Trabajo Multidisciplinar del Hospital La Paz-Carlos III en Enfermedad por Virus Zika, Initial experience with imported Zika virus infection in Spain.
      ), Switzerland (
      • Gyurech D.
      • Schilling J.
      • Schmidt-Chanasit J.
      • Cassinotti P.
      • Kaeppeli F.
      • Dobec M.
      False positive dengue NS1 antigen test in a traveller with an acute Zika virus infection imported into Switzerland.
      ), and the Netherlands (
      • Duijster J.W.
      • Goorhuis A.
      • van Genderen P.J.
      • Visser L.G.
      • Koopmans M.P.
      • Reimerink J.H.
      • Grobusch M.P.
      • van der Eijk A.A.
      • van den Kerkhof J.H.
      • Reusken C.B.
      • Hahne S.J.
      • Dutch ZIKV Study Team
      Zika virus infection in 18 travellers returning from Surinam and the Dominican Republic, the Netherlands, November 2015–March 2016.
      ). Using species distribution modeling techniques, a recent study has predicted that a large portion of the tropical and sub-tropical regions of the globe has suitable environmental conditions but has not yet reported symptomatic cases of ZIKV infection (Fig. 3), with over two billion people living in these areas (
      • Messina J.P.
      • Kraemer M.U.
      • Brady O.J.
      • Pigott D.M.
      • Shearer F.M.
      • Weiss D.J.
      • Golding N.
      • Ruktanonchai C.W.
      • Gething P.W.
      • Cohn E.
      • Brownstein J.S.
      • Khan K.
      • Tatem A.J.
      • Jaenisch T.
      • Murray C.J.
      • Marinho F.
      • Scott T.W.
      • Hay S.I.
      Mapping global environmental suitability for Zika virus.
      ). Thus, the presence of a much larger susceptible population in suitable environments for ZIKV, combined with its travel-mediated rapid dissemination, has raised concerns about the high risk of introducing and establishing new autochthonous transmission in these areas (
      • Bogoch I.I.
      • Brady O.J.
      • Kraemer M.U.
      • German M.
      • Creatore M.I.
      • Kulkarni M.A.
      • Brownstein J.S.
      • Mekaru S.R.
      • Hay S.I.
      • Groot E.
      • Watts A.
      • Khan K.
      Anticipating the international spread of Zika virus from Brazil.
      ,
      • Fauci A.S.
      • Morens D.M.
      Zika virus in the Americas — yet another arbovirus threat.
      ,
      • Gatherer D.
      • Kohl A.
      Zika virus: a previously slow pandemic spreads rapidly through the Americas.
      ,
      • Musso D.
      • Gubler D.J.
      Zika virus.
      ). The situation is particularly challenging in the US, because ZIKV is likely to spread further, from Florida to other southern states (Fig. 3, inset), given the estimated range of the two major competent mosquito vectors, A. aegypti and A. albopictus (
      • CDC
      Potential Range in US.
      ). For the same reason, it is very likely that ZIKV will become endemic in the Americas.
      Fig. 3
      Fig. 3Global environmental suitability for ZIKV. Highlighted are the countries that are highly environmentally suitable, having a suitable area of more than 10,000 km2, but not yet reporting symptomatic ZIKV cases (Source:
      • Messina J.P.
      • Kraemer M.U.
      • Brady O.J.
      • Pigott D.M.
      • Shearer F.M.
      • Weiss D.J.
      • Golding N.
      • Ruktanonchai C.W.
      • Gething P.W.
      • Cohn E.
      • Brownstein J.S.
      • Khan K.
      • Tatem A.J.
      • Jaenisch T.
      • Murray C.J.
      • Marinho F.
      • Scott T.W.
      • Hay S.I.
      Mapping global environmental suitability for Zika virus.
      ). An inset shows an estimated range for A. aegypti and A. albopictus in the US.
      Source: Centers for Disease Control and Prevention (https://www.cdc.gov/zika/vector/range.html).

