West Nile Virus

Last Update: April 2003

Author: F. A. Leighton

Reviewer: H. Artsob

The Virus

West Nile virus (WN) is classified as a Flavivirus (Flaviviridae). Its name comes from West Nile Province, Uganda, where the first case of disease due to this virus was recognized in 1937. It is a single-stranded, enveloped, RNA virus. It is closely related to two other important arboviruses, St Louis Encephalitis virus, which occurs in Canada, and Japanese Encephalitis Virus, which occurs widely in eastern and southern Asia.

Geographic Distribution

Until the summer of 1999, West Nile virus (WN) was known to occur only in Africa, Europe and southwestern Asia. Its east-west range in the Old World is from India to Portugal; the north-south range is from South Africa to middle European latitudes (France, Ukraine, Belarus). WN was recognized for the first time in North America in August of 1999 in the area in and around New York City. It is assumed that WN arrived in North America shortly before this first recognized outbreak, and that it was imported by accident in association with some form of commerce or human travel. In 1999, most virus activity was in the area close to New York City, but virus was detected up to 200 km away. In 2000, WN increased its range several hundred kilometres in all directions from its area of distribution in 1999, and by the end of the summer of 2000 it had reached all counties in New York State that border Ontario and Quebec. It was not detected in Canada in 2000. In 2001, the range of the virus increased markedly, extending south to Florida, west to the west side of the Mississippi River and north into southern Ontario (see map below). In 2002, the range of the virus in North America increased again, including the entire United States from the Atlantic to the Rocky Mountains and a few foci of activity in NW Washington State. In Canada, the virus was found in Nova Scotia, Quebec, Ontario, Manitoba and Saskatchewan (see map below). The long-term distribution of West Nile virus in North America is yet to be determined. It can be anticipated that, over the next several decades, the virus will become incorporated into one or more North American ecosystems in which it may persist indefinitely and from which it may extend its range from time to time in response to environmental variables. At present, the virus is a new species infecting many hosts which have no pre-existing resistance to it because of no previous exposure. Thus it has spread widely and occuppied many areas and habitats that may, with time, cease to support it.

Map of WN in Canada, 2001 Map of WN in Canada 2005


WN normally cycles among wild birds and several kinds of arthropods, primarily mosquitoes. The virus reproduces itself efficiently in several species of wild bird and there is enough virus in the blood of these birds to infect arthropods. This does not appear to be the case for wild or domestic mammals. Many species of mammal become infected, but the virus does not become sufficiently numerous in their blood to infect arthropods and thus perpetuate the life cycle of the virus.

In Canada, West Nile virus appears to follow an annual cycle typical of other viruses that also primarily infect mosquitos and birds but, can, like WN, occasionally infect humans and other mammals (for example, Western Equine Encephalitis and Eastern Equine Encephalitis viruses). The virus first becomes active in the spring, either because dormant adult mosquitoes that were infected the previous year become active or because infected migratory birds bring the virus with them to their northern summer homes. The virus-infected mosquitoes feed on birds and infect them, or un-infected mosquitoes feed on newly-arrived infected birds and become infected. In either case, mosquitoes begin to transmit virus from bird to bird, and each infected bird then can infect several more mosquitoes. During spring and summer, this cycle of transmission of virus between birds and bird-feeding mosquitoes results in a build-up of the total number of virus-infected birds and of virus-infected mosquitoes. During this time, only birds and bird-feeding mosquitoes are infected. Mosquitoes that feed on humans and other mammals generally do not also feed on birds. However, late n the summer, certain species of mosquito that will feed both on birds and mammals reach their peak of annual activity. It is at this time, late in summer, when the liklihood is highest that one of these species of mosquito may feed first on a virus-infected bird and then on a person (or other mammal) and thus infect that person with WN. Thus, the period of greatest risk of infection for people, horses and other mammals is in late summer and early fall.

In Canada, the species of mosquito most likely to carry West Nile virus from birds to people or other mammals differs in different parts of the country. The species thus far most implicated in transmitting infection to people are Culex pipiens, Culex tarsalis, and Culex restuans. Aedes vexans also may be important in some environments.

In the Old World, several species of mosquitoes, and of ticks, are known to become infected with WN and to transmit the virus to vertebrate hosts. Mosquitoes appear to be the most important arthropod hosts and vectors of infection. In South Africa, Egypt and Israel, the mosquito Culex univittatus appears to be the most important arthropod host, with 0.5 to 2 infected individuals found per 1000 females tested. Soft ticks of the genera Ornithodorus and Argas collected at colony sites of colonial water birds also have been found to be infected, and these ticks may be part of natural cycles of the virus in some instances. Mosquitoes are considered the primary arthropod hosts and vectors of infection for birds and mammals.

