D411 - West Nile virus

West Nile virus (WNV) is a single-stranded RNA virus of the family Flaviviridae. It is carried by mosquitoes and can cause encephalitis or meningitis in humans and other animals. In most countries, mosquitoes belonging to the genus Culex are the primary vectors for WNV transmission. The WNV is maintained in an enzootic cycle involving mosquitoes and birds. After passing through three aquatic stages (egg, larva, pupa), adult mosquitoes begin to emerge in the spring in temperate regions. Viral amplification occurs in the bird-mosquito-bird cycle until early fall, when female mosquitoes begin diapause and infrequently bite. Many environmental factors affect this viral amplification cycle (ex. weather or climate, host and vector predators and parasites, and host immune status). Dogs, cats, and domestic birds are all possible hosts for WNV infection, however instances of naturally occurring infection are very rare. Infection with West Nile Virus has been previously demonstrated in dogs but none were clinically ill. In 2002, West Nile virus was isolated from three cats in the United States. The three cats were severely ill and died subsequently. WNV has been isolated from dead birds in at least 138 avian species. Although certain species are more vulnerable to the virus then others (crows and jays), most infected birds do survive. Birds are able to maintain long-term infection and consequently, migratory birds are considered to be instrumental in transporting the virus from region to region. The prevalence of infection in captive species is rare but does exist. According to the United States Center for Disease Control, the West Nile Virus has been isolated from cockatiels, cockatoos, lorikeet species, macaws, finches and domestic chickens. The virus is maintained in the mosquito's salivary glands. During blood feeding, the virus is injected into the animal, multiplies and may cause clinical signs in a susceptible animal. Most infections are inapparent or mild. Symptoms of WNV infection include fever, depression, incoordination, muscle weakness or spasms, seizures or paralysis.

WNV infection can be inferred by immunoglobulin M (IgM) capture enzyme-linked immunosorbent assay (ELISA); however, the assay cannot readily differentiate between WNV and other members of this serocomplex. Serologic confirmation of WNV infection in animals is possible only through detection of the presence of WNV-specific neutralizing antibody in either cerebrospinal fluid (CSF) or serum by the plaque reduction neutralization (PRNT) assay. Virus isolation in cell culture from CSF, serum, or tissues, followed by virus identification in an immunofluorescence assay with WNV-specific monoclonal antibodies can also yield unambiguous results. However, both PRNT and virus isolation assays require up to a week for completion, and isolation requires viable virus in samples. Recently, a highly sensitive and specific reverse transcriptase (RT)-PCR assay for the identification of WNV became available. This test offers veterinarians a timely and definitive method of diagnosis for West Nile Virus infection that is significantly more accurate then conventional methods of virus detection.