      3. Genetic diversity

      During the seven decades since its accidental discovery in 1947, >500 ZIKV isolates have been obtained sporadically in Africa, Asia, the Pacific Islands, and the Americas, but only a very limited number of these strains have been fully sequenced (
      • Baronti C.
      • Piorkowski G.
      • Charrel R.N.
      • Boubis L.
      • Leparc-Goffart I.
      • de Lamballerie X.
      Complete coding sequence of Zika virus from a French Polynesia outbreak in 2013.
      ,
      • Barzon L.
      • Pacenti M.
      • Berto A.
      • Sinigaglia A.
      • Franchin E.
      • Lavezzo E.
      • Brugnaro P.
      • Palu G.
      Isolation of infectious Zika virus from saliva and prolonged viral RNA shedding in a traveller returning from the Dominican Republic to Italy, January 2016.
      ,
      • Berthet N.
      • Nakoune E.
      • Kamgang B.
      • Selekon B.
      • Descorps-Declere S.
      • Gessain A.
      • Manuguerra J.C.
      • Kazanji M.
      Molecular characterization of three Zika flaviviruses obtained from sylvatic mosquitoes in the Central African Republic.
      ,
      • Cunha M.S.
      • Esposito D.L.
      • Rocco I.M.
      • Maeda A.Y.
      • Vasami F.G.
      • Nogueira J.S.
      • de Souza R.P.
      • Suzuki A.
      • Addas-Carvalho M.
      • Barjas-Castro Mde L.
      • Resende M.R.
      • Stucchi R.S.
      • Boin Ide F.
      • Katz G.
      • Angerami R.N.
      • da Fonseca B.A.
      First complete genome sequence of Zika virus (Flaviviridae, Flavivirus) from an autochthonous transmission in Brazil.
      ,
      • Ellison D.W.
      • Ladner J.T.
      • Buathong R.
      • Alera M.T.
      • Wiley M.R.
      • Hermann L.
      • Rutvisuttinunt W.
      • Klungthong C.
      • Chinnawirotpisan P.
      • Manasatienkij W.
      • Melendrez M.C.
      • Maljkovic Berry I.
      • Thaisomboonsuk B.
      • Ong-Ajchaowlerd P.
      • Kaneechit W.
      • Velasco J.M.
      • Tac-An I.A.
      • Villa D.
      • Lago C.B.
      • Roque Jr., V.G.
      • Plipat T.
      • Nisalak A.
      • Srikiatkhachorn A.
      • Fernandez S.
      • Yoon I.K.
      • Haddow A.D.
      • Palacios G.F.
      • Jarman R.G.
      • Macareo L.R.
      Complete genome sequences of Zika virus strains isolated from the blood of patients in Thailand in 2014 and the Philippines in 2012.
      ,
      • Giovanetti M.
      • Faria N.R.
      • Nunes M.R.
      • de Vasconcelos J.M.
      • Lourenco J.
      • Rodrigues S.G.
      • Vianez Jr., J.L.
      • da Silva S.P.
      • Lemos P.S.
      • Tavares F.N.
      • Martin D.P.
      • do Rosario M.S.
      • Siqueira I.C.
      • Ciccozzi M.
      • Pybus O.G.
      • de Oliveira T.
      • Alcantara Jr., L.C.
      Zika virus complete genome from Salvador, Bahia, Brazil.
      ,
      • Kuno G.
      • Chang G.J.
      Full-length sequencing and genomic characterization of Bagaza, Kedougou, and Zika viruses.
      ,
      • Ladner J.T.
      • Wiley M.R.
      • Prieto K.
      • Yasuda C.Y.
      • Nagle E.
      • Kasper M.R.
      • Reyes D.
      • Vasilakis N.
      • Heang V.
      • Weaver S.C.
      • Haddow A.
      • Tesh R.B.
      • Sovann L.
      • Palacios G.
      Complete genome sequences of five Zika virus isolates.
      ,
      • Lanciotti R.S.
      • Kosoy O.L.
      • Laven J.J.
      • Velez J.O.
      • Lambert A.J.
      • Johnson A.J.
      • Stanfield S.M.
      • Duffy M.R.
      Genetic and serologic properties of Zika virus associated with an epidemic, Yap State, Micronesia, 2007.
      ,
      • Lednicky J.
      • Beau De Rochars V.M.
      • El Badry M.
      • Loeb J.
      • Telisma T.
      • Chavannes S.
      • Anilis G.
      • Cella E.
      • Ciccozzi M.
      • Rashid M.
      • Okech B.
      • Salemi M.
      • Morris Jr., J.G.
      Zika virus outbreak in Haiti in 2014: molecular and clinical data.
      ,
      • Liu L.
      • Wu W.
      • Zhao X.
      • Xiong Y.
      • Zhang S.
      • Liu X.
      • Qu J.
      • Li J.
      • Nei K.
      • Liang M.
      • Shu Y.
      • Hu G.
      • Ma X.
      • Li D.
      Complete genome sequence of Zika virus from the first imported case in mainland China.
      ,
      • Zhu Z.
      • Chan J.F.
      • Tee K.M.
      • Choi G.K.
      • Lau S.K.
      • Woo P.C.
      • Tse H.
      • Yuen K.Y.
      Comparative genomic analysis of pre-epidemic and epidemic Zika virus strains for virological factors potentially associated with the rapidly expanding epidemic.
      ). As shown in Fig. 4, a phylogenetic analysis using the nucleotide sequence of all 29 complete or near-complete ZIKV genomes retrievable from GenBank (as of May 2016) revealed that the geographically and temporally distinct ZIKV strains can be grouped into two major genetic lineages, African and Asian, with all eighteen 2015–2016 American epidemic strains derived from a common ancestor of the Asian lineage (
      • Yun S.I.
      • Song B.H.
      • Frank J.C.
      • Julander J.G.
      • Polejaeva I.A.
      • Davies C.J.
      • White K.L.
      • Lee Y.M.
      Complete genome sequences of three historically important, spatiotemporally distinct, and genetically divergent strains of Zika virus: MR-766, P6-740, and PRVABC-59.
      ). This spatiotemporal relationship is consistent with recent reports (
      • Calvet G.
      • Aguiar R.S.
      • Melo A.S.
      • Sampaio S.A.
      • de Filippis I.
      • Fabri A.
      • Araujo E.S.
      • de Sequeira P.C.
      • de Mendonca M.C.
      • de Oliveira L.
      • Tschoeke D.A.
      • Schrago C.G.
      • Thompson F.L.
      • Brasil P.
      • Dos Santos F.B.
      • Nogueira R.M.
      • Tanuri A.
      • de Filippis A.M.
      Detection and sequencing of Zika virus from amniotic fluid of fetuses with microcephaly in Brazil: a case study.
      ,
      • Enfissi A.
      • Codrington J.
      • Roosblad J.
      • Kazanji M.
      • Rousset D.
      Zika virus genome from the Americas.
      ,
      • Faria N.R.
      • Azevedo Rdo S.
      • Kraemer M.U.
      • Souza R.
      • Cunha M.S.
      • Hill S.C.
      • Theze J.
      • Bonsall M.B.
      • Bowden T.A.
      • Rissanen I.
      • Rocco I.M.
      • Nogueira J.S.
      • Maeda A.Y.
      • Vasami F.G.
      • Macedo F.L.
      • Suzuki A.
      • Rodrigues S.G.
      • Cruz A.C.
      • Nunes B.T.
      • Medeiros D.B.
      • Rodrigues D.S.
      • Nunes Queiroz A.L.
      • da Silva E.V.
      • Henriques D.F.
      • Travassos da Rosa E.S.
      • de Oliveira C.S.
      • Martins L.C.
      • Vasconcelos H.B.
      • Casseb L.M.
      • Simith Dde B.
      • Messina J.P.
      • Abade L.
      • Lourenco J.
      • Carlos Junior Alcantara L.
      • de Lima M.M.
      • Giovanetti M.
      • Hay S.I.
      • de Oliveira R.S.
      • Lemos Pda S.
      • de Oliveira L.F.
      • de Lima C.P.
      • da Silva S.P.
      • de Vasconcelos J.M.
      • Franco L.
      • Cardoso J.F.
      • Vianez-Junior J.L.
      • Mir D.
      • Bello G.
      • Delatorre E.
      • Khan K.
      • Creatore M.
      • Coelho G.E.
      • de Oliveira W.K.
      • Tesh R.
      • Pybus O.G.
      • Nunes M.R.
      • Vasconcelos P.F.
      Zika virus in the Americas: early epidemiological and genetic findings.
      ,
      • Faye O.
      • Freire C.C.
      • Iamarino A.
      • Faye O.
      • de Oliveira J.V.
      • Diallo M.
      • Zanotto P.M.
      • Sall A.A.
      Molecular evolution of Zika virus during its emergence in the 20th century.
      ,
      • Haddow A.D.
      • Schuh A.J.
      • Yasuda C.Y.
      • Kasper M.R.
      • Heang V.
      • Huy R.
      • Guzman H.
      • Tesh R.B.
      • Weaver S.C.
      Genetic characterization of Zika virus strains: geographic expansion of the Asian lineage.
      ,
      • Lanciotti R.S.
      • Lambert A.J.
      • Holodniy M.
      • Saavedra S.
      • Signor Ldel C.
      Phylogeny of Zika virus in western hemisphere, 2015.
      ,
      • Mlakar J.
      • Korva M.
      • Tul N.
      • Popovic M.
      • Poljsak-Prijatelj M.
      • Mraz J.
      • Kolenc M.
      • Resman Rus K.
      • Vesnaver Vipotnik T.
      • Fabjan Vodusek V.
      • Vizjak A.
      • Pizem J.
      • Petrovec M.
      • Avsic Zupanc T.
      Zika virus associated with microcephaly.
      ,
      • Wang L.
      • Valderramos S.G.
      • Wu A.
      • Ouyang S.
      • Li C.
      • Brasil P.
      • Bonaldo M.
      • Coates T.
      • Nielsen-Saines K.
      • Jiang T.
      • Aliyari R.
      • Cheng G.
      From mosquitos to humans: genetic evolution of Zika virus.
      ). Moreover, the most recent data from phylogenetic and molecular clock analyses suggest that the 2015–2016 American epidemic began with a single introduction of ZIKV into the Americas during May–December 2013, along with the increase in air traffic from ZIKV-endemic areas to Brazil and the emergence of ZIKV in the Pacific Islands (
      • Faria N.R.
      • Azevedo Rdo S.
      • Kraemer M.U.
      • Souza R.
      • Cunha M.S.
      • Hill S.C.
      • Theze J.
      • Bonsall M.B.
      • Bowden T.A.
      • Rissanen I.
      • Rocco I.M.
      • Nogueira J.S.
      • Maeda A.Y.
      • Vasami F.G.
      • Macedo F.L.
      • Suzuki A.
      • Rodrigues S.G.
      • Cruz A.C.
      • Nunes B.T.
      • Medeiros D.B.
      • Rodrigues D.S.
      • Nunes Queiroz A.L.
      • da Silva E.V.
      • Henriques D.F.
      • Travassos da Rosa E.S.
      • de Oliveira C.S.
      • Martins L.C.
      • Vasconcelos H.B.
      • Casseb L.M.
      • Simith Dde B.
      • Messina J.P.
      • Abade L.
      • Lourenco J.
      • Carlos Junior Alcantara L.
      • de Lima M.M.
      • Giovanetti M.
      • Hay S.I.
      • de Oliveira R.S.
      • Lemos Pda S.
      • de Oliveira L.F.
      • de Lima C.P.
      • da Silva S.P.
      • de Vasconcelos J.M.
      • Franco L.
      • Cardoso J.F.
      • Vianez-Junior J.L.
      • Mir D.
      • Bello G.
      • Delatorre E.
      • Khan K.
      • Creatore M.
      • Coelho G.E.
      • de Oliveira W.K.
      • Tesh R.
      • Pybus O.G.
      • Nunes M.R.
      • Vasconcelos P.F.
      Zika virus in the Americas: early epidemiological and genetic findings.
      ). Despite its considerable degree of genetic variation, little is known about the impact of viral genetic variation on the pathogenicity of ZIKV of the African and Asian lineages, as well as among different strains within the Asian lineage (
      • Wang L.
      • Valderramos S.G.
      • Wu A.
      • Ouyang S.
      • Li C.
      • Brasil P.
      • Bonaldo M.
      • Coates T.
      • Nielsen-Saines K.
      • Jiang T.
      • Aliyari R.
      • Cheng G.
      From mosquitos to humans: genetic evolution of Zika virus.
      ). Thus, more research is needed to extend our understanding of the evolution and diversity of ZIKV and to explore the biological effects of viral genetic variation on the outcome of ZIKV infection.
      Fig. 4
      Fig. 4An unrooted phylogenetic tree based on the nucleotide sequence of complete or near-complete genomes of all 29 available ZIKVs (as of May 2016). Multiple sequence alignments were performed, and the unrooted phylogenetic tree was reconstructed, as described in . Two major genetic clusters are indicated: African lineage (green circle disc) and Asian lineage (orange circle disc). Also, the tree shows that all of the eighteen 2015–2016 American epidemic strains (red circle disc) belong to the Asian lineage. The ZIKV strains used in our phylogenetic analysis are listed at the bottom of the tree, with a description in parenthesis of the year, country, host of isolation, and their GenBank accession numbers.
      Note that the genome of MR-766 has been fully sequenced by four independent groups, and their nucleotide sequences are not identical, likely because of variations in the cultivation history of the virus.

      4. Transmission

      ZIKV is transmitted to humans primarily through the bite of infected mosquitoes, but it can also be passed from mother to child during pregnancy or spread through sexual contact, breastfeeding, or blood transfusion. The multiple modes of ZIKV transmission make it difficult to develop control strategies against the pathogen.

