Overview

West Nile virus is spread to people by infected mosquitoes. The virus can cause inflammation of the brain (encephalitis) that in a few cases can be fatal. West Nile encephalitis is not spread from person to person.

Mosquitoes also can spread the virus to horses, birds, and some other animals (cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and raccoons), but animals cannot spread the virus to other animals or to people.

Mosquitoes become infected when they feed on infected birds. The virus is stored in the mosquito's salivary glands; infected mosquitoes transmit West Nile virus to humans and animals while biting to take blood. The virus then multiplies in the blood and travels to the brain. It interferes with normal central nervous system functioning and causes inflammation of brain tissue.

In temperate areas of the world (between 23.5¡ and 66.5¡ north and south latitudes), West Nile encephalitis cases occur mainly in the late summer or early fall. Most of North America falls within these latitudes. The latitude of Miami, Florida, for example, is 25¡ north, while that of Nome, Alaska, is 61¡ north. In tropical climates (the area between 23.5¡ north and south latitudes), West Nile virus can be transmitted year-round.

However, even in areas where mosquitoes do carry the virus, very few mosquitoesÑmuch less than 1%Ñare infected. If the mosquito is infected, less than 1% of people who are bitten and become infected will become severely ill. The chance that you will become severely ill from any one-time mosquito bite is extremely small. The risk of severe illness is highest among people older than 50. Of those with severe illness, 3% to 15% die of the infection.

West Nile virus was first found in the West Nile District of Uganda in 1937. It has since spread to the rest of Africa, Europe, the Middle East, and western and central Asia. The first outbreak in the United States occurred in 1999 in the northeast. It has since been found in Florida.

As of Aug. 1, 2001, the Centers for Disease Control and Prevention (CDC) reported that in 2000 there were 21 cases of West Nile virus through September, including 2 deaths in the New York City area. In 1999, the CDC reported 62 cases of severe disease, including 7 deaths, in the New York area. No estimates are available for the number of cases of West Nile encephalitis worldwide.

SYMPTOMS

The time from infection to the onset of symptoms (incubation period) for West Nile encephalitis usually is 3 to 15 days. Most infections are mild. Symptoms include:

• Fever.
• Headache and body aches.
• Skin rash (occasionally).
• Swollen lymph nodes (occasionally).

More severe infections may cause:

• Headache.
• High fever.
• Stiff neck.
• Disorientation.
• Stupor.
• Tremors and convulsions.
• Muscle weakness or paralysis.
• Coma.

EXAM AND TESTS

If a mosquito-borne illness is suspected, your doctor will take a medical history to assess your risk for West Nile virus. People who live in or travel to areas where the virus has been found are at risk of getting West Nile encephalitis. Those who are older than 50 have the highest risk of severe disease.

If your doctor or another health professional thinks you are at high risk, or if you have symptoms of West Nile encephalitis, a blood sample will be taken. Your blood will be tested for antibodies to the virus, a sign that you have been infected

Treatment Overview

Severe cases of West Nile virus encephalitis are treated with supportive care in a hospital. Supportive care involves helping the body fight illness on its own, rather than treating the cause of illness directly. It often is used when no treatment exists for an illness, as is the case with some viruses.

Supportive treatment for West Nile virus includes intravenous fluids, respiratory support (ventilator), and prevention of secondary infections, such as pneumonia or a urinary tract infection.

HOME TREATMENT

If you have flu-like symptoms that continue for more than 2 or 3 days, or if you have any of the more severe symptoms of West Nile virus encephalitis, call your health professional immediately.

If your doctor determines that you have a mild infection and can stay at home until you get better, make sure to drink plenty of fluids and get lots of rest.

No vaccine is available to prevent West Nile virus infection. But you can take steps to lower your risk of mosquito bites:

• Stay indoors at dawn and dusk and in the early evening.
• Wear long-sleeved shirts and long pants whenever you are outdoors.
• Spray clothing with an insect repellent containing permethrin or DEET since mosquitoes may bite through thin clothing.
• Apply insect repellent sparingly to exposed skin. An effective repellent will contain 35% DEET (N, N-diethyl-meta-toluamide). DEET in high concentrations (greater than 35%) does not provide any additional protection.
• Avoid applying repellent to the hands of children. Repellents may irritate the eyes and mouth.
• Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's directions for use, as printed on the product.

For further information visit the Centers For Disease Control Website:

http://www.cdc.gov/ncidod/dvbid/westnile/index.htm