West Nile Fever

West Nile fever is a viral infection that can result in inflammation of the brain, called encephalitis (en-seh-fuh-LYE-tis). The virus that causes it spreads to humans by way of infected mosquitoes.

What Is West Nile Fever?

West Nile fever (WNF) is caused by West Nile virus (WNV), which is part of the flavivirus family. First discovered in Africa, WNV can infect animals and humans, although animals (mainly birds, but also horses, cats, and bats) are the primary hosts for the virus.

Most of the time, people with WNF become only mildly ill. In some cases, however, WNF can develop into a life-threatening disease. If the virus passes into the brain, the infection can cause serious inflammation and complications affecting the nervous system. Of those infected, people 50 years of age or older have the greatest risk of developing severe disease.

Do Many People Contract West Nile Fever?

WNF is found most frequently in Africa, the Middle East, Western Europe, and Asia. It was not found in the Western Hemisphere until 1999, when the first case appeared in the United States. Between 1999 and 2008, presence of the virus was documented in 39 states and the District of Columbia. In 2007, the Centers for Disease Control (CDC) reported a total of 3,630 WNF cases in the United States. WNF tends to occur more often in the summer and early fall, but the vast majority of cases likely go unreported because they cause only mild illness, if any. Of the 3,630 total in 2007, 124 (3 percent) resulted in death from the discase. Those numbers represented a decrease compared with 2006, when the CDC reported 4,269 cases and 177 deaths. Some experts believed that the U.S. population was starting to develop immunity to the virus.

Is West Nile fever Contagious?

Generally, a person cannot contract WNF from another infected person or from an infected animal, although transmission of the virus through a blood transfusion* has been confirmed in some cases. Likewise, infected people cannot spread the virus to animals. Scientists think that the virus is transmitted almost exclusively by the bite of an infected mosquito. The chances of becoming ill with WNF actually are very small. Of all the mosquitoes in any area where infected mosquitoes have been found, less than 1 percent carry the virus.

The transmission cycle begins when a mosquito bites an infected bird and takes in blood that contains WNV. If the mosquito then bites a human, it can transmit the virus to that person. Scientists have found no evidence that humans can contract the discase by handling live or dead birds or any other animal that has been infected with the virus. Still, experts recommend that people never handle a dead animal with bare hands, instead always using disposable gloves and placing the dead animal in a plastic bag when discarding it.