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July 27, 2007 -- The U.S. West Nile virus season is off to its fastest start ever. By July 24, the mosquito-borne bug sickened 122 people and killed three, the CDC says.
It's too soon to know if this means 2007 will be a record year for West Nile virus. But in 2003 -- the worst year in the brief eight-year U.S. history of the virus -- there were only nine official cases this early in the year. The 2003 caseload eventually reached nearly 10,000, with 264 deaths.
"It is looking sort of similar to 2003 at this point," CDC researcher Emily Zielinski-Gutierrez, DrPH, tells WebMD. "The question is not just how many cases, but where the focal points of activity will be."
West Nile virus doesn't spread evenly across the U.S. Last year's epicenter was Boise, Idaho. This year, the virus seems to be disproportionately severe in the Dakotas.
"The Dakotas and the central plain states have just been hit hard year after year," Zielinski-Gutierrez says. "If you look at the impact on these rural counties, with people getting sick and hospitalized and needing mosquito control without a huge population base to fund it, that represents a huge challenge to those more rural states."
West Nile virus first appeared in New York in 1999. It spread inexorably until it finally crossed the country in 2002. From 2004 through 2006, the virus relaxed its hold in most Southeastern states. That's probably because the virus replicates a bit better in the mosquito species found in the Central and Western states.
Another factor is agriculture. The mosquitoes that spread West Nile in the Central states have an affinity for irrigation water. And farm workers, who are outside from morning until dark, are highly exposed to mosquito bites.
Four out of five people who get West Nile virus have no symptoms at all. The other 20% get a fever that can last for only a few days, or for several weeks. One in 150 infections, however, invades the brain or other parts of the nervous system (neuroinvasive). The result can be death or permanent polio-like paralysis.
Because so many infections have no symptoms, the CDC's official case count is a huge underestimate. But nearly every case of West Nile encephalitis or meningitis (affecting the brain or lining around the brain) is reported -- so by multiplying the number of these cases by 150, it's possible to get a rough estimate of the true number of infections.
So far this year, there have been 42 cases of neuroinvasive West Nile infection. That suggests there have already been about 6,300 total infections with West Nile. By the end of worst year, 2003, there were 2,866 reports of neuroinvasive disease, suggesting there were nearly 430,000 infections.
Whether this year will be as bad as that depends largely on the weather. Birds carry West Nile virus, and mosquitoes get the bug after feeding on birds. The virus then has to grow inside the mosquito to levels infectious to humans. This happens most efficiently in hot weather.
If August and September are relatively cool, the bug might not be a big problem. But that's just speculation. The U.S. history of West Nile virus is too short for scientists to make accurate predictions.
One prediction can be made for sure: You will have a much lower risk of West Nile infection if you avoid mosquito bites.
"From the early numbers, it looks like we have a pretty busy West Nile season going," Zielinski-Gutierrez says. "Get out that repellent. The take-home message at this point for the American public is, 'Whoa -- there seems to be a lot of West Nile out there. When I go out there I'd better do something to keep from getting mosquito bites.'"