By John P. Roche




West Nile virus is a flavivirus, a member of a family of RNA viruses that also includes St. Louis encephalitis virus and Japanese encephalitis virus. Like St. Louis encephalitis virus, West Nile virus can cause swelling of the brain and spinal cord in humans who contract the disease. Also like St. Louis encephalitis virus, West Nile virus is carried by mosquitoes, and is spread to people when humans are bitten by infected mosquitoes.
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, 62 people became ill and seven of these people died from West Nile in New York. In 2000, only 16 people became ill with West Nile in North America, and only one of these died. However, West Nile’s geographic range expanded considerably in 2000, with samples positive for West Nile virus found in birds as far south as North Carolina and as far north as northern New York State. [In 2015, 146 people died in the U.S. from West Nile virus encephalitis.]


West Nile virus is contracted from bites of infected mosquitoes of several species, including the common house mosquito Culux pipiens. The Connecticut Department of Public Health estimates that in an area where West Nile virus exists, generally fewer than one in five hundred mosquitoes carry the virus. Moreover, even if a human is bitten by an infected mosquito, according to Connecticut Department of Health estimates, the individual’s probability of becoming ill from the virus is only about one in three hundred. Using these estimates as a reference point, that makes the overall chance of any single mosquito bite producing illness from the disease (in an area where West Nile virus is present) at only about one in 150,000.
One in 150,000 is a low probability, much lower than one’s annual chances of dying in a fire (1 in 83,333*), or by drowning (1 in 65,116*). However, each time a person is bitten by a mosquito in an area where West Nile is present, the person’s cumulative probability of becoming ill from the virus increases. For example, if a person were bitten 100 times during the summer by mosquitoes in a West Nile-affected area, the person’s cumulative chance of illness would increase to one in 1,500. That would put their annual probability of becoming ill from West Nile somewhat less than the annual risk of a person dying from cancer (1 in 519*). If, however, a person is only bitten by mosquitoes ten times in a summer, their risk of becoming ill with West Nile virus would be only one in 15,000, which is less than a person’s annual probability of suffering mortality from Alzheimer’s disease (1 in 12,458*). [*Data from the Harvard Center for Risk Analysis (]
The individual risk of suffering severe effects from West Nile virus, if it is contracted, varies among individuals. The elderly and individuals with weakened immune systems are the most vulnerable to the disease. Of the people who do become infected with West Nile virus, most experience relatively minor symptoms or no symptoms at all. In a study in Queens New York (the 1999 epicenter of the disease) in 2000, 677 people were randomly sampled and 2.6% were found to have been infected at some time with West Nile virus. Only 20% of the infected individuals (one half of one percent of people sampled) experienced any symptoms from the 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.


There is no specific treatment for West Nile virus other than treating the symptoms, and there is no vaccine that provides protection against the virus. 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 (g., from trucks) adult mosquitoes with chemical insecticides, such as resmethrin

--Aerial spraying (g., from helicopters) adult mosquitoes with chemical insecticides, such as resmethrin.


These methods vary in terms of their economic and potential health-related costs. Potentially higher-cost strategies must be weighed carefully against the actual risks that West Nile virus and other mosquito-borne diseases pose to citizens. Mosquito source reduction by individual people, however, in the form of reducing stagnant water sites, is a highly effective, virtually zero-cost option that can be easily implemented by many citizens.

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.


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 U.S. Geological Survey, National Wildlife Health Center:
The Centers for Disease Control:
The New York City Department of Health:
Cornell University Environmental Risk Analysis Program:
(Reprinted from the Initiative for Vector and Insect Science website.)