West Nile Virus

A virus that can cause a fatal ENCEPHALITIS, commonly found in humans and birds in Africa, Eastern Europe, West Asia, and the Middle East. Until 1999 the virus had not been documented in the Western Hemisphere. In 1999, 62 cases of severe disease, including seven deaths, occurred in the New York area, followed by 21 more cases the next year, including two deaths. In 2003, 9,858 people caught the virus and 262 died. The United States also suffered the biggest reported outbreak of West Nile encephalitis in the world in 2002.

Symptoms
West Nile virus rarely kills, but about one in 150 people who get it will develop its potentially deadly encephalitis or MENINGITIS. Most people who are infected with the virus will not get sick; about a fifth of those will develop a fever, headache, body aches, and sometimes a rash and swollen lymph glands. Symptoms for West Nile encephalitis or meningitis include headache, high fever, neck stiffness, disorientation, and sometimes paralysis.

Cause
The virus is transmitted to humans via the bite of infected mosquitoes, which become infected when they feed on infected birds. The virus is located in the mosquito’s salivary glands. During feeding, the virus may be injected, multiplying in a person’s blood and crossing the blood-brain barrier to reach the brain, where it inflames brain tissue and interferes with central nervous system function. Among those with severe illness due to West Nile virus, the fatality rates range from 3 percent to 15 percent.

However, even in areas where mosquitoes do carry the virus, very few mosquitoes (less than 1 percent) are infected. If the mosquito is infected, less than 1 percent of people who get bitten and become infected will be severely ill. The chances that any one person will become severely ill from a single mosquito bite are extremely small.

The virus is not spread by touching or kissing a person who has the disease, or from a health-care worker who has treated someone with the disease. Although ticks infected with West Nile virus have been found in Asia and Africa, their role in the transmitting the virus is uncertain. There is no information to suggest that ticks played any role in the cases identified in the United States.

While experts do not know where the U.S. virus originated, it is most closely related genetically to strains found in the Middle East. West Nile virus was first isolated from a feverish woman in the West Nile district of Uganda in 1937. The virus became recognized as a cause of severe inflammation of the spinal cord and brain in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s. It first appeared in North America in 1999, with encephalitis reported in humans and horses.

West Nile virus has been described in Africa, Europe, the Middle East, west and central Asia, and most recently, North America. Recent outbreaks of West Nile virus encephalitis in humans have occurred in the Democratic Republic of the Congo in 1998, Russia in 1999, the United States in 1999–2001, and Israel in 2000.

Although the vast majority of infections have been identified in birds, the virus can infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. While there is no evidence that a person can get the virus from handling live or dead infected birds, people should avoid bare-handed contact when handling any dead animals and use gloves or double plastic bags to place the carcass in a garbage can. Normal veterinary infection control precautions should be followed when caring for a horse suspected to have this or any viral infection.

Prevention
To prevent being bitten by infected mosquitoes, people should:

• stay indoors at dawn, dusk, and in the early evening
• wear long-sleeved shirts and long pants outdoors
• spray clothing with repellents containing permethrin or DEET
• apply insect repellent sparingly to exposed skin. An effective repellent contains 35 percent DEET (higher concentrations provide no additional protection).