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West Nile Virus in 15 States

August Is Peak Month for West Nile Illness; Case Reports Are Rising
WebMD Health News
Reviewed by Laura J. Martin, MD

Aug. 16, 2011 -- If you've been bitten by a mosquito and feel ill, it could be West Nile virus.

Ever since its 1999 appearance in the U.S., reports of West Nile illness have peaked in August. With a two- to 14-day incubation period, the next infection might be coming from a mosquito hovering outside your door.

Three West Nile deaths have been reported so far this year -- two in the last week. So far there have been 32 serious cases of West Nile encephalitis, meningitis, or polio-like flaccid paralysis. The CDC estimates that for every such case there are 26.5 cases of milder West Nile fever -- meaning an estimated 848 cases this year.

But it's hard to know from such preliminary counts exactly how bad this year's West Nile season will be, says Marc Fischer, MD, MPH, chief of surveillance of the CDC branch that tracks mosquito- and tick-borne diseases.

"Usually, nationally, West Nile cases peak this time of year, sort of middle to late August," Fischer tells WebMD. "But there can be several weeks' delay before reports come in to CDC so it's difficult to say what kind of a season it is yet."

All kinds of factors limit the accuracy of reporting. Some depend on which parts of the country are getting the wet-then-hot weather pattern mosquitoes love. Some depend on mosquito outbreaks, which can be very localized and which may overwhelm local health departments in a time of funding cutbacks. And some depend on whether doctors test for West Nile virus, as the symptoms of mild or even more severe illness mimic other diseases.

In 2007, case reports got off to their fastest start ever, but the year didn't turn out to be anywhere near as bad as the peak year of 2003, when there were nearly 10,000 case reports and 232 deaths.

West Nile has spread from coast to coast. Last year, there were 1,021 reported cases of West Nile illness. The CDC estimates that for every reported case, there are 140 infections. Four out of five of these infections result in no symptoms. Only one in five people become ill, with the odds of illness after infection rising with a person's age.

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