ATLANTA--Last summer's outbreak of West Nile encephalitis in the New York metropolitan area has prompted the Centers for Disease Control and Prevention (CDC) to issue guidelines for surveillance, prevention, and control of the West Nile virus.[1] The outbreak included 62 laboratory-positive cases; there were seven deaths.

The outbreak caused public panic both because the West Nile virus is new to this hemisphere and because the 11% case fatality rate was much higher than usual. "Historically, the virus hasn't caused much mortality," pointed out Duane Gubler, ScD, director of the Division of Vector-Borne Infectious Diseases at the CDC. However, the case fatality rate was about 10% in a 1996 outbreak in Romania, he noted. Mortality was also higher than usual during an outbreak in Russia last year, though the exact death toll in that episode remains unknown, "so we don't know whether this increased mortality is going to be the trend for the future or whether these were unusual events," Dr. Gubler told PULMONARY REVIEWS.

WILL IT STRIKE AGAIN?

West Nile virus, a flavivirus similar to those that cause dengue and yellow fever, is most commonly found in Africa, Eastern Europe, West Asia, and the Middle East. It is transmitted by mosquitoes, which acquire it by biting infected birds. After an incubation period of 10 days to two weeks, the mosquitoes can pass the virus to humans and other animals.

Human-to-human transmission of West Nile virus does not appear possible, nor does animal-to-human transmission. In humans, the virus incubates for five to 15 days before symptoms appear.

It is unknown if further outbreaks of West Nile virus will occur in the United States. "We don't know if the virus has persisted through the winter," Dr. Gubler said, although genetic evidence of the virus was recently found in hibernating mosquitoes in New York City (see sidebar). "We also don't know how far it may have spread from the New York metropolitan area." he added.

The CDC therefore recommends increased surveillance in areas most likely to have an outbreak. That includes all states from Massachusetts to Texas along the Atlantic and Gulf coasts.

In northern states, surveillance should start early in the spring to coincide with the start of mosquito activity in those states, according to the CDC. Surveillance should continue through the winter in southern states, where mosquitoes are active year-round.

Specific surveillance measures should include monitoring of West Nile virus in mosquitoes and birds, especially crows. In last year's outbreak, the death of birds was an early, and overlooked, clue to the virus's presence in this country. "We're also encouraging enhanced surveillance for neurologic disease in humans and large animals, such as horses," Dr. Gubler added.

Such surveillance is necessary because severe West Nile virus infection may cause encephalitis leading to a variety of neurologic symptoms, including headache, disorientation, stupor, coma, tremors, convulsions, and paralysis. It may also sometimes cause death. In mild cases, which are by far the most common, patients frequently develop a fever, body aches, skin rash, and swollen lymph nodes. Often, the symptoms resemble a severe case of the flu.

Because of the neurologic symptoms, health care providers should report cases of viral encephalitis and, if resources permit, aseptic meningitis in humans to their state health departments and the CDC. Veterinarians should report neurologic disease in horses and other large animals. In addition, state public health and veterinary laboratories should confirm suspected cases of West Nile virus infection with appropriate assays.

"Treatment for the infection is supportive only, just like for other flaviviral encephalitis," Dr. Gubler noted. "There's no antiviral agent that's effective against it." Severe cases, he added, may require admission to the intensive care unit.

PREVENTING TRANSMISSION

Mosquito control, typically through locally funded programs that destroy larvae, is the best way to prevent West Nile virus transmission, according to the CDC. Public education about modes of transmission and ways to reduce the risk of virus exposure is also essential.

To minimize virus exposure, the CDC recommends the following strategies:

• Stay indoors at dawn, dusk, and in the early evening to avoid mosquitoes.
• Wear long-sleeved shirts and long pants when outdoors.
• Apply insect repellent to exposed skin (an effective repellent should contain 20% to 30% N,N-diethyl-m-toluamide, or deet).
• Apply a repellent containing permethrin or deet to clothing (mosquitoes can bite through thin fabrics).

However, the CDC cautions the public not to be overzealous in protecting against the West Nile virus. For example, it warns against putting insect repellent on children's hands because they may put their hands in their eyes or mouths and cause irritation. Also, parents should not put repellent on children under age 3 years, according to the CDC.

More research is necessary to understand and control the West Nile virus. Currently, the highest priorities are to determine if the live virus is still present in the United States and, if it is, where the next outbreak is most likely to occur. Other important research areas include developing more effective mosquito surveillance and control measures and laboratory diagnostic tests as well as studying the long-term effects of West Nile virus infection in humans.

"We're also encouraging the development of antivirals and vaccines for West Nile virus and other flaviviruses," added Dr. Gubler. "But neither of those is anywhere near fruition."

--Timothy Begany

Reference
1. Guidelines for surveillance, prevention, and control of West Nile virus infection--United States. MMWR Morb Mortal Wkly Rep. 2000;49:25-28.

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