(Last updated June 2012)
West Nile virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus found in the United States. The virus can infect humans, birds, mosquitoes, horses, and some other mammals.
West Nile fever is a case of mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.
More severe disease caused by a person’s being infected with this virus can be West Nile encephalitis, West Nile meningitis, or West Nile meningoencephalitis. Encephalitis refers to an inflammation of the brain; meningitis is an inflammation of the membrane around the brain and the spinal cord; and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.
West Nile virus has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but until 1999 had not previously been documented in the Western Hemisphere. It is not known where the U.S. virus originated, but genetically, it is most closely related to strains found in the Middle East.
The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere.
In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round.
One of the species of mosquitoes found to carry West Nile virus is the Culex species, which survives through the winter, or overwinters, in the adult stage. The virus, along with the mosquitoes, have been documented in Western Pennsylvania in the past. This does not mean that if you are bitten by a mosquito, you will get the disease.
All residents of areas where virus activity has been identified are at risk of getting West Nile encephalitis; persons over 50 years of age have the highest risk of severe disease. It is unknown if immunocompromised persons are at increased risk for WNV disease.
Most people who are infected with the West Nile virus will not have any type of illness. It is estimated that 20 percent of the people who become infected will develop West Nile fever—mild symptoms including fever, headache, and body aches, occasionally with a skin rash on the trunk of the body and swollen lymph glands.
The symptoms of severe infection (West Nile encephalitis or meningitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that one in 150 persons infected with the West Nile virus will develop a more severe form of disease.
Usually three to fourteen days.
Symptoms of West Nile fever will generally last a few days, although even some healthy people report having been sick for several weeks. Symptoms of severe disease (encephalitis or meningitis) may last several weeks, although neurological effects may be permanent.
Here are preventive measures that you and your family can take:
Prevention and control of West Nile virus and other arboviral diseases are most effectively accomplished through integrated vector management programs. These programs should include surveillance for West Nile virus activity in mosquito vectors, birds, horses, other animals, and humans, and implementation of appropriate mosquito-control measures to reduce mosquito populations when necessary. Additionally, when virus activity is detected in an area, residents should be alerted and advised to increase measures to reduce contact with mosquitoes.
No, but several companies are working toward developing a vaccine.
Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito’s salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.
No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than 1 percent of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.
No. West Nile encephalitis is not transmitted from person to person. For example, you cannot get West Nile virus from touching or kissing a person who has the disease or from a health-care worker who has treated someone with the disease.
—This information is from the Bayer Advanced website.