      4.1 Vector-borne transmission

      ZIKV is an arthropod-borne virus (arbovirus) that is transmitted by mosquito vectors, with two distinct transmission cycles (
      • Weaver S.C.
      • Costa F.
      • Garcia-Blanco M.A.
      • Ko A.I.
      • Ribeiro G.S.
      • Saade G.
      • Shi P.Y.
      • Vasilakis N.
      Zika virus: history, emergence, biology, and prospects for control.
      ): (i) a sylvatic cycle, involved in the maintenance of ZIKV between non-human primates and arboreal mosquitoes in forests; and (ii) an urban cycle, involved in the transmission of ZIKV between humans and urban mosquitoes in towns (Fig. 5). Occasionally, the virus is presumably transmitted by arboreal mosquitoes from non-human primates to humans when they are in close proximity. ZIKV has been isolated in Africa and Asia from a number of different mosquito species in the genus Aedes (e.g., A. aegypti, A. africanus, A. albopictus, A. apicoargenteus, A. furcifer, A. luteocephalus, A. opok, and A. vittatus), which can potentially act as vectors for viral transmission in a given environment of those endemic areas (
      • Akoua-Koffi C.
      • Diarrassouba S.
      • Benie V.B.
      • Ngbichi J.M.
      • Bozoua T.
      • Bosson A.
      • Akran V.
      • Carnevale P.
      • Ehouman A.
      Investigation surrounding a fatal case of yellow fever in Cote d'Ivoire in 1999.
      ,
      • Berthet N.
      • Nakoune E.
      • Kamgang B.
      • Selekon B.
      • Descorps-Declere S.
      • Gessain A.
      • Manuguerra J.C.
      • Kazanji M.
      Molecular characterization of three Zika flaviviruses obtained from sylvatic mosquitoes in the Central African Republic.
      ,
      • Cornet M.
      • Robin Y.
      • Chateau R.
      • Heme G.
      • Adam C.
      Isolement d'arbovirus au Senegal oriental a partir de moustiques (1972–1977) et note surl'epidemiologie des virus transmis par les Aedes, en particulier du virus amaril.
      ,
      • Diallo D.
      • Sall A.A.
      • Diagne C.T.
      • Faye O.
      • Faye O.
      • Ba Y.
      • Hanley K.A.
      • Buenemann M.
      • Weaver S.C.
      • Diallo M.
      Zika virus emergence in mosquitoes in southeastern Senegal, 2011.
      ,
      • Dick G.W.
      • Kitchen S.F.
      • Haddow A.J.
      Zika virus. I. Isolations and serological specificity.
      ,
      • Fagbami A.H.
      Zika virus infections in Nigeria: virological and seroepidemiological investigations in Oyo State.
      ,
      • Grard G.
      • Caron M.
      • Mombo I.M.
      • Nkoghe D.
      • Mboui Ondo S.
      • Jiolle D.
      • Fontenille D.
      • Paupy C.
      • Leroy E.M.
      Zika virus in Gabon (Central Africa) — 2007: a new threat from Aedes albopictus?.
      ,
      • Haddow A.J.
      • Williams M.C.
      • Woodall J.P.
      • Simpson D.I.
      • Goma L.K.
      Twelve isolations of Zika virus from Aedes (Stegomyia) Africanus (Theobald) taken in and above a Uganda forest.
      ,
      • Marchette N.J.
      • Garcia R.
      • Rudnick A.
      Isolation of Zika virus from Aedes aegypti mosquitoes in Malaysia.
      ,
      • McCrae A.W.
      • Kirya B.G.
      Yellow fever and Zika virus epizootics and enzootics in Uganda.
      ,
      • Weinbren M.P.
      • Williams M.C.
      Zika virus: further isolations in the Zika area, and some studies on the strains isolated.
      ). Also, A. hensilli and A. polynesiensis were suggested to be vectors in the Yap (
      • Ledermann J.P.
      • Guillaumot L.
      • Yug L.
      • Saweyog S.C.
      • Tided M.
      • Machieng P.
      • Pretrick M.
      • Marfel M.
      • Griggs A.
      • Bel M.
      • Duffy M.R.
      • Hancock W.T.
      • Ho-Chen T.
      • Powers A.M.
      Aedes hensilli as a potential vector of chikungunya and Zika viruses.
      ) and French Polynesia (
      • Cao-Lormeau V.M.
      • Roche C.
      • Teissier A.
      • Robin E.
      • Berry A.L.
      • Mallet H.P.
      • Sall A.A.
      • Musso D.
      Zika virus, French Polynesia, South Pacific, 2013.
      ,
      • Musso D.
      • Nilles E.J.
      • Cao-Lormeau V.M.
      Rapid spread of emerging Zika virus in the Pacific area.
      ) outbreaks, respectively. In most cases, however, their vector competence in ZIKV transmission has not been fully investigated (
      • Diagne C.T.
      • Diallo D.
      • Faye O.
      • Ba Y.
      • Faye O.
      • Gaye A.
      • Dia I.
      • Faye O.
      • Weaver S.C.
      • Sall A.A.
      • Diallo M.
      Potential of selected Senegalese Aedes spp. mosquitoes (Diptera: Culicidae) to transmit Zika virus.
      ). Based on previous studies with other mosquito-borne flaviviruses (DENV and YFV), the vector competence is apparently influenced not only by the genetic variation in the virus and vector (
      • Gubler D.J.
      • Nalim S.
      • Tan R.
      • Saipan H.
      • Sulianti Saroso J.
      Variation in susceptibility to oral infection with dengue viruses among geographic strains of Aedes aegypti.
      ) but also by the population density of the vectors in a particular environment (
      • Miller B.R.
      • Monath T.P.
      • Tabachnick W.J.
      • Ezike V.I.
      Epidemic yellow fever caused by an incompetent mosquito vector.
      ). This observation is also likely true for ZIKV (
      • Diagne C.T.
      • Diallo D.
      • Faye O.
      • Ba Y.
      • Faye O.
      • Gaye A.
      • Dia I.
      • Faye O.
      • Weaver S.C.
      • Sall A.A.
      • Diallo M.
      Potential of selected Senegalese Aedes spp. mosquitoes (Diptera: Culicidae) to transmit Zika virus.
      ).
      Fig. 5
      Fig. 5Vector-borne transmission of ZIKV. There are two mosquito-driven transmission cycles: (1) a sylvatic cycle, in which the virus cycles between non-human primates and arboreal mosquitoes; and (2) an urban cycle, in which the virus cycles between humans and urban mosquitoes. Under certain circumstances, ZIKV can presumably be transmitted from non-human primates to humans via arboreal mosquitoes.
      In an urban cycle, the transmission of ZIKV is believed to be mediated predominantly by two Aedes species mosquitoes: A. aegypti, recognized by a bright lyre-shaped dorsal pattern with white bands on its legs, and A. albopictus, characterized by a single longitudinal dorsal stripe with white bands on its legs (
      • ECDC
      Main Diagnostic Morphological Characters for Adults of IMS.
      ). Of these two species, A. aegypti is considered to be the primary vector associated with ZIKV outbreaks, as supported by (1) isolating or detecting the virus in a pool of A. aegypti mosquitoes (
      • Akoua-Koffi C.
      • Diarrassouba S.
      • Benie V.B.
      • Ngbichi J.M.
      • Bozoua T.
      • Bosson A.
      • Akran V.
      • Carnevale P.
      • Ehouman A.
      Investigation surrounding a fatal case of yellow fever in Cote d'Ivoire in 1999.
      ,
      • Diallo D.
      • Sall A.A.
      • Diagne C.T.
      • Faye O.
      • Faye O.
      • Ba Y.
      • Hanley K.A.
      • Buenemann M.
      • Weaver S.C.
      • Diallo M.
      Zika virus emergence in mosquitoes in southeastern Senegal, 2011.
      ,
      • Marchette N.J.
      • Garcia R.
      • Rudnick A.
      Isolation of Zika virus from Aedes aegypti mosquitoes in Malaysia.
      ); (2) showing the susceptibility of A. aegypti mosquitoes to infection with the virus (
      • Boorman J.P.
      • Porterfield J.S.
      A simple technique for infection of mosquitoes with viruses; transmission of Zika virus.
      ,
      • Cornet M.
      • Robin Y.
      • Adam C.
      • Valade M.
      • Calvo M.A.
      Comparison between experimental transmission of yellow fever and Zika viruses in Aedes aegypti.
      ,
      • Li M.I.
      • Wong P.S.
      • Ng L.C.
      • Tan C.H.
      Oral susceptibility of Singapore Aedes (Stegomyia) aegypti (Linnaeus) to Zika virus.
      ); and (3) demonstrating the transmission of the virus from artificially fed A. aegypti mosquitoes to rhesus monkeys and mice (
      • Boorman J.P.
      • Porterfield J.S.
      A simple technique for infection of mosquitoes with viruses; transmission of Zika virus.
      ). As a secondary vector for ZIKV, A. albopictus has been suggested to be involved in the urban transmission in Gabon, where five human sera and two A. albopictus pools collected in an urban setting in 2007 were positive for ZIKV (
      • Grard G.
      • Caron M.
      • Mombo I.M.
      • Nkoghe D.
      • Mboui Ondo S.
      • Jiolle D.
      • Fontenille D.
      • Paupy C.
      • Leroy E.M.
      Zika virus in Gabon (Central Africa) — 2007: a new threat from Aedes albopictus?.
      ), and in Singapore, where high dissemination and transmission rates of ZIKV in local A. albopictus have been reported (
      • Wong P.S.
      • Li M.Z.
      • Chong C.S.
      • Ng L.C.
      • Tan C.H.
      Aedes (Stegomyia) albopictus (Skuse): a potential vector of Zika virus in Singapore.
      ). Because of its high invasive ability and wide geographic distribution in tropical and temperate regions, there is a high potential for A. albopictus to become a major vector for the urban transmission of ZIKV across the globe.
      Both A. aegypti and A. albopictus are usually active during daylight hours (
      • Ponlawat A.
      • Harrington L.C.
      Blood feeding patterns of Aedes aegypti and Aedes albopictus in Thailand.
      ,
      • Scott T.W.
      • Takken W.
      Feeding strategies of anthropophilic mosquitoes result in increased risk of pathogen transmission.
      ) and are widely distributed throughout the tropical and subtropical regions of the world, with the habitat of A. albopictus extending further into cool temperate regions (
      • Kraemer M.U.
      • Sinka M.E.
      • Duda K.A.
      • Mylne A.Q.
      • Shearer F.M.
      • Barker C.M.
      • Moore C.G.
      • Carvalho R.G.
      • Coelho G.E.
      • Van Bortel W.
      • Hendrickx G.
      • Schaffner F.
      • Elyazar I.R.
      • Teng H.J.
      • Brady O.J.
      • Messina J.P.
      • Pigott D.M.
      • Scott T.W.
      • Smith D.L.
      • Wint G.R.
      • Golding N.
      • Hay S.I.
      The global distribution of the arbovirus vectors Aedes aegypti and Ae. albopictus.
      ,
      • Paupy C.
      • Delatte H.
      • Bagny L.
      • Corbel V.
      • Fontenille D.
      Aedes albopictus, an arbovirus vector: from the darkness to the light.
      ,
      • Thomas S.M.
      • Obermayr U.
      • Fischer D.
      • Kreyling J.
      • Beierkuhnlein C.
      Low-temperature threshold for egg survival of a post-diapause and non-diapause European aedine strain, Aedes albopictus (Diptera: Culicidae).
      ). In the US, both A. aegypti and A. albopictus are present in practically the entire southern and southeastern half of the country (Fig. 3, inset map), according to a recent estimate by the Centers for Disease Control and Prevention (
      • CDC
      Potential Range in US.
      ), reaching their maximal abundance in June–October each year (
      • Monaghan A.J.
      • Morin C.W.
      • Steinhoff D.F.
      • Wilhelmi O.
      • Hayden M.
      • Quattrochi D.A.
      • Reiskind M.
      • Lloyd A.L.
      • Smith K.
      • Schmidt C.A.
      • Scalf P.E.
      • Ernst K.
      On the seasonal occurrence and abundance of the Zika virus vector mosquito Aedes aegypti in the contiguous United States.
      ). A recent study has shown that although they are susceptible to ZIKV infection, American populations of both A. aegypti and A. albopictus collected from Florida have unexpectedly low levels of vector competence (the mosquito's ability to biologically transmit ZIKV) (
      • Chouin-Carneiro T.
      • Vega-Rua A.
      • Vazeille M.
      • Yebakima A.
      • Girod R.
      • Goindin D.
      • Dupont-Rouzeyrol M.
      • Lourenco-de-Oliveira R.
      • Failloux A.B.
      Differential susceptibilities of Aedes aegypti and Aedes albopictus from the Americas to Zika virus.
      ). However, A. aegypti (and to a lesser extent A. albopictus) is thought to have a high level of vectorial capacity (a measurement of the efficiency of ZIKV transmission) because it primarily bites humans, infects several persons during a single blood meal, and lives in close association with humans (
      • Gubler D.J.
      The global emergence/resurgence of arboviral diseases as public health problems.
      ). Collectively, there is real potential for the spread of ZIKV across portions of the southern US over the next few years.
      Moreover, ZIKV has been isolated or detected occasionally in other mosquitoes, such as Anopheles coustani, Culex perfuscus, and Mansonia uniformis (
      • Diallo D.
      • Sall A.A.
      • Diagne C.T.
      • Faye O.
      • Faye O.
      • Ba Y.
      • Hanley K.A.
      • Buenemann M.
      • Weaver S.C.
      • Diallo M.
      Zika virus emergence in mosquitoes in southeastern Senegal, 2011.
      ,
      • Faye O.
      • Faye O.
      • Diallo D.
      • Diallo M.
      • Weidmann M.
      • Sall A.A.
      Quantitative real-time PCR detection of Zika virus and evaluation with field-caught mosquitoes.
      ). These mosquito species may contribute to the transmission of ZIKV in particular environments, but the mere isolation of the virus from mosquitoes or its detection in the mosquitoes does not necessarily identify it as an actual vector. Further field and experimental studies are needed to better define vector competence for ZIKV and to determine whether there are any other mosquito vectors or animal hosts.

