West Nile virus

West Nile virus (WNV), a member of the Japanese encephalitis complex, is a neurotropic flavivirus virus that produces damage of varying severity in human, animal and avian hosts. The virus is amplified in birds and transmitted to humans usually by Culex mosquitoes. Most cases of WNV are subclinical, with overt clinical illness affecting 1:100 to 1:150 cases. Meningoencephalitis is the most common diagnosis in hospitalized WNV patients, affecting 50–84%. In the elderly the mortality rate may range as high as 10% though it is much lower in the current outbreak in the USA. The epidemiological cycle of WNV is shown in Figure 4.1.

West Nile virus was first isolated from a febrile woman in the West Nile District of Uganda in 1937 (Smithburn et al., 1940); in 1950 it was found that the virus was present in a large percentage of normal individuals in the vicinity of Cairo, Egypt. The majority of the children from whom the sera were collected appeared to be normal; there was no evidence that children with viremia were severely ill. In 1950 more than 70% of the Cairo inhabitants aged 4 years and over had antibodies to WNV (Melnick et al. 1950).