Skeeter Alert

Is this the year West Nile strikes with a vengeance? Back in the spring, public-health officials were divided over how bad this summer's outbreak would be. Last year the mosquito-borne disease resulted in more than 4,000 cases and nearly 300 deaths, and heavy spring rains led some local experts to predict a mosquito baby boom and an explosion of cases in 2003. But the Centers for Disease Control and Prevention (CDC) was more cautious. The outbreak was likely to be as bad as last year's, said the CDC, but because diseases like West Nile spread so unpredictably, nobody could say for sure whether the disease peaked last summer or was still getting worse.

Now we're headed into the virus's late-summer sweet spot, and the CDC reported last week that the number of West Nile cases has begun to shoot up right on schedule. By week's end, 470 cases and 10 deaths had been reported in the U.S., in 25 states from Florida to Wyoming. That's three times as many cases as in the previous week and nearly twice as many compared with the same time last year.

But the CDC was right to hedge its bets. The number of West Nile deaths was actually slightly higher last year at this time. That, say CDC officials, probably reflects the fact that more people are being tested and milder cases are being reported. "A quarter to a third of the cases last year were considered mild. This year it's up to half," says Dr. Tony Marfin, a CDC scientist who specializes in West Nile disease.

The virus's primary hot zone has also changed. Last year the most dangerous part of the country was the lower Mississippi River basin, including Mississippi and Louisiana. This year the action has shifted to Colorado, which had only 14 cases and no deaths in all of 2002 but already has nearly 250 cases and six deaths so far this year. Part of that shift is due to the nature of mosquito populations, which tend to go through boom-and-bust cycles. Colorado's happens to be booming this summer.

But it also reflects the fact that West Nile has been moving inexorably westward since it arrived in the Northeast in 1999. Diseases tend to expand their range until something stops them, and because this one is transmitted mostly by mosquitoes, which live in every state, there's not much stopping it. (It can also be transmitted by blood transfusion, but on July 1, the Food and Drug Administration authorized two experimental tests now being used on the nation's blood supply.)

There's still a chance that the outbreak will peter out this summer, but not much of one. "At this point," says Stephen Ostroff, deputy director of the CDC's National Center for Infectious Diseases, "we have every reason to believe that the 2003 outbreak will be just as severe as last year's, and that one was unprecedented." With a few hundred deaths a year, West Nile isn't likely to become a major killer anytime soon. But it's not going to be easily eradicated either.