      4.2 Non-vector-borne transmission

      Direct human-to-human transmission of ZIKV has been documented to occur perinatally, sexually, and through breastfeeding or blood transfusion: (i) ZIKV can be passed from an infected mother to her fetus during pregnancy, as evidenced not only by the detection of viral RNA in the amniotic fluid, urine, or serum of mothers whose fetuses had brain abnormalities (
      • Brasil P.
      • Pereira Jr., J.P.
      • Moreira M.E.
      • Ribeiro Nogueira R.M.
      • Damasceno L.
      • Wakimoto M.
      • Rabello R.S.
      • Valderramos S.G.
      • Halai U.A.
      • Salles T.S.
      • Zin A.A.
      • Horovitz D.
      • Daltro P.
      • Boechat M.
      • Raja Gabaglia C.
      • Carvalho de Sequeira P.
      • Pilotto J.H.
      • Medialdea-Carrera R.
      • Cotrim da Cunha D.
      • Abreu de Carvalho L.M.
      • Pone M.
      • Machado Siqueira A.
      • Calvet G.A.
      • Rodrigues Baião A.E.
      • Neves E.S.
      • Nassar de Carvalho P.R.
      • Hasue R.H.
      • Marschik P.B.
      • Einspieler C.
      • Janzen C.
      • Cherry J.D.
      • Bispo de Filippis A.M.
      • Nielsen-Saines K.
      Zika virus infection in pregnant women in Rio de Janeiro.
      ,
      • Calvet G.
      • Aguiar R.S.
      • Melo A.S.
      • Sampaio S.A.
      • de Filippis I.
      • Fabri A.
      • Araujo E.S.
      • de Sequeira P.C.
      • de Mendonca M.C.
      • de Oliveira L.
      • Tschoeke D.A.
      • Schrago C.G.
      • Thompson F.L.
      • Brasil P.
      • Dos Santos F.B.
      • Nogueira R.M.
      • Tanuri A.
      • de Filippis A.M.
      Detection and sequencing of Zika virus from amniotic fluid of fetuses with microcephaly in Brazil: a case study.
      ,
      • Driggers R.W.
      • Ho C.Y.
      • Korhonen E.M.
      • Kuivanen S.
      • Jaaskelainen A.J.
      • Smura T.
      • Rosenberg A.
      • Hill D.A.
      • DeBiasi R.L.
      • Vezina G.
      • Timofeev J.
      • Rodriguez F.J.
      • Levanov L.
      • Razak J.
      • Iyengar P.
      • Hennenfent A.
      • Kennedy R.
      • Lanciotti R.
      • du Plessis A.
      • Vapalahti O.
      Zika virus infection with prolonged maternal viremia and fetal brain abnormalities.
      ,
      • Jouannic J.M.
      • Friszer S.
      • Leparc-Goffart I.
      • Garel C.
      • Eyrolle-Guignot D.
      Zika virus infection in French Polynesia.
      ,
      • Meaney-Delman D.
      • Hills S.L.
      • Williams C.
      • Galang R.R.
      • Iyengar P.
      • Hennenfent A.K.
      • Rabe I.B.
      • Panella A.
      • Oduyebo T.
      • Honein M.A.
      • Zaki S.
      • Lindsey N.
      • Lehman J.A.
      • Kwit N.
      • Bertolli J.
      • Ellington S.
      • Igbinosa I.
      • Minta A.A.
      • Petersen E.E.
      • Mead P.
      • Rasmussen S.A.
      • Jamieson D.J.
      Zika virus infection among U.S. pregnant travelers — August 2015–February 2016.
      ,
      • Oliveira Melo A.S.
      • Malinger G.
      • Ximenes R.
      • Szejnfeld P.O.
      • Alves Sampaio S.
      • Bispo de Filippis A.M.
      Zika virus intrauterine infection causes fetal brain abnormality and microcephaly: tip of the iceberg?.
      ,
      • Sarno M.
      • Sacramento G.A.
      • Khouri R.
      • do Rosario M.S.
      • Costa F.
      • Archanjo G.
      • Santos L.A.
      • Nery Jr., N.
      • Vasilakis N.
      • Ko A.I.
      • de Almeida A.R.
      Zika virus infection and stillbirths: a case of hydrops fetalis, hydranencephaly and fetal demise.
      ,
      • Villamil-Gomez W.E.
      • Mendoza-Guete A.
      • Villalobos E.
      • Gonzalez-Arismendy E.
      • Uribe-Garcia A.M.
      • Castellanos J.E.
      • Rodriguez-Morales A.J.
      Diagnosis, management and follow-up of pregnant women with Zika virus infection: a preliminary report of the ZIKERNCOL cohort study on Sincelejo, Colombia.
      ), but also by probing of viral RNA/proteins/particles in the brain, placenta, or serum of newborns with microcephaly and those aborted (
      • Besnard M.
      • Lastere S.
      • Teissier A.
      • Cao-Lormeau V.
      • Musso D.
      Evidence of perinatal transmission of Zika virus, French Polynesia, December 2013 and February 2014.
      ,
      • Butler D.
      First Zika-linked birth defects detected in Colombia.
      ,
      • Driggers R.W.
      • Ho C.Y.
      • Korhonen E.M.
      • Kuivanen S.
      • Jaaskelainen A.J.
      • Smura T.
      • Rosenberg A.
      • Hill D.A.
      • DeBiasi R.L.
      • Vezina G.
      • Timofeev J.
      • Rodriguez F.J.
      • Levanov L.
      • Razak J.
      • Iyengar P.
      • Hennenfent A.
      • Kennedy R.
      • Lanciotti R.
      • du Plessis A.
      • Vapalahti O.
      Zika virus infection with prolonged maternal viremia and fetal brain abnormalities.
      ,
      • Faria N.R.
      • Azevedo Rdo S.
      • Kraemer M.U.
      • Souza R.
      • Cunha M.S.
      • Hill S.C.
      • Theze J.
      • Bonsall M.B.
      • Bowden T.A.
      • Rissanen I.
      • Rocco I.M.
      • Nogueira J.S.
      • Maeda A.Y.
      • Vasami F.G.
      • Macedo F.L.
      • Suzuki A.
      • Rodrigues S.G.
      • Cruz A.C.
      • Nunes B.T.
      • Medeiros D.B.
      • Rodrigues D.S.
      • Nunes Queiroz A.L.
      • da Silva E.V.
      • Henriques D.F.
      • Travassos da Rosa E.S.
      • de Oliveira C.S.
      • Martins L.C.
      • Vasconcelos H.B.
      • Casseb L.M.
      • Simith Dde B.
      • Messina J.P.
      • Abade L.
      • Lourenco J.
      • Carlos Junior Alcantara L.
      • de Lima M.M.
      • Giovanetti M.
      • Hay S.I.
      • de Oliveira R.S.
      • Lemos Pda S.
      • de Oliveira L.F.
      • de Lima C.P.
      • da Silva S.P.
      • de Vasconcelos J.M.
      • Franco L.
      • Cardoso J.F.
      • Vianez-Junior J.L.
      • Mir D.
      • Bello G.
      • Delatorre E.
      • Khan K.
      • Creatore M.
      • Coelho G.E.
      • de Oliveira W.K.
      • Tesh R.
      • Pybus O.G.
      • Nunes M.R.
      • Vasconcelos P.F.
      Zika virus in the Americas: early epidemiological and genetic findings.
      ,
      • Martines R.B.
      • Bhatnagar J.
      • Keating M.K.
      • Silva-Flannery L.
      • Muehlenbachs A.
      • Gary J.
      • Goldsmith C.
      • Hale G.
      • Ritter J.
      • Rollin D.
      • Shieh W.J.
      • Luz K.G.
      • Ramos A.M.
      • Davi H.P.
      • Kleber de Oliveria W.
      • Lanciotti R.
      • Lambert A.
      • Zaki S.
      Notes from the field: evidence of Zika virus infection in brain and placental tissues from two congenitally infected newborns and two fetal losses — Brazil, 2015.
      ,
      • Mlakar J.
      • Korva M.
      • Tul N.
      • Popovic M.
      • Poljsak-Prijatelj M.
      • Mraz J.
      • Kolenc M.
      • Resman Rus K.
      • Vesnaver Vipotnik T.
      • Fabjan Vodusek V.
      • Vizjak A.
      • Pizem J.
      • Petrovec M.
      • Avsic Zupanc T.
      Zika virus associated with microcephaly.
      ,
      • Sarno M.
      • Sacramento G.A.
      • Khouri R.
      • do Rosario M.S.
      • Costa F.
      • Archanjo G.
      • Santos L.A.
      • Nery Jr., N.
      • Vasilakis N.
      • Ko A.I.
      • de Almeida A.R.
      Zika virus infection and stillbirths: a case of hydrops fetalis, hydranencephaly and fetal demise.
      ). (ii) ZIKV can be sexually transmitted from an infected person to his or her partners, as shown by the detection of the virus in patient's semen, often in high titer, and transmission of the virus between both sexes, with the male-to-female transmission occurring more frequently than female-to-male or male-to-male transmission (
      • Armstrong P.
      • Hennessey M.
      • Adams M.
      • Cherry C.
      • Chiu S.
      • Harrist A.
      • Kwit N.
      • Lewis L.
      • McGuire D.O.
      • Oduyebo T.
      • Russell K.
      • Talley P.
      • Tanner M.
      • Williams C.
      • Zika Virus Response Epidemiology and Laboratory Team
      Travel-associated Zika virus disease cases among U.S. residents — United States, January 2015–February 2016.
      ,
      • Atkinson B.
      • Hearn P.
      • Afrough B.
      • Lumley S.
      • Carter D.
      • Aarons E.J.
      • Simpson A.J.
      • Brooks T.J.
      • Hewson R.
      Detection of Zika virus in semen.
      ,
      • Deckard D.T.
      • Chung W.M.
      • Brooks J.T.
      • Smith J.C.
      • Woldai S.
      • Hennessey M.
      • Kwit N.
      • Mead P.
      Male-to-male sexual transmission of Zika virus — Texas, January 2016.
      ,
      • Foy B.D.
      • Kobylinski K.C.
      • Chilson Foy J.L.
      • Blitvich B.J.
      • Travassos da Rosa A.
      • Haddow A.D.
      • Lanciotti R.S.
      • Tesh R.B.
      Probable non-vector-borne transmission of Zika virus, Colorado, USA.
      ,
      • Frank C.
      • Cadar D.
      • Schlaphof A.
      • Neddersen N.
      • Gunther S.
      • Schmidt-Chanasit J.
      • Tappe D.
      Sexual transmission of Zika virus in Germany, April 2016.
