The West Nile Virus, an arbovirus, is a member of the family Flaviviridae which consists of three genera: Flavivirus, Pestivirus, and Hepacivirus (Peterson 2001). The West Nile Virus is included in the group Flaviviruses which are the only mammalian plus-strand RNA virus genomes that do not have a 3' poly (A) tract (Hubalek 2001). The West Nile Virus, classified under the Flaviviruses, as mentioned above, has a 40 to 60nm icosohedral core composed of multiple copies of a 12-kDa capsid protein (Marfin 2001). This capsid encloses a single-stranded, positive-sense RNA of approximately 12,000 nucleotides and is enclosed in a host cell-derived envelope that has been modified by the insertion of two integral membrane glycoproteins, E (envelope) and prM (membrane) (Peterson 2001). The prM protein is cleaved to the M protein by cellular protease, and the M protein is then incorporated into the mature virion late into the virus maturation. The WN genome also encodes seven nonstructural proteins (NS1, NS2a, NS2b, NS3, NS4a, NS4b, and NS5) that are important to the structure of the intracellular replication machinery, and the E-glycoprotein which is the viral hemagglutinin and assists in virus-host cell binding (Peterson 2001; Borowski 2001). The West Nile Virus is also further separated genetically into two lineages (1 and 2) based on the signature amino acid substitutions or deletions in their envelope proteins (Peterson 2001; Hubalek 2001). Lineage 1 viruses have included most epidemic strains from Africa, United States, Romania, Israel, Volgograd, etc.; while lineage 2 viruses have been limited to endemic enzootic infections in Africa (Marfin 2001; Hubalek 2001).
Studies have shown that the West Nile Virus replicates in a wide variety of cell cultures, including chicken, duck, and mouse embryo cells and continuous cell lines from monkeys, humans, pigs, rodents, amphibians, and insects, but has not caused obvious cytopathology in many cell lines (Beasley 2002).
