Zika virus (ZIKV) is a member of the Spondweni serocomplex within the genus Flavivirus, family Flaviviridae. Zika virus was first isolated from a sentinel rhesus monkey placed in the Zika Forest near Lake Victoria, Uganda in April 1947; a second isolation from the mosquito Aedes africanus followed at the same site in January 1948. Since that time, sporadic isolations have been made from humans and a variety of mosquito species in both Africa and Asia, with studies of human and animal seroprevalence confirming this distribution. Zika virus is most likely maintained in a sylvatic cycle involving non-human primates and mosquitoes, with cyclic epizootics in monkeys reported in Uganda. In the sylvatic transmission cycle, humans likely serve as incidental hosts. However, in areas without non-human primates, humans probably serve as primary ampli- fication hosts and potentially as reservoir hosts if their viremia is sufficient in duration and magnitude. Although it is thought that enzootic ZIKV is maintained primarily in a monkey/ mosquito transmission cycle, antibodies have been detected in numerous other animal species including water buffalo, elephants, goats, hippos, impala, kongoni, lions, sheep, rodents, wildebeest, and zebras. Figure 1 shows the schematic model of the Zika Virus:
Fig. 1 The Schematic Model of the Zika Virus
Human case reports of clinically diagnosed ZIKV infections include self-limiting acute febrile illnesses with fever, headache, myalgia and rash. This clinical picture could easily be mistaken for dengue (DEN) or chikungunya (CHIK) fevers, two common arboviral infections which both produce similar clinical presentations. The dengue (DEN) and chikungunya (CHIK) fevers are much more commonly diagnosed in tropical Africa and Asia than ZIKV. DENV and CHIKV infections are familiar to local clinicians and most diagnostic laboratories can detect them. In contrast, few physicians are aware of ZIKV and few laboratories test for clinical infection. Consequently, most ZIKV infections are probably missed or incorrectly diagnosed, as suggested by the high prevalence of ZIKV antibodies found in serosurveys of human populations in Africa and Asia. A recent epidemic on Yap Island, Federated States of Micronesia, and a pediatric case of ZIKV infection in Cambodia demonstrate that ZIKV is also capable of causing human disease and may be expanding its geographic distribution.
Zika virus has a positive-sense, single-stranded RNA genome approximately 11 kilobases in length. The genome contains 59 and 39 untranslated regions flanking a single open reading frame (ORF) that encodes a polyprotein that is cleaved into three structural proteins: the capsid (C), premembrane/membrane (prM), and envelope (E), and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, 2K, NS4B, and NS5). A previous genetic study using nucleotide sequences derived from the NS5 gene indicated three ZIKV lineages: East African (one strain examined), West African (three strains examined), and Asian (one strain examined).