      ,
      • Freour T.
      • Mirallie S.
      • Hubert B.
      • Splingart C.
      • Barriere P.
      • Maquart M.
      • Leparc-Goffart I.
      Sexual transmission of Zika virus in an entirely asymptomatic couple returning from a Zika epidemic area, France, April 2016.
      ,
      • Hills S.L.
      • Russell K.
      • Hennessey M.
      • Williams C.
      • Oster A.M.
      • Fischer M.
      • Mead P.
      Transmission of Zika virus through sexual contact with travelers to areas of ongoing transmission — continental United States, 2016.
      ,
      • Mansuy J.M.
      • Dutertre M.
      • Mengelle C.
      • Fourcade C.
      • Marchou B.
      • Delobel P.
      • Izopet J.
      • Martin-Blondel G.
      Zika virus: high infectious viral load in semen, a new sexually transmitted pathogen?.
      ,
      • Musso D.
      • Roche C.
      • Robin E.
      • Nhan T.
      • Teissier A.
      • Cao-Lormeau V.M.
      Potential sexual transmission of Zika virus.
      ,
      • Venturi G.
      • Zammarchi L.
      • Fortuna C.
      • Remoli M.E.
      • Benedetti E.
      • Fiorentini C.
      • Trotta M.
      • Rizzo C.
      • Mantella A.
      • Rezza G.
      • Bartoloni A.
      An autochthonous case of Zika due to possible sexual transmission, Florence, Italy, 2014.
      ). In addition to semen, it is important to note that ZIKV has also been detected in urine, saliva, and nasopharyngeal swabs, potentially facilitating the non-vector-borne transmission of ZIKV (
      • Atkinson B.
      • Hearn P.
      • Afrough B.
      • Lumley S.
      • Carter D.
      • Aarons E.J.
      • Simpson A.J.
      • Brooks T.J.
      • Hewson R.
      Detection of Zika virus in semen.
      ,
      • Barzon L.
      • Pacenti M.
      • Berto A.
      • Sinigaglia A.
      • Franchin E.
      • Lavezzo E.
      • Brugnaro P.
      • Palu G.
      Isolation of infectious Zika virus from saliva and prolonged viral RNA shedding in a traveller returning from the Dominican Republic to Italy, January 2016.
      ,
      • Besnard M.
      • Lastere S.
      • Teissier A.
      • Cao-Lormeau V.
      • Musso D.
      Evidence of perinatal transmission of Zika virus, French Polynesia, December 2013 and February 2014.
      ,
      • Brasil P.
      • Pereira Jr., J.P.
      • Moreira M.E.
      • Ribeiro Nogueira R.M.
      • Damasceno L.
      • Wakimoto M.
      • Rabello R.S.
      • Valderramos S.G.
      • Halai U.A.
      • Salles T.S.
      • Zin A.A.
      • Horovitz D.
      • Daltro P.
      • Boechat M.
      • Raja Gabaglia C.
      • Carvalho de Sequeira P.
      • Pilotto J.H.
      • Medialdea-Carrera R.
      • Cotrim da Cunha D.
      • Abreu de Carvalho L.M.
      • Pone M.
      • Machado Siqueira A.
      • Calvet G.A.
      • Rodrigues Baião A.E.
      • Neves E.S.
      • Nassar de Carvalho P.R.
      • Hasue R.H.
      • Marschik P.B.
      • Einspieler C.
      • Janzen C.
      • Cherry J.D.
      • Bispo de Filippis A.M.
      • Nielsen-Saines K.
      Zika virus infection in pregnant women in Rio de Janeiro.
      ,
      • Fonseca K.
      • Meatherall B.
      • Zarra D.
      • Drebot M.
      • MacDonald J.
      • Pabbaraju K.
      • Wong S.
      • Webster P.
      • Lindsay R.
      • Tellier R.
      First case of Zika virus infection in a returning Canadian traveler.
      ,
      • Gourinat A.C.
      • O'Connor O.
      • Calvez E.
      • Goarant C.
      • Dupont-Rouzeyrol M.
      Detection of Zika virus in urine.
      ,
      • Korhonen E.M.
      • Huhtamo E.
      • Smura T.
      • Kallio-Kokko H.
      • Raassina M.
      • Vapalahti O.
      Zika virus infection in a traveller returning from the Maldives, June 2015.
      ,
      • Kutsuna S.
      • Kato Y.
      • Takasaki T.
      • Moi M.
      • Kotaki A.
      • Uemura H.
      • Matono T.
      • Fujiya Y.
      • Mawatari M.
      • Takeshita N.
      • Hayakawa K.
      • Kanagawa S.
      • Ohmagari N.
      Two cases of Zika fever imported from French Polynesia to Japan, December 2013 to January 2014.
      ,
      • Leung G.H.
      • Baird R.W.
      • Druce J.
      • Anstey N.M.
      Zika virus infection in Australia following a monkey bite in Indonesia.
      ,
      • Maria A.T.
      • Maquart M.
      • Makinson A.
      • Flusin O.
      • Segondy M.
      • Leparc-Goffart I.
      • Le Moing V.
      • Foulongne V.
      Zika virus infections in three travellers returning from South America and the Caribbean respectively, to Montpellier, France, December 2015 to January 2016.
      ,
      • Musso D.
      • Roche C.
      • Nhan T.X.
      • Robin E.
      • Teissier A.
      • Cao-Lormeau V.M.
      Detection of Zika virus in saliva.
      ,
      • Musso D.
      • Roche C.
      • Robin E.
      • Nhan T.
      • Teissier A.
      • Cao-Lormeau V.M.
      Potential sexual transmission of Zika virus.
      ,
      • Roze B.
      • Najioullah F.
      • Ferge J.L.
      • Apetse K.
      • Brouste Y.
      • Cesaire R.
      • Fagour C.
      • Fagour L.
      • Hochedez P.
      • Jeannin S.
      • Joux J.
      • Mehdaoui H.
      • Valentino R.
      • Signate A.
      • Cabie A.
      • GBS Zika Working Group
      Zika virus detection in urine from patients with Guillain-Barre syndrome on Martinique, January 2016.
      ,
      • Shinohara K.
      • Kutsuna S.
      • Takasaki T.
      • Moi M.L.
      • Ikeda M.
      • Kotaki A.
      • Yamamoto K.
      • Fujiya Y.
      • Mawatari M.
      • Takeshita N.
      • Hayakawa K.
      • Kanagawa S.
      • Kato Y.
      • Ohmagari N.
      Zika fever imported from Thailand to Japan, and diagnosed by PCR in the urine.
      ). (iii) ZIKV RNA/particles have been detected in breast milk, suggesting a potential risk of viral transmission through breastfeeding (
      • Besnard M.
      • Lastere S.
      • Teissier A.
      • Cao-Lormeau V.
      • Musso D.
      Evidence of perinatal transmission of Zika virus, French Polynesia, December 2013 and February 2014.
      ,
      • Dupont-Rouzeyrol M.
      • Biron A.
      • O'Connor O.
      • Huguon E.
      • Descloux E.
      Infectious Zika viral particles in breastmilk.
      ). (iv) ZIKV is likely to be transmitted through transfusions of blood from donors who have been infected with the virus (
      • Aubry M.
      • Finke J.
      • Teissier A.
      • Roche C.
      • Broult J.
      • Paulous S.
      • Despres P.
      • Cao-Lormeau V.M.
      • Musso D.
      Seroprevalence of arboviruses among blood donors in French Polynesia, 2011–2013.
      ,
      • Aubry M.
      • Richard V.
      • Green J.
      • Broult J.
      • Musso D.
      Inactivation of Zika virus in plasma with amotosalen and ultraviolet A illumination.
      ,
      • Faria N.R.
      • Azevedo Rdo S.
      • Kraemer M.U.
      • Souza R.
      • Cunha M.S.
      • Hill S.C.
      • Theze J.
      • Bonsall M.B.
      • Bowden T.A.
      • Rissanen I.
      • Rocco I.M.
      • Nogueira J.S.
      • Maeda A.Y.
      • Vasami F.G.
      • Macedo F.L.
      • Suzuki A.
      • Rodrigues S.G.
      • Cruz A.C.
      • Nunes B.T.
      • Medeiros D.B.
      • Rodrigues D.S.
      • Nunes Queiroz A.L.
      • da Silva E.V.
      • Henriques D.F.
      • Travassos da Rosa E.S.
      • de Oliveira C.S.
      • Martins L.C.
      • Vasconcelos H.B.
      • Casseb L.M.
      • Simith Dde B.
      • Messina J.P.
      • Abade L.
      • Lourenco J.
      • Carlos Junior Alcantara L.
      • de Lima M.M.
      • Giovanetti M.
      • Hay S.I.
      • de Oliveira R.S.
      • Lemos Pda S.
      • de Oliveira L.F.
      • de Lima C.P.
      • da Silva S.P.
      • de Vasconcelos J.M.
      • Franco L.
      • Cardoso J.F.
      • Vianez-Junior J.L.
      • Mir D.
      • Bello G.
      • Delatorre E.
      • Khan K.
      • Creatore M.
      • Coelho G.E.
      • de Oliveira W.K.
      • Tesh R.
      • Pybus O.G.
      • Nunes M.R.
      • Vasconcelos P.F.
      Zika virus in the Americas: early epidemiological and genetic findings.
      ,
      • Musso D.
      • Nhan T.
      • Robin E.
      • Roche C.
      • Bierlaire D.
      • Zisou K.
      • Shan Yan A.
      • Cao-Lormeau V.M.
      • Broult J.
      Potential for Zika virus transmission through blood transfusion demonstrated during an outbreak in French Polynesia, November 2013 to February 2014.
      ). (v) Direct transmission, although uncommon, can take place through the skin or mucous membranes, as exemplified by the index patient, who had a high viral load of 2 × 108 genome copies per ml in serum (
      • Swaminathan S.
      • Schlaberg R.
      • Lewis J.
      • Hanson K.E.
      • Couturier M.R.
      Fatal Zika virus infection with secondary nonsexual transmission.
      ). Although these non-vector-borne transmissions of ZIKV are believed to be uncommon, further studies are necessary to identify the risk factors involved in these routes of viral transmission.

