Within weeks, if not days, nature will quietly snuff out a killer in Illinois.
The first hard frost will put West Nile in the deep freeze, halting a statewide scourge that has infected 691 people in Illinois and killed 43 — by far the deadliest outbreak since the virus was first detected in the United States in 1999. The cold will kill the mosquitoes that carry the virus.
But health officials know the relief could be as fleeting as frost on an autumn morning. So they will spend the winter studying everything — including the outbreak's geography, its timing and its victims — that might help them ward off the virus next year.
As of Tuesday, 3,242 human cases of West Nile virus and 176 deaths had been reported in the United States this year, according to the Centers for Disease Control and Prevention.
Experts are uncertain why the problem is so bad in Illinois, but their theories include bird migration patterns, a heavy concentration of mosquito-infested cemeteries, and a landscape that includes lots of forests and marshes.
Elsewhere around the country, Michigan was the hardest hit state behind Illinois. It has had 455 cases and 33 deaths, followed by Ohio with 371 cases and 17 deaths, and Louisiana with 310 cases and 16 deaths. But even in warm Louisiana, cases are tapering off with the arrival of fall and could drop to zero or close to it by the end of November.
``Unless the weather is rather warm, we don't expect many cases in the dead of winter,'' said Dr. Raoult Ratard, Louisiana state epidemiologist,
Over the winter, experts will analyze where in Illinois the disease hit hardest. The Chicago area has logged more than two-thirds of the cases, while downstate Canton, a city of just 14,000, had two deaths.
They will also look at the timing of the outbreak, which registered its first human case in early August, peaked in early September and began tapering off in early October.
And they will research West Nile's victims. Horses proved most vulnerable, with nearly 1,000 confirmed cases and a 42 percent mortality rate. For reasons that are still a mystery, other livestock and pets were virtually unscathed.
In the meantime, health officials are hoping history repeats itself. In 1975, a mosquito-borne outbreak of St. Louis encephalitis in Illinois rivaled this year's West Nile crisis, with 578 confirmed cases. The next year, that fell to 19, then to zero by 1977.
One theory is that birds — a primary carrier of the virus — develop an immunity and no longer pass the disease to mosquitoes.
Even if nature fails, Tom Schafer, spokesman for the Illinois Department of Public Health, said he expects West Nile numbers to drop next year. They key to prevention is public awareness, which increased as the toll climbed, he said.
``I think next year it won't be as hard to get Illinoisans' attention,'' he said. ``I don't think that happened this year until August.''
Public awareness is key because the best defense against West Nile is a community that mobilizes to stamp out mosquito breeding grounds, said Rex Still of the Fulton County Health Department in Canton.
Some people there and across Illinois are cleaning up their back yards, emptying bird baths and turning over wheelbarrows. Neighbors and watchful public officials have turned in some homeowners, who have been ordered to get rid of standing water.
Still said most people respond voluntarily, but the threat of $500-a-day fines proved convincing in at least one case where a property owner balked.
Such preventive measures may be key, because state money might be tight next summer.
Illinois provided about $3.5 million in emergency funding this year, which local governments used for such things as educational campaigns and pesticides. But with the state facing a budget crisis, Schafer said, ``the likelihood of the state having a large pool of money available next year is fairly slim.''
Some parts of the state, mostly in the Chicago area, already have mosquito-abatement districts that can levy taxes to raise money to fight mosquitoes, but most of the state's 102 counties are not covered by such districts. Some, like Fulton County, are considering starting them.