West Nile Virus

5 August 2004

For the past month or so, a search of Google News for ‘virus’ has yielded mainly articles on computer viruses or West Nile virus.

Why are there so many articles on West Nile virus? Summer is the prime season for transmission of this virus, which is spread to humans by mosquitoes.

In temperate climates (e.g. New York City), the mosquito season extends from April through October, when mosquitoes breed in polluted, standing water. The virus is maintained in birds (the reservoir of the virus) and transmitted among birds by mosquitoes. This bird-mosquito-bird pattern is known as an enzootic cycle.

West Nile virus may cause a serious disease in humans called viral encephalitis (inflammation of the brain). The virus was first isolated in the West Nile district of Uganda in 1937. It had not been isolated in the Western Hemisphere before 1999, when cases of West Nile virus encephalitis were identified in New York City. Since that first outbreak, the virus has spread across the United States, and is now invading Canada, Mexico, and the West Indies. All the Google News ‘hits’ on West Nile virus concern first isolations of the virus this season. For example: West Nile virus turns up in 42 Illinois counties, and West Nile virus found in three Bay Area Houston cities.

A fascinating, and unanswered question, is why the virus arrived in New York City in 1999. The New York strain of West Nile virus is very similar to a virus isolated from a goose in Israel in 1999. This virus might have been transported across the Atlantic Ocean in an infected bird, mosquito, human, or horse.

An excellent review article on West Nile virus and the disease it causes can be found in the Journal of Clinical Investigation.