      5. Virology

      ZIKV, a 50-nm enveloped particle, contains an inner nucleocapsid composed of a linear plus-strand genomic RNA and multiple copies of the viral capsid (C) protein and the outer host cell-derived lipid bilayer bearing 180 copies each of two proteins: the viral membrane (M, a cleavage product of prM) protein and the envelope (E) protein (
      • Kostyuchenko V.A.
      • Lim E.X.
      • Zhang S.
      • Fibriansah G.
      • Ng T.S.
      • Ooi J.S.
      • Shi J.
      • Lok S.M.
      Structure of the thermally stable Zika virus.
      ,
      • Sirohi D.
      • Chen Z.
      • Sun L.
      • Klose T.
      • Pierson T.C.
      • Rossmann M.G.
      • Kuhn R.J.
      The 3.8 A resolution cryo-EM structure of Zika virus.
      ). In the case of the American ZIKV strain PRVABC-59 (GenBank accession no. KX377337) isolated from a patient in Puerto Rico in 2015 (
      • Lanciotti R.S.
      • Lambert A.J.
      • Holodniy M.
      • Saavedra S.
      • Signor Ldel C.
      Phylogeny of Zika virus in western hemisphere, 2015.
      ), the genomic RNA is 10,807 nucleotides (nt) long and comprises a single 10,272-nt open reading frame (ORF) flanked by a 107-nt 5′ noncoding region (NCR) and a 428-nt 3′ NCR (
      • Yun S.I.
      • Song B.H.
      • Frank J.C.
      • Julander J.G.
      • Polejaeva I.A.
      • Davies C.J.
      • White K.L.
      • Lee Y.M.
      Complete genome sequences of three historically important, spatiotemporally distinct, and genetically divergent strains of Zika virus: MR-766, P6-740, and PRVABC-59.
      ) (Fig. 6A ). The ORF encodes a large polyprotein of 3423 amino acids (aa), which is predicted to be cleaved by viral and cellular proteases into at least 10 individual proteins, designated in an N- to C-terminal direction (
      • Kim J.K.
      • Kim J.M.
      • Song B.H.
      • Yun S.I.
      • Yun G.N.
      • Byun S.J.
      • Lee Y.M.
      Profiling of viral proteins expressed from the genomic RNA of Japanese encephalitis virus using a panel of 15 region-specific polyclonal rabbit antisera: implications for viral gene expression.
      ,
      • Yun S.I.
      • Song B.H.
      • Frank J.C.
      • Julander J.G.
      • Polejaeva I.A.
      • Davies C.J.
      • White K.L.
      • Lee Y.M.
      Complete genome sequences of three historically important, spatiotemporally distinct, and genetically divergent strains of Zika virus: MR-766, P6-740, and PRVABC-59.
      ): C (122 aa), prM (168 aa), E (504 aa), NS1 (352 aa), NS2A (226 aa), NS2B (130 aa), NS3 (617 aa), NS4A (150 aa), NS4B (251 aa), and NS5 (903 aa). The three N-terminal structural proteins are necessary for the formation of infectious virions (
      • Kostyuchenko V.A.
      • Lim E.X.
      • Zhang S.
      • Fibriansah G.
      • Ng T.S.
      • Ooi J.S.
      • Shi J.
      • Lok S.M.
      Structure of the thermally stable Zika virus.
      ,
      • Sirohi D.
      • Chen Z.
      • Sun L.
      • Klose T.
      • Pierson T.C.
      • Rossmann M.G.
      • Kuhn R.J.
      The 3.8 A resolution cryo-EM structure of Zika virus.
      ), and the seven C-terminal nonstructural proteins are essential for the replication of the viral genomic RNA (
      • Brinton M.A.
      Replication cycle and molecular biology of the West Nile virus.
      ,
      • Villordo S.M.
      • Gamarnik A.V.
      Genome cyclization as strategy for flavivirus RNA replication.
      ,
      • Westaway E.G.
      • Mackenzie J.M.
      • Khromykh A.A.
      Replication and gene function in Kunjin virus.
      ,
      • Yun S.I.
      • Lee Y.M.
      Japanese encephalitis virus: molecular biology and vaccine development.
      ). Viral RNA replication is catalyzed by the two viral nonstructural proteins: (1) NS3 has an N-terminal serine protease domain, which requires NS2B as a cofactor for its activity (
      • Lei J.
      • Hansen G.
      • Nitsche C.
      • Klein C.D.
      • Zhang L.
      • Hilgenfeld R.
      Crystal structure of Zika virus NS2B-NS3 protease in complex with a boronate inhibitor.
      ,
      • Phoo W.W.
      • Li Y.
      • Zhang Z.
      • Lee M.Y.
      • Loh Y.R.
      • Tan Y.B.
      • Ng E.Y.
      • Lescar J.
      • Kang C.
      • Luo D.
      Structure of the NS2B-NS3 protease from Zika virus after self-cleavage.
      ); it also has a C-terminal RNA helicase domain that also possesses nucleoside triphosphatase and RNA 5′-triphosphatase activities (
      • Jain R.
      • Coloma J.
      • Garcia-Sastre A.
      • Aggarwal A.K.
      Structure of the NS3 helicase from Zika virus.
      ). (2) NS5 has an N-terminal methyltransferase domain (
      • Coloma J.
      • Jain R.
      • Rajashankar K.R.
      • Garcia-Sastre A.
      • Aggarwal A.K.
      Structures of NS5 methyltransferase from Zika virus.
      ,
      • Zhang C.
      • Feng T.
      • Cheng J.
      • Li Y.
      • Yin X.
      • Zeng W.
      • Jin X.
      • Li Y.
      • Guo F.
      • Jin T.
      Structure of the NS5 methyltransferase from Zika virus and implications in inhibitor design.
      ) and a C-terminal RNA-dependent RNA polymerase domain (
      • Adiga R.
      Phylogenetic analysis of the NS5 gene of Zika virus.
      ,
      • Cox B.D.
      • Stanton R.A.
      • Schinazi R.F.
      Predicting Zika virus structural biology: challenges and opportunities for intervention.
      ).
      Fig. 6
      Fig. 6Genome structure and replication cycle of ZIKV. (A) A schematic diagram of the ZIKV genomic RNA. The single open reading frame (ORF) encoded in the viral genome is shown as a long gray barrel, with the three structural (green) and seven nonstructural (magenta) proteins defined below the ORF. NCR, non-coding region. Open arrowheads indicate the cleavage sites for cellular signalases, and solid arrowheads mark the cleavage sites for the viral serine protease (NS2B-NS3). A gray arrowhead indicates the cleavage site for furin or a furin-like protease. The cleavage at the NS1-NS2A junction is mediated by an unknown protease(s). (B) Proposed life cycle of ZIKV. The eight major steps of the viral life cycle are: attachment, endocytosis, membrane fusion, translation, RNA replication, assembly, maturation, and release.
      Source:
      • Yun S.I.
      • Lee Y.M.
      Japanese encephalitis: the virus and vaccines.
      .
      There are very few direct experimental data available concerning the replication cycle of ZIKV, but the current understanding of the molecular biology of other flaviviruses provides a clear path forward for ZIKV research (
      • Lindenbach B.D.
      • Murray C.L.
      • Thiel H.J.
      • Rice C.M.
      Flaviviridae.
      ). As schematically illustrated in Fig. 6B, flaviviruses enter their host cells initially by binding nonspecifically to the cell surface (
      • Chen Y.
      • Maguire T.
      • Hileman R.E.
      • Fromm J.R.
      • Esko J.D.
      • Linhardt R.J.
      • Marks R.M.
      Dengue virus infectivity depends on envelope protein binding to target cell heparan sulfate.
      ,
      • Davis C.W.
      • Nguyen H.Y.
      • Hanna S.L.
      • Sanchez M.D.
      • Doms R.W.
      • Pierson T.C.
      West Nile virus discriminates between DC-SIGN and DC-SIGNR for cellular attachment and infection.
      ,
      • Navarro-Sanchez E.
      • Altmeyer R.
      • Amara A.
      • Schwartz O.
      • Fieschi F.
      • Virelizier J.L.
      • Arenzana-Seisdedos F.
      • Despres P.
      Dendritic-cell-specific ICAM3-grabbing non-integrin is essential for the productive infection of human dendritic cells by mosquito-cell-derived dengue viruses.
      ,
      • Pokidysheva E.
      • Zhang Y.
      • Battisti A.J.
      • Bator-Kelly C.M.
      • Chipman P.R.
      • Xiao C.
      • Gregorio G.G.
      • Hendrickson W.A.
      • Kuhn R.J.
      • Rossmann M.G.
      Cryo-EM reconstruction of dengue virus in complex with the carbohydrate recognition domain of DC-SIGN.
      ,
      • Tassaneetrithep B.
      • Burgess T.H.
      • Granelli-Piperno A.
      • Trumpfheller C.
      • Finke J.
      • Sun W.
      • Eller M.A.
      • Pattanapanyasat K.
      • Sarasombath S.
      • Birx D.L.
      • Steinman R.M.
      • Schlesinger S.
      • Marovich M.A.
      DC-SIGN (CD209) mediates dengue virus infection of human dendritic cells.
      ), followed by internalization specifically through clathrin-mediated endocytosis in a viral glycoprotein E-dependent manner (
      • Perera-Lecoin M.
      • Meertens L.
      • Carnec X.
      • Amara A.
      Flavivirus entry receptors: an update.
      ,
      • Pierson T.C.
      • Kielian M.
      Flaviviruses: braking the entering.
      ). When the virions are delivered to endosomes, the E glycoprotein undergoes a series of low pH-induced structural changes, enabling the fusion of the viral membrane with the endosomal membrane and the release of the viral genome into the cytoplasm (
      • Harrison S.C.
      Viral membrane fusion.
      ,
      • Kaufmann B.
      • Rossmann M.G.
      Molecular mechanisms involved in the early steps of flavivirus cell entry.
      ,
      • Smit J.M.
      • Moesker B.
      • Rodenhuis-Zybert I.
      • Wilschut J.
      Flavivirus cell entry and membrane fusion.
      ,
      • Stiasny K.
      • Fritz R.
      • Pangerl K.
      • Heinz F.X.
      Molecular mechanisms of flavivirus membrane fusion.
      ). Viral RNA replication takes place in close association with virus-induced intracellular membrane structures known as replication complexes, which contain the viral RNA and proteins, as well as presumably a set of the host cell factors required for viral RNA synthesis (
      • Gillespie L.K.
      • Hoenen A.
      • Morgan G.
      • Mackenzie J.M.
      The endoplasmic reticulum provides the membrane platform for biogenesis of the flavivirus replication complex.
      ,
      • Hsu N.Y.
      • Ilnytska O.
      • Belov G.
      • Santiana M.
      • Chen Y.H.
      • Takvorian P.M.
      • Pau C.
      • van der Schaar H.
      • Kaushik-Basu N.
      • Balla T.
      • Cameron C.E.
      • Ehrenfeld E.
      • van Kuppeveld F.J.
      • Altan-Bonnet N.
      Viral reorganization of the secretory pathway generates distinct organelles for RNA replication.
      ,
      • Welsch S.
      • Miller S.
      • Romero-Brey I.
      • Merz A.
      • Bleck C.K.
      • Walther P.
      • Fuller S.D.
      • Antony C.
      • Krijnse-Locker J.
      • Bartenschlager R.
      Composition and three-dimensional architecture of the dengue virus replication and assembly sites.
      ). Assembly of immature virions is initiated by budding of a viral genomic RNA-C protein complex into the endoplasmic reticulum (
      • Zhang Y.
      • Corver J.
      • Chipman P.R.
      • Zhang W.
      • Pletnev S.V.
      • Sedlak D.
      • Baker T.S.
      • Strauss J.H.
      • Kuhn R.J.
      • Rossmann M.G.
      Structures of immature flavivirus particles.
      ,
      • Zhang Y.
      • Kaufmann B.
      • Chipman P.R.
      • Kuhn R.J.
      • Rossmann M.G.
      Structure of immature West Nile virus.
      ), where it is enveloped by the lipid bilayer with the viral prM and E proteins embedded (
      • Lorenz I.C.
      • Allison S.L.
      • Heinz F.X.
      • Helenius A.
      Folding and dimerization of tick-borne encephalitis virus envelope proteins prM and E in the endoplasmic reticulum.
      ). These immature virions undergo a maturation process while moving through the host secretory pathway (
      • Mackenzie J.M.
      • Westaway E.G.
      Assembly and maturation of the flavivirus Kunjin virus appear to occur in the rough endoplasmic reticulum and along the secretory pathway, respectively.
      ) and particularly the trans-Golgi network, going through post-translational modifications that include the cleavage of prM by furin or a furin-like protease to produce the mature M protein (
      • Stadler K.
      • Allison S.L.
      • Schalich J.
      • Heinz F.X.
      Proteolytic activation of tick-borne encephalitis virus by furin.
      ,
      • Yu I.M.
      • Zhang W.
      • Holdaway H.A.
      • Li L.
      • Kostyuchenko V.A.
      • Chipman P.R.
      • Kuhn R.J.
      • Rossmann M.G.
      • Chen J.
      Structure of the immature dengue virus at low pH primes proteolytic maturation.
      ). Finally, the completely or partially mature virions are released from the cell via exocytosis (
      • Pierson T.C.
      • Diamond M.S.
      Degrees of maturity: the complex structure and biology of flaviviruses.
      ). For future studies, it is important to understand the similarities and differences in molecular biology between ZIKV and other relatively well-characterized flaviviruses (e.g., JEV, WNV, DENV, and YFV), with respect to viral replication and pathogenesis.

