West Nile Virus


West Nile Virus is a mosquito-borne arbovirus in the group of the flaviviruses. It belongs taxonomically to the Japanese encephalitis serocomplex that includes the closely related St. Louis encephalitis, Japanese encephalitis, Kunjin and Murray Valley encephalitis viruses, as well as others. In humans, WNV often causes asymptomatic infection or mild febrile disease, sometimes with a rash. However, it can also cause severe illness in the form of encephalitis with death occurring in a small percentage of patients. People at greater risk generally are the elderly, and those who are immune compromised. There is also evidence of WNV transmission through blood transfusions and organ transplants.

Reported epidemics occurred in Israel during 1951-1954 and in 1957. European epidemics of WNV encephalitis have occurred in southern France in 1962, in southeastern Romania in 1996, and in south central Russia in 1999. The largest recorded epidemic caused by WNV occurred in South Africa in 1974.

The first recognized outbreak of West Nile Virus infections in the Americas occurred in New York City (NYC) in the fall of 1999. Genetically, the WNV 1999 NYC strain closely resembles the 1998 Israel strain. The method of importation of the WNV into NYC is unknown, but it may have arrived via an infected human, bird (including a migratory bird), or mosquito. The 1999 NYC outbreak was unexpected and underscored the ease with which human and animal pathogens can move between the world's population centers.

In 2000, WNV activity was reported in 12 US states. There were 21 human cases, resulting in 2 deaths. All cases were in the Greater NY City area. There were also 60 equine cases of which 23 died or were euthanised. In 2001, a total 27 states and the District of Columbia had confirmed findings of WNV in a mosquito, bird, or mammal. Virus activity was detected further south and west than in the previous year. WNV was also discovered in birds and mosquitoes in Ontario. A total of 66 human cases were reported, with 9 deaths. There were 651 cases diagnosed in horses, mostly in Florida. The spread of the virus in 2002 resulted in the largest arboviral outbreak ever recorded in the Western Hemisphere. WNV activity was reported in 44 US states, as far west as California and Washington, and in 5 Canadian provinces. The outbreak resulted in nearly 15 000 reported equine cases and over 4 500 human cases with 288 deaths.

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: Dr. Paul Innes - Veterinary Scientist, Epidemiology/OMAFRA
Creation Date: March 2003
Last Reviewed: March 2003