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West Nile Virus

mosquito infected encephalitis spread

West Nile virus is a member of the family of viruses that is called Flaviviridae. The virus is similar to other members of this viral family, is passed to humans from birds by the bite of a mosquito, and is capable of causing disease in humans. For example, another mosquito borne flavivirus causes St. Louis encephalitis in humans.

West Nile virus is endemic in Africa. Periodic outbreaks have occurred in Europe for decades. The virus has become more prominent in Europe and North America in the past decade. The virus was first discovered in North America in September 1999, during an investigation of an encephalitis outbreak in New York City.

Encephalitis is a swelling of the brain. The malady occurs in domestic animals such as horses, dogs, cats, wild animals, and wild birds. The virus can be transferred from an infected bird to humans by the bite of a mosquito. When transferred to a human, the viral infection produces encephalitis and inflammation of nerve cells of the spinal cord (meningitis).

In 1999, 62 cases of the disease were reported in New York City. Seven people died. The following spring, 21 more cases occurred, and two of the people died. It is thought that infected mosquitos that survived the winter were responsible for the renewed outbreak. In 1999 and 2000, the West Nile virus was confined to the northeastern region of the United States. However, since then, the virus has spread to most of the continental United States (42 of the 48 states) and Canada. In the summer of 2001, dead birds infected with the virus were found as far north as Toronto, Canada, as far south as the northern portion of Florida and Louisiana, and as far west as Milwaukee, Wisconsin. An encephalitis outbreak that occurred in Louisiana garnered a great deal of media attention and led to fears of a mass outbreak of encephalitis. This has not occurred. As of November 2002, there are 3,559 confirmed cases of encephalitis and 211 deaths in the United States. Most of these cases occurred during 2002.

Infected migratory birds may have aided the rapid spread of the virus. As well, the presence of the virus or infected mosquitoes in transported equipment, luggage, and on people is thought to be a factor in the spread.

The symptoms of infection begin three to 15 days following the bite of an infected mosquito. Most people experience only mild symptoms that mimic the flu (i.e., fever, headache, body aches). Others will also develop a mild rash or have swollen lymph glands.

In about 3–15% of those who are infected with the virus, the infection is more serious. This is particularly the case for the elderly or those whose immune systems are not functioning properly. These are the people who are at risk for developing encephalitis or meningitis. Symptoms appear suddenly, and include a severe headache, high fever, stiff neck, vomiting, confusion, and loss of consciousness. Even after recovery, a person may have muscle weakness and brain related complications for a long time.

The origin of the virus dates back to 1937, when the virus was isolated from a woman in the West Nile District of Uganda. This is the basis for the name of the virus. The disease causing nature of the virus for humans was discovered in the 1950s, and in animals during the 1960s. It is unclear whether the virus radiated out from Uganda, or whether it has long been present in North America and was previously undetected. However, the pattern of detection suggested that the virus spread globally from one region.

A Culex mosquito, a major carrier of West Nile virus in the southern United States. AP/Wide World Photos. Reproduced by permission.

The mosquito is responsible for spread of the West Nile virus. Many species of mosquito can become infected with the virus. Three species in particular have been most commonly associated with outbreaks. These species are Culex pipiens, Culex restuans, and Culex quinquefasciatus.

Mosquitoes acquire the virus when they obtain a blood meal from an infected animal or a bird. The virus resides in the salivary glands of the mosquito and can be passed into a human from which the mosquito subsequently obtains a blood meal. A period of time, thought to be approximately two weeks, must pass before a mosquito is able to transmit the virus to a human or animal.

The number of bird species involved in transmission of the West Nile virus is not known. It is known that over 70 species of birds can be infected with the virus, and that the infection kills many crows, blue jays, magpies, and ravens. The recent outbreaks in North America have been associated mainly with crows.

The virus enters the host's bloodstream and, by a mechanism that is not yet known, is able to cross the barrier between the blood and the brain. Multiplication of the virus in brain tissue disrupts the cells, causing the nervous system to malfunction and the brain tissue to become inflamed.

Despite the publicity of public fear surrounding the spread of the West Nile virus in North America, the chance of acquiring West Nile virus via a mosquito bite is small. The available data from surveys of mosquito populations indicates that less than 1% of mosquitoes carry the virus, even in areas where the virus is known to be present.

Because the mosquito will not survive cold northerly winters in the wild, the spread of the virus in colder climates is slow. However, if mosquitoes can find protection from the winter, such as in buildings or sewer pipelines, than over wintering of the mosquitoes can occur. Indeed, in New York, infections diminished in the winters of 1999 and 2000, but increased in the spring.

Another climatic factor may be hot and dry spring and summer seasons. The dry conditions limit the open water supplies that are frequented by mosquitoes. The large mosquito populations that gather at the available water supplies make it easier for the virus to become established in large numbers of the insects.

The mosquito to human route of infection is the only route to have been confirmed thus far. While ticks can be infected with the virus, a tick-borne outbreak of the disease has not been documented in humans. Person to person contact cannot occur. Even exchange of body fluids like saliva from an infected to an uninfected person will not transmit the virus. The claim that the virus can be transmitted by sexual contact has been refuted.

In 2002, several people became ill after receiving blood from a donor who was subsequently found to be infected with West Nile virus. This has sparked concern that the virus can be transferred via blood donation. Currently, blood is not routinely screened for the presence of the virus.

Currently no vaccine to the West Nile virus exists for humans, although researchers have recently reported on the success of a weakened West Nile virus-dengue virus construct in stimulating an immune response in experimental animals. A vaccine for horses has been developed, but is not widely available yet. As of 2002, prevention of infection consists of the use of chemical mosquito repellent and insecticide, the use of protective clothing, and simply avoiding being outside at places or times when mosquitoes are typically present.



Despommier, D. West Nile Story. New York: Apple Trees Productions, LLC, 2001.

White, D. J., and D. L. Morse. West Nile Virus: Detection, Surveillance, and Control. Baltimore: Johns Hopkins University Press, 2002.


Pletnev, A. G., R. Putnak, J. Speicher, et al. "West Nile Virus/Dengue Type 4 Virus Chimeras that are Reduced in Neurovirulence and Peripheral Virulence Without Loss of Immunogenicity or Protective Efficacy." Proceedings of the National Academy of Sciences 99 (March 2002): 3036–3041.


Centers for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases. P.O. Box 2087, Fort Collins, CO 80522. (888) 246–2675. <http://www.cdc.gov/ncidod/dvbid/westnile/>.

Health Canada. 0904A Brooke Claxton Bldg., Tunney's Pasture, Ottawa, ON K1A 0K9. (613) 957–2991. <http://www.hc-sc.gc.ca/pphb-dgspsp/publicat/info/wnv_e.html.>

Brian Hoyle

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