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 is spread to humans by 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 older than 50 have the greatest risk of developing severe disease.

Do Many People Contract WNF?

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. Since then, presence of the virus has been documented in 39 states and the District of Columbia. 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.

Is WNF Contagious?

Generally, a person cannot contract WNF from another infected person or from an infected animal (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, fewer 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. There is no evidence that humans can contract the disease by handling live or dead birds or any other animal that has been infected with the virus. Still, it is never a good idea to handle dead animals with bare hands; experts recommend that people always use disposable gloves and place the dead animal in a plastic bag when disposing of it.

How Do People Know They Have WNF?

The first symptoms of WNV infection are usually fever, headache, and body aches, sometimes accompanied by a rash and swollen lymph nodes ^* . Serious cases of the disease may cause more severe symptoms, including high fever, stiff neck, muscle weakness, convulsions ^* , confusion, paralysis ^* , and coma ^* . Very severe cases can result in death, but this is rare. Symptoms usually begin 3 to 15 days after infection.

How Is WNF Diagnosed and Treated?

If WNF is suspected, the first thing a doctor will do is take a history, which means asking a person about prior health, when symptoms began, and recent travels and activities. This may help determine if the person might have been exposed to an infected mosquito. A blood test can confirm the presence of the virus.

For mild cases of WNF, there is no specific treatment. A doctor usually will recommend rest and over-the-counter medications, such as acetaminophen (uh-see-teh-MIH-noh-fen), to ease fever and aches. Severe cases of WNF may require hospitalization and more specialized care, such as intravenous (in-tra-VEE-nus) fluids (fluids given directly into a vein) to prevent or treat dehydration ^* in someone who is too sick to drink or who is vomiting. A person who is having trouble breathing may be put on a ventilator ^* .

How long WNV illness lasts depends on the severity of the infection. If a person has a mild infection, symptoms often will go away in about a week. Recovery from serious infection may take several weeks to months. Most people who are infected with WNV do not become very sick. Only about 1 percent of all infected people become severely ill. Of these severe cases, up to 15 percent are fatal. Elderly people have the highest risk of developing serious complications from the disease.

Can WNF Be Prevented?

There is no vaccine for WNF, so the best way to prevent the spread of the virus is to prevent mosquito bites. To do so, experts recommend that people avoid being outside at times when mosquitoes are most active (dawn, dusk, and early evening), and that they wear long sleeves and long pants and use insect repellent when outside. When using repellent, it is very important to follow the instructions on the package, especially for children.

In the United States, WNV often has been traced to areas where dead birds have been found. By tracking the disease and looking for patterns of infection, public health officials are better able to prevent future outbreaks. Experts advise people to contact the local or state health department if a dead bird is found in an area where WNV has been reported; a representative will collect the bird for testing.