WEST NILE VIRUS

Symptoms develop 3-14 days after infection. No effective treatment is known.

The disease can be diagnosed by employing an ELISA test detecting IgM antibodies against the virus. However, several related viruses also cause encephalitis and result in similar antibodies.

The virus is mostly maintained in birds. Female mosquitos, mainly of the species Culex pipiens, Culex restuan and Culex quinquefasciatus, bite infected birds, carry the virus in their salivary glands, and infect other birds when they bite again. It is unknown which mosquito species transmit the virus from birds to mammals. In mammals the virus does not multiply as readily, and it is believed that mosquitos biting infected mammals do not further transmit the virus.

It was initially believed that direct human-to-human transmission is impossible, but in 2002 the Centers for Disease Control and Prevention (CDC) discovered the transmission of West Nile virus through blood transfusion or organ transplants as well as through breast milk and prenatal infection.

There is no vaccine for humans. A vaccine for horses based on killed virus exists; some zoos have given this vaccine to their birds, although its efficacy there is unknown.

The first appearance of West Nile virus in North America in 1999, with encephalitis reported in humans and horses, and the subsequent spread in the United States, may be an important milestone in the evolving history of this virus. The US outbreak began in the New York City area, and the virus is believed to have entered via an air traveler. Since the first North American cases in 1999, the virus has been reported throughout the United States and Canada east of the Rocky Mountains. There have been human cases and horse cases, and many birds--especially crows and other corvids--are infected.

A very high level of media coverage through 2001/2002 raised public fears of West Nile virus, even though common diseases such as influenza take far more lives each year. This disproportionate coverage is most likely the result of the novelty of the disease and the successive announcements of the disease's initial appearance in new areas.

Environmentalists have condemned attempts to control the transmitting mosquitos by spraying pesticide, saying that the detrimental health effects of spraying outweigh the relatively few lives which may be saved, and that there are more environmentally friendly ways of controlling mosquitoes. There are also questions about the effectiveness of insecticide spraying, because mosquitos that are resting or flying above the level of spraying will not be killed; the commonest vector in the northeastern U.S., Culex pipiens, is a canopy feeder.

Canada: One human death occurred in 1999. In 2002, ten human deaths out of 416 confirmed and probable cases were reported by Canadian health officials. In 2003, to Nov. 27, 10 deaths and 1314 confirmed and probable cases were reported. Cases were reported in 2003 in Nova Scotia, Quebec, Ontario, Manitoba, Saskatchewan, Alberta, British Columbia, and the Yukon.

• Family: Flaviviridae
• Genus: Flavivirus Japanese Encephalitis Antigenic Complex
• Complex includes: Alfuy, Cacipacore, Japanese encephalitis, Koutango, Kunjin, Murray Valley encephalitis, St. Louis encephalitis, Rocio, Stratford, Usutu, West Nile, and Yaounde viruses.
• Flaviviruses: share a common size (40-60 nanometres), symmetry (enveloped, icosahedral nucleocapsid), nucleic acid (positive-sense, single stranded RNA approximately 10,000-11,000 bases), and appearance in the electron microscope.

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