Classification: ICD-066.3; ICD-10 A92.3
Syndromes and synonyms: None.
Agent: West Nile virus (WNV), a flavivirus, with an envelope and RNA plus-strand genome. Sequencing and phylogenetic analysis of full-length genomes has resulted in a division of WNV strains into four distinct lineages, with lineage 1 strains further separated into three clades (1a, 1b, and 1c). Kunjin virus and Murray Valley encephalitis virus are subtypes of WNV that are endemic in parts of Oceania.
Reservoir: Wild birds. Over 100 bird species can be infected by WNV infection. Many birds get infected and amplify the virus but do not die. The robin is probably the main amplifying host, and rarely has overt disease. American crows, blue jays, and hawks are susceptible to death. The number WNV bird deaths can predict subsequent disease severity in humans and equines.
Vector: Mosquitoes of more than 40 species, principally of the genus Culex, but also Anopheles, Aedes and others. Some vector species can shift their feeding preference from birds to humans, depending on the season (e.g. Culex pipiens). The main vectors in the USA are: Culex pipiens (Eastern), Culex tarsalis (Midwest and West), and Culex quinquefasciatus (Southeast). In Europe two populations of Culex pipiens exist: one that feeds on birds and the other on humans.
Transmission
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