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Has a new virus come to town? What is West Nile virus, and why all the fuss about it? West Nile virus is an infection spread by mosquitoes that can cause inflammation of the brain and its coverings. It is not a new virus, but has been known in the US only since the summer of 1999. That year, 62 cases of severe disease, including 7 deaths, occurred in the New York City area. Each year since then, the virus has moved a bit further westward. In the summer of 2002, West Nile virus was identified in Colorado for the first time.
West Nile virus has been commonly found in humans and birds and other animals in Africa, Eastern Europe, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus, which causes outbreaks every summer in the United States.
People become infected with West Nile virus by the bite of mosquitoes infected with the virus. The mosquitoes acquire the infection when they feed on infected birds, which may circulate the virus in their blood for a few days. Even in areas where the virus is circulating in animals, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than 1% of people who are bitten and become infected will become severely ill. The chances that a person will become severely ill from any one mosquito bite are extremely small. West Nile virus is not transmitted person to person. It cannot be passed by touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease. It has been passed, however, in transplanted organs and blood from persons who were incubating the disease but who were not known to be ill at the time.
Not all human cases are symptomatic — most cases are not clinically apparent. After an incubation period of 3 to 14 days, approximately 1 in 5 infected persons will develop a mild illness with fever. One in 150 will develop encephalitis and/or meningitis. Signs and symptoms may include stiff neck, light sensitivity, abnormal reflexes, muscle and joint aches. A few people will develop paralysis similar to polio. People older than 50 years are most likely to develop severe neurologic disease or death.
Because West Nile can sicken or kill animals, there has been concern that handling a dead bird or caring for a sick horse could transmit the disease. There have been no documented cases of this occurring, but it is wise to avoid barehanded 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 of having this or any viral infection.
What else can you do to prevent becoming infected with West Nile virus? Avoid mosquito bites by wearing long-sleeved shirts and long pants when you are outdoors, particularly at dawn, dusk, and in the early evening, which are prime mosquito-biting times. Use insect repellent containing 30-35% DEET on exposed skin. For children use no stronger than 10% DEET, and avoid putting it on the hands of children who may touch their eyes and mouths. Place mosquito netting over infant carriers whenever you are outdoors with infants. Install or repair window and door screens so mosquitoes cannot get indoors. Drain standing water in your environment—look around flower pots, clogged rain gutters, hot tub covers, discarded tires, and other containers that may hold stagnant water.
At this time, work is being done on a vaccine for humans. A vaccine for horses already exists.
West Nile is a potentially dangerous, even deadly virus, and it is spreading westward in the United States. But a little common sense and prevention can go a long way toward protecting you and your family from acquiring it.
E. Patricia Gill, M.D. is a primary care physician in internal medicine and a specialist in infectious disease. She board certified in both internal medicine and infectious disease. She joined Longmont Clinic in April 1995.
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