Disease in Animals

Wildlife: In the Old World, disease due to WN has been reported only in one Rock Dove (pigeon) found in Egypt. Fatal disease was produced experimentally in Hooded Crows. Many species of birds and mammals have antibodies against the virus, which indicates that infection is quite common. Disease appears to be very rare. However, in North America, the virus has proven to cause fatal disease in many species of native birds and a few species of mammals. Hundreds of thousands of American Crows have died of WN infection and other members of the crow family also appear highly susceptible to severe disease when they become infected. On the other hand, many species clearly survive infection with little or no evidence of disease and, in many species, some individuals become ill and may die when infected while many other infected individuals suffer no illness at all. A regularly up-dated list of animal species for which at least one individual is known to have died with West Nile virus infection is provided by the U.S. National Wildlife Health Laboratory.

Domestic Animals: In the Old World, infection with WN has been documented in a long list of domestic mammals and birds, but sickness and death have been reported only in horses and geese. Disease due to WN is rare. Disease in horses has occurred in France and Egypt and has taken the form of encephalitis (infection and inflammation of the brain) in all cases. Experimental infections of horses have not resulted in serious disease. In North America in 2002, over 14,000 horses with clinical illness due to WN were reported. A vaccine now is availabe to protect horses from disease due to WN. Although a few individuals of several different domestic species have become ill or died due to infection with WN, most individuals of domestic species other than horses seem quite resistant to infection. Even among horses, only a very small proportion of infected individuals develop disease after infection.

Human Disease

In the Old World, infection of people with WN appears to be fairly common and disease is quite rare. In one survey in Egypt, 40-55% of young children and 90% of adults had antibodies against the virus. When disease occurs, it has ranged from a flu-like illness to encephalitis (infection and inflammation of the brain) that can be fatal. Severe disease is more likely to develop in elderly people than in younger age groups.

The pattern of human infection and disease in North America since 1999 has been similar to that noted in the Old World. Although precise numbers are not yet known for North America, it appears that 80% or so of people infected with the virus suffer no disease or illness of any kind. These people also are immune to further infections. Some 20 % of infected people may develop some form of mild illness from which nearly all will recover completely. However, a very small number of infected people will develop severe, potentially fatal disease that takes the form of encephalitis.

People most often become infected with WN from the bite of an infected mosquito. Prevention of infection is best achieved by preventing mosquito bites, especially in late summer and in areas where WN is known to be active. Health Canada maintains a web site that provides advice regarding personal protection from infection with WN.

Surveillance and Management

In Canada, a national surveillance program for WN has been in place since May of 2000. To date, the earliest, most sensitive and most effective method to detect the presence of WN activitiy in an area has been to test birds found dead in the wild for the presence of virus in their tissues. This approach is used throughout North America and thus far has been more effective than has surveillance based on testing mosquitoes or maintaining sentinel flocks of captive birds. The Canadian wild bird surveillance program is operated by the Canadian Cooperative Wildlife Health Centre in cooperation with Health Canada and provincial governments across the country. The surveillance program tests only members of the crow family (crows, jays, ravens, magpies) and only birds found freshly dead during the mosquito season. In 2000, 2288 birds were examined in the surveillance program and no WN was detected. In 2001, 3911 birds were tested in the surveillance program and 128 (3.3%) were found to have WN infection. In 2002, 3478 birds were tested and 563 (16%) were found to have WN infection.

General References

General Information about West Nile Virus can be found in the following:

Hayes, C.G. 1988. West Nile fever. In: Monath T.P. (ed.) The Arboviruses: Epidemiology and Ecology. Boca Raton: CRC Press, Inc. p.59-88.

Hubálek, Z. and J. Halouzka. 1999. West Nile fever - a reemerging mosquito-borne viral disease in Europe. Emerging Infectious Diseases 5(5): 643-650.

Komar, N. 2000. West Nile viral encephalitis. Revue Scientifique et Technique de l' Office International des Épizooties. 19(1):166-176.

Komar, N. et al. 2003. Experimental Infection of North American Birds with the New York 1999 Strain of West Nile Virus. Emerging Infectious Diseases 9:311-322.

Peiris, J.S.M., and Amerasinghe, F.P. 1994. West Nile fever. In: Beran, G.W.(Editor-in-chief) Handbook of Zoonoses. 2nd Edition. Section B Viral. CRC Press Inc. Boca Raton.p.139-148.

Rappole, John H., Scott R. Derrickson and Zdenek Hubálek. 2000. Migratory Birds and Spread of West Nile Virus in the Western Hemisphere. Emerging Infectious Diseases 6:(4)v

Nash, D. et al. 2001. The Outbreak of West Nile Virus Infection in the New York City Area in 1999. New England Journal of Medicine 344: 1807-1814.

Tyler, K. L. (2001).. West Nile Virus Encephalitis in America. New England Journal of Medicine 344: 1858-1859

Other Information about West Nile virus

Links to Other Sources of Information About Arboviruses

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