      6. Clinical presentation

      ZIKV can produce a wide variety of clinical symptoms in humans. A growing body of evidence suggests that in some severe cases, ZIKV causes neurological diseases, such as Guillain-Barré syndrome in ZIKV-infected adults and microcephaly in infants born to ZIKV-infected women.

      6.1 Common signs and symptoms

      ZIKV infections are symptomatic in only ~20–25% of the infected individuals who develop a mild and self-limited illness, with an incubation period of 4–10 days (
      • Bearcroft W.G.
      Zika virus infection experimentally induced in a human volunteer.
      ,
      • CDC
      Zika Virus.
      ,
      • Cerbino-Neto J.
      • Mesquita E.C.
      • Souza T.M.
      • Parreira V.
      • Wittlin B.B.
      • Durovni B.
      • Lemos M.C.
      • Vizzoni A.
      • Bispo de Filippis A.M.
      • Sampaio S.A.
      • Goncalves Bde S.
      • Bozza F.A.
      Clinical manifestations of Zika virus infection, Rio de Janeiro, Brazil, 2015.
      ,
      • Duffy M.R.
      • Chen T.H.
      • Hancock W.T.
      • Powers A.M.
      • Kool J.L.
      • Lanciotti R.S.
      • Pretrick M.
      • Marfel M.
      • Holzbauer S.
      • Dubray C.
      • Guillaumot L.
      • Griggs A.
      • Bel M.
      • Lambert A.J.
      • Laven J.
      • Kosoy O.
      • Panella A.
      • Biggerstaff B.J.
      • Fischer M.
      • Hayes E.B.
      Zika virus outbreak on Yap Island, Federated States of Micronesia.
      ,
      • Lazear H.M.
      • Diamond M.S.
      Zika virus: new clinical syndromes and its emergence in the western hemisphere.
      ,
      • Musso D.
      • Nhan T.
      • Robin E.
      • Roche C.
      • Bierlaire D.
      • Zisou K.
      • Shan Yan A.
      • Cao-Lormeau V.M.
      • Broult J.
      Potential for Zika virus transmission through blood transfusion demonstrated during an outbreak in French Polynesia, November 2013 to February 2014.
      ). In symptomatic cases, the common symptoms are nonspecific and resemble those of a flu-like syndrome, including transient low-grade fever, itchy maculopapular rash, arthritis or arthralgia, and nonpurulent conjunctivitis; at a lesser frequency, retro-orbital pain, headache, myalgia, edema, and vomiting are seen (
      • Brasil P.
      • Pereira Jr., J.P.
      • Moreira M.E.
      • Ribeiro Nogueira R.M.
      • Damasceno L.
      • Wakimoto M.
      • Rabello R.S.
      • Valderramos S.G.
      • Halai U.A.
      • Salles T.S.
      • Zin A.A.
      • Horovitz D.
      • Daltro P.
      • Boechat M.
      • Raja Gabaglia C.
      • Carvalho de Sequeira P.
      • Pilotto J.H.
      • Medialdea-Carrera R.
      • Cotrim da Cunha D.
      • Abreu de Carvalho L.M.
      • Pone M.
      • Machado Siqueira A.
      • Calvet G.A.
      • Rodrigues Baião A.E.
      • Neves E.S.
      • Nassar de Carvalho P.R.
      • Hasue R.H.
      • Marschik P.B.
      • Einspieler C.
      • Janzen C.
      • Cherry J.D.
      • Bispo de Filippis A.M.
      • Nielsen-Saines K.
      Zika virus infection in pregnant women in Rio de Janeiro.
      ,
      • Cao-Lormeau V.M.
      • Roche C.
      • Teissier A.
      • Robin E.
      • Berry A.L.
      • Mallet H.P.
      • Sall A.A.
      • Musso D.
      Zika virus, French Polynesia, South Pacific, 2013.
      ,
      • Duffy M.R.
      • Chen T.H.
      • Hancock W.T.
      • Powers A.M.
      • Kool J.L.
      • Lanciotti R.S.
      • Pretrick M.
      • Marfel M.
      • Holzbauer S.
      • Dubray C.
      • Guillaumot L.
      • Griggs A.
      • Bel M.
      • Lambert A.J.
      • Laven J.
      • Kosoy O.
      • Panella A.
      • Biggerstaff B.J.
      • Fischer M.
      • Hayes E.B.
      Zika virus outbreak on Yap Island, Federated States of Micronesia.
      ,
      • Macnamara F.N.
      Zika virus: a report on three cases of human infection during an epidemic of jaundice in Nigeria.
      ,
      • Musso D.
      • Nhan T.
      • Robin E.
      • Roche C.
      • Bierlaire D.
      • Zisou K.
      • Shan Yan A.
      • Cao-Lormeau V.M.
      • Broult J.
      Potential for Zika virus transmission through blood transfusion demonstrated during an outbreak in French Polynesia, November 2013 to February 2014.
      ,
      • Zammarchi L.
      • Stella G.
      • Mantella A.
      • Bartolozzi D.
      • Tappe D.
      • Gunther S.
      • Oestereich L.
      • Cadar D.
      • Munoz-Fontela C.
      • Bartoloni A.
      • Schmidt-Chanasit J.
      Zika virus infections imported to Italy: clinical, immunological and virological findings, and public health implications.
      ). Other clinical manifestations observed with acute ZIKV infection include hematospermia (
      • Foy B.D.
      • Kobylinski K.C.
      • Chilson Foy J.L.
      • Blitvich B.J.
      • Travassos da Rosa A.
      • Haddow A.D.
      • Lanciotti R.S.
      • Tesh R.B.
      Probable non-vector-borne transmission of Zika virus, Colorado, USA.
      ,
      • Musso D.
      • Roche C.
      • Robin E.
      • Nhan T.
      • Teissier A.
      • Cao-Lormeau V.M.
      Potential sexual transmission of Zika virus.
      ), hearing difficulties (
      • Tappe D.
      • Nachtigall S.
      • Kapaun A.
      • Schnitzler P.
      • Gunther S.
      • Schmidt-Chanasit J.
      Acute Zika virus infection after travel to Malaysian Borneo, September 2014.
      ), thrombocytopenia, and subcutaneous bleeding (
      • Chraibi S.
      • Najioullah F.
      • Bourdin C.
      • Pegliasco J.
      • Deligny C.
      • Resiere D.
      • Meniane J.C.
      Two cases of thrombocytopenic purpura at onset of Zika virus infection.
      ,
      • Duijster J.W.
      • Goorhuis A.
      • van Genderen P.J.
      • Visser L.G.
      • Koopmans M.P.
      • Reimerink J.H.
      • Grobusch M.P.
      • van der Eijk A.A.
      • van den Kerkhof J.H.
      • Reusken C.B.
      • Hahne S.J.
      • Dutch ZIKV Study Team
      Zika virus infection in 18 travellers returning from Surinam and the Dominican Republic, the Netherlands, November 2015–March 2016.
      ,
      • Karimi O.
      • Goorhuis A.
      • Schinkel J.
      • Codrington J.
      • Vreden S.G.
      • Vermaat J.S.
      • Stijnis C.
      • Grobusch M.P.
      Thrombocytopenia and subcutaneous bleedings in a patient with Zika virus infection.
      ,
      • Sharp T.M.
      • Munoz-Jordan J.
      • Perez-Padilla J.
      • Bello-Pagan M.I.
      • Rivera A.
      • Pastula D.M.
      • Salinas J.L.
      • Martinez Mendez J.H.
      • Mendez M.
      • Powers A.M.
      • Waterman S.
      • Rivera-Garcia B.
      Zika virus infection associated with severe thrombocytopenia.
      ). The symptoms generally appear along with the viremia and disappear spontaneously within a week, but arthralgia may persist for up to a month (
      • Foy B.D.
      • Kobylinski K.C.
      • Chilson Foy J.L.
      • Blitvich B.J.
      • Travassos da Rosa A.
      • Haddow A.D.
      • Lanciotti R.S.
      • Tesh R.B.
      Probable non-vector-borne transmission of Zika virus, Colorado, USA.
      ).

