Skip to main content

Secondary navigation:


biology department

Birds carrying West Nile virusWhat are West Nile Virus and West Nile Encephalitis?

West Nile virus is a flavivirus, a member of a family of RNA viruses that also includes St. Louis encephalitis virus and yellow fever. Like St. Louis encephalitis virus, West Nile virus can cause swelling of the brain and spinal cord in some humans who become infected. Also like St. Louis encephalitis virus, West Nile virus is spread to people when humans are bitten by infected mosquitoes. Symptoms of infection include fever, severe headache, and muscular pain.

In New York in August, 1999, public health personnel at Flushing Hospital in Queens noticed a series of cases of undefined encephalitis in humans. On August 23, Flushing Hospital's chief of infectious diseases contacted the New York City Department of Health, asking for help in identifying the cause of the encephalitis cases. Concurrently, birds were found dead in Queens, the Bronx, and Nassau County including captive birds in an outdoor cage at the Bronx Zoo. The cases of human encephalitis were initially diagnosed as St. Louis encephalitis, but continued tests by scientists at the University of California, Irvine and the Centers for Disease Control determined, using genome sequencing, that the disease was caused by West Nile virus. In 1999 in North America, 62 people suffered acute illness from the virus and seven of these people died (affecting approximately one in 129,000, and one in 1,114,000, respectively, in the New York City population). In 2000 in North America, 21 people suffered acute illness and two of these died.

Between 1999 and 2001, the CDC reports that there were a total of 149 confirmed cases of West Nile virus-related illness and 18 fatalities in the United States. In 2002, 4156 cases of West Nile virus in the U.S. were reported to the CDC, 284 of which were fatal. In 2003, 9862 U.S. cases were reported, 264 of which were fatal. As of October 19, 2004, 2151 U.S. cases of West Nile virus have been reported to the CDC for 2004, 68 of which were fatal.

How do people contract West Nile Virus?

Birds serve as the primary reservoir for West Nile virus and the virus is spread to humans via the bites of infected mosquitoes of several species, including the common house mosquito, Culux pipiens. Estimates of the prevalence of West Nile virus infection rates in humans in endemic areas vary. In seroprevalence studies in the New York area, rates of human infection ranged from 0."5%" (Staten Island, NY, 2000; Fairfield County, CT, 2000) to 0."12%" (Suffolk County, NY, 2000) to 2."6%" (Queens County, NY, 1999). Of those infected with the virus, the CDC estimates that less than one percent suffer severe illness. Only a fraction of people that are seropositive for the virus notice symptoms; e.g., in the 1999 Queens serosurvey, only "20%" of infected people were symptomatic. Investigators from the New York City Health Department estimate that as many as 8200 people may have been infected in New York City in 1999.

Note that in an area where mosquitoes are infected with West Nile, less than one percent of mosquitoes are estimated to carry the virus, but also keep in mind that a person's cumulative risk of infection increases with each subsequent mosquito bite. The individual risk of suffering severe effects such as encephalitis from a West Nile virus infection varies among individuals; elderly individuals are the most likely to suffer severe effects. Nevertheless, for anyone, the risk of becoming ill from West Nile virus is vastly smaller than the risk of suffering from other, more familiar diseases. For example, in 1999, seven people in the United States died from West Nile virus whereas 63,730 died from influenza and pneumonia, 124,181 died from chronic lower respiratory diseases, and 549,838 died from cancer.

What is the geographic distribution of West Nile Virus?

West Nile was unknown in North America until 1999, but has been known in the Old World since 1937 (the year in which it was discovered in a woman in the West Nile district of Uganda). In the 1990's, outbreaks of West Nile virus have occurred in Romania, Algeria, Russia, the Czech Republic, the Democratic Republic of the Congo, Israel, and, beginning in 1999, in the United States. It is not known how West Nile virus reached North America. It could have arrived in a migratory bird or in an infected person (or mosquito) that traveled to the New York area in an airliner. The geographic range of West Nile virus in the U.S. expanded dramatically between 1999 and 2003; in 2003, human cases have been reported from 45 states.

How can West Nile Virus be treated or controlled?

There is no specific treatment for West Nile virus other than treating the symptoms, and there is as of yet no vaccine. Therefore, the primary means of controlling the disease is by reducing the number of times people are bitten by mosquitoes. In addition, decreasing the number of times people are bitten by mosquitoes also reduces people's risk of contracting other mosquito-borne diseases, such as Eastern Equine Encephalitis.

Mosquito Reduction. One way to reduce the incidence of mosquito bites is to reduce the number of mosquitoes in an area. Potential methods of decreasing mosquito populations include the following:

  • Reducing pools of stagnant water (for example, in old tires, clogged house gutters, and unattended bird baths)
  • Managing bodies of water to increase the numbers of mosquito predators Larviciding of mosquito-breeding areas with biological control agents, such as the bacterium Bacillus thuringiensi, that reduce the number of mosquito larvae
  • Larviciding mosquito-breeding areas with chemical control agents, such as methoprene, that reduce the number of mosquito larvae
  • Surface spraying (e.g.,from trucks) adult mosquitoes with chemical insecticides, such as resmethrin
  • Aerial spraying (e.g.,from helicopters) adult mosquitoes with chemical insecticides, such as resmethrin.

Mosquito source reduction by individuals, in the form of reducing stagnant water sites, is a highly effective, low-cost option that can be easily implemented by many citizens. You can do your part by contributing to mosquito source reduction on your property, and by encouraging your neighbors to do so as well.

Personal Protection. Another way to reduce the frequency of mosquito bites is by simple personal protection. People can wear long sleeves, wear long pants, avoid being outdoors at times when mosquitoes are most active (such as during the evening), and avoid areas where mosquitoes are seasonally most abundant (such as in swamps). Personal protection is a low-cost, high-effectiveness risk-reducing strategy.

Where can you find out more about West Nile Virus?

  • CDC. 2001. West Nile Virus Activity--Eastern United States, 2001. MMWR Morb Mortal Wkly Rep. July 27, 2001; 50(29); 617-619.
  • CDC. 2001. Human West Nile virus surveillance--Connecticut, New Jersey, and New York, 2000. MMWR Morb Mortal Wkly Rep. Apr 13, 2001; 50(14):265-268.
  • CDC. 2001. Serosurveys for West Nile virus infection--New York and Connecticut counties, 2000. MMWR Morb Mortal Wkly Rep. Jan 26, 2001; 50(3):37-39.
  • Sampson, B. A., Ambrosi, C., Charlot, A., Reiber, K., Veress, J. F., Armbrustmacher, V. 2000. The pathology of human West Nile Virus infection. Human Pathology 31: 527-531.
  • Rappole, J. H., Derrickson, S. R., Hubalek, Z. 2000 Migratory birds and spread of West Nile virus in the Western Hemisphere. Emerging Infectious Diseases 6: 319-328.
  • Lanciotti, R. S., Roehrig, J. T., Deubel, V., Smith, J., Parker, M., Steele, K., Crise, B., Volpe, K. E., Crabtree, M. B., Scherret, J. H., Hall, R. A., MacKenzie, J. S., Cropp, C. B., Panigrahy, B., Ostlund, E., Schmitt, B., Malkinson, M., Banet, C., Weissman, J., Komar, N., Savage, H. M., Stone, W., McNamara, T., Gubler, D. J. 1999. Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States. Science 286: 2333-2337.

The Centers for Disease Control

--Prepared by John P. Roche--