      6.2 Guillain-Barré syndrome

      Guillain-Barré syndrome is an autoimmune disease in which the immune system attacks part of the peripheral nervous system, causing tingling, muscle weakness, paralysis, and even death (
      • Creange A.
      Guillain–Barre syndrome: 100 years on.
      ,
      • Goodfellow J.A.
      • Willison H.J.
      Guillain–Barre syndrome: a century of progress.
      ). Previously, this neuromuscular complication had been associated with infection by other arboviruses, such as DENV (
      • Carod-Artal F.J.
      • Wichmann O.
      • Farrar J.
      • Gascon J.
      Neurological complications of dengue virus infection.
      ,
      • Ralapanawa D.M.
      • Kularatne S.A.
      • Jayalath W.A.
      Guillain–Barre syndrome following dengue fever and literature review.
      ,
      • Simon O.
      • Billot S.
      • Guyon D.
      • Daures M.
      • Descloux E.
      • Gourinat A.C.
      • Molko N.
      • Dupont-Rouzeyrol M.
      Early Guillain–Barre syndrome associated with acute dengue fever.
      ,
      • Verma R.
      • Sahu R.
      • Holla V.
      Neurological manifestations of dengue infection: a review.
      ) and chikungunya virus (
      • Wielanek A.C.
      • Monredon J.D.
      • Amrani M.E.
      • Roger J.C.
      • Serveaux J.P.
      Guillain–Barre syndrome complicating a chikungunya virus infection.
      ). The temporal and geographic association of ZIKV with Guillain-Barré syndrome was initially observed during the 2013–2014 outbreak reported in French Polynesia (
      • ECDC
      Rapid Risk Assessment: Zika Virus Infection Outbreak, French Polynesia.
      ,
      • Musso D.
      • Nilles E.J.
      • Cao-Lormeau V.M.
      Rapid spread of emerging Zika virus in the Pacific area.
      ,
      • Oehler E.
      • Watrin L.
      • Larre P.
      • Leparc-Goffart I.
      • Lastere S.
      • Valour F.
      • Baudouin L.
      • Mallet H.
      • Musso D.
      • Ghawche F.
      Zika virus infection complicated by Guillain-Barre syndrome — case report, French Polynesia, December 2013.
      ) and subsequently during the 2015–2016 outbreak that is still ongoing in the Americas (
      • Broutet N.
      • Krauer F.
      • Riesen M.
      • Khalakdina A.
      • Almiron M.
      • Aldighieri S.
      • Espinal M.
      • Low N.
      • Dye C.
      Zika virus as a cause of neurologic disorders.
      ,
      • Roze B.
      • Najioullah F.
      • Ferge J.L.
      • Apetse K.
      • Brouste Y.
      • Cesaire R.
      • Fagour C.
      • Fagour L.
      • Hochedez P.
      • Jeannin S.
      • Joux J.
      • Mehdaoui H.
      • Valentino R.
      • Signate A.
      • Cabie A.
      • GBS Zika Working Group
      Zika virus detection in urine from patients with Guillain-Barre syndrome on Martinique, January 2016.
      ,
      • Thomas D.L.
      • Sharp T.M.
      • Torres J.
      • Armstrong P.A.
      • Munoz-Jordan J.
      • Ryff K.R.
      • Martinez-Quinones A.
      • Arias-Berrios J.
      • Mayshack M.
      • Garayalde G.J.
      • Saavedra S.
      • Luciano C.A.
      • Valencia-Prado M.
      • Waterman S.
      • Rivera-Garcia B.
      Local transmission of Zika virus — Puerto Rico, November 23, 2015–January 28, 2016.
      ). During the previous French Polynesia outbreak, the incidence of Guillain-Barré syndrome was estimated to be ~20-fold higher than its basal incidence of 1–2 cases per 100,000 population per year (
      • Oehler E.
      • Watrin L.
      • Larre P.
      • Leparc-Goffart I.
      • Lastere S.
      • Valour F.
      • Baudouin L.
      • Mallet H.
      • Musso D.
      • Ghawche F.
      Zika virus infection complicated by Guillain-Barre syndrome — case report, French Polynesia, December 2013.
      ,
      • Sejvar J.J.
      • Baughman A.L.
      • Wise M.
      • Morgan O.W.
      Population incidence of Guillain–Barre syndrome: a systematic review and meta-analysis.
      ). More definitively, a recent case-control study revealed that anti-ZIKV IgM or IgG was detected in 41 (98%) of 42 patients with Guillain-Barré syndrome, and all had neutralizing antibodies against ZIKV, as compared to 54 (55%) of 98 controls: age-, sex- and residence-matched patients with a non-febrile illness (
      • Cao-Lormeau V.M.
      • Blake A.
      • Mons S.
      • Lastere S.
      • Roche C.
      • Vanhomwegen J.
      • Dub T.
      • Baudouin L.
      • Teissier A.
      • Larre P.
      • Vial A.L.
      • Decam C.
      • Choumet V.
      • Halstead S.K.
      • Willison H.J.
      • Musset L.
      • Manuguerra J.C.
      • Despres P.
      • Fournier E.
      • Mallet H.P.
      • Musso D.
      • Fontanet A.
      • Neil J.
      • Ghawche F.
      Guillain–Barre syndrome outbreak associated with Zika virus infection in French Polynesia: a case–control study.
      ). The same study also showed that patients with Guillain-Barré syndrome had electrophysiological characteristics consistent with the acute motor axonal neuropathy type of the disease (
      • Cao-Lormeau V.M.
      • Blake A.
      • Mons S.
      • Lastere S.
      • Roche C.
      • Vanhomwegen J.
      • Dub T.
      • Baudouin L.
      • Teissier A.
      • Larre P.
      • Vial A.L.
      • Decam C.
      • Choumet V.
      • Halstead S.K.
      • Willison H.J.
      • Musset L.
      • Manuguerra J.C.
      • Despres P.
      • Fournier E.
      • Mallet H.P.
      • Musso D.
      • Fontanet A.
      • Neil J.
      • Ghawche F.
      Guillain–Barre syndrome outbreak associated with Zika virus infection in French Polynesia: a case–control study.
      ). Thus far, ZIKV-induced Guillain-Barré syndrome has been transient, and most patients have recovered fully. Currently, the mechanism by which ZIKV infection leads to Guillain-Barré syndrome is unknown but is under active investigation.

      6.3 Microcephaly

      Microcephaly is a neurological condition in which the brain of a baby does not develop properly, causing the head to be smaller than normal (
      • Klase Z.A.
      • Khakhina S.
      • Schneider Ade B.
      • Callahan M.V.
      • Glasspool-Malone J.
      • Malone R.
      Zika fetal neuropathogenesis: etiology of a viral syndrome.
      ,
      • Panchaud A.
      • Stojanov M.
      • Ammerdorffer A.
      • Vouga M.
      • Baud D.
      Emerging role of Zika virus in adverse fetal and neonatal outcomes.
      ,
      • Tang B.L.
      Zika virus as a causative agent for primary microencephaly: the evidence so far.
      ). It is divided into two types: (1) primary or congenital microcephaly, which is present in utero or at birth; and (2) secondary or postnatal microcephaly, which develops after birth (
      • Alcantara D.
      • O'Driscoll M.
      Congenital microcephaly.