By Tim Bonfield
The Cincinnati Enquirer

One upside to this year's rainy summer: a sharp decline in the number of people infected by the West Nile virus.

Health experts say the rain, along with heightened awareness of the disease, were the major reasons why the mosquito-borne illness has been much less intense in the Tristate than last year.

"Major West Nile outbreaks have tended to occur in drought years. Last year, there was very little rain from May through September, so we were primed for that outbreak. This year was a different story," said Bob Restifo, chief of the Ohio Department of Health's vector-borne disease program.

Nationwide, West Nile infections are up compared to last year. Even here, infections remain a risk until the season's first killing frost, which usually occurs in October. But cases in the region are down.

Last year, Ohio had 441 probable human cases of West Nile infection and 31 deaths - the nation's third-highest total. This year, through Sept. 18, Ohio had 28 human cases and three deaths, according to the federal Centers for Disease Control and Prevention.

Hamilton County's first human case was reported a week ago, about a month later than the first reported case last year.

In Kentucky, six human cases and no deaths were reported through Sept. 18, including one case in Grant County. Last year, the state reported 75 human cases and five deaths.

Indiana officials reported six human cases this year, including one in Switzerland County. None has been fatal. Last year, Indiana reported 293 cases and 11 deaths.

The three biggest outbreaks this year have been in Colorado, South Dakota and Nebraska.

"This is a disease that we've watched migrate from the East Coast toward the West Coast. It's about to make the leap across the Rocky Mountains," said Tim Ingram, Hamilton County health commissioner.

Last year's regional drought-like conditions were ideal for the Culex pipiens mosquitoes, which are known to be strong carriers of West Nile virus.

This year's frequent rainstorms tended to wash out the storm drains, catch basins and containers where Culex prefer to lay eggs, Restifo said.

The rainy weather offered better breeding conditions for other types of mosquitoes, such as the Aedes vexans and Ochlerotatus trivittatus species, which are known to breed quickly after heavy rains and floods.

The type of mosquito matters because scientists have learned that some species are more efficient than others at spreading the disease.

By harvesting and studying the salivary glands of the tiny bugs, researchers have learned that the West Nile virus is much less likely to be carried in the saliva of Aedes mosquito than the Culex bugs.

That means those bugs are less likely to transmit the disease when they bite people, even if the bug is infected, Restifo said.

Officials are confident that most people have not become immune to West Nile virus.

Earlier this year, in Cleveland, health officials collected blood samples from more than 1,200 people.

Testing revealed that even in areas with the highest concentrations of infected bugs, no more than 6.4 percent of the population were carrying antibodies to the West Nile virus.

That means immunity did not play a major role in explaining why the number of infections dropped this year, Restifo said.

Aside from the weather, stepped-up public awareness campaigns helped reduce the number of cases, health officials say.

In Greater Cincinnati, public health officials repeatedly urged the public to use repellant containing DEET and to check their property for signs of standing water.

More villages and townships trained staff to apply larvae-killing dunks in storm sewers, retention ponds and other public areas where mosquitoes might breed.

Neighbors were quick to call public health officials about potential problems.

And when infected birds and mosquitoes were detected, teams were dispatched to warn neighbors and conduct searches for breeding sites in half-mile areas.

Managers at camping and outdoor supply stores say they did not experience a big run on bug spray and other protective items.

People planning camping trips did buy more head nets and sleeping nets from the Sun & Ski Sports store in Kenwood this year, but not much more bug spray than usual.

"It wasn't as much of an increase as I expected," said Josh Garry, store manager.

Even so, public health officials say awareness programs need to continue.

"West Nile virus is a low-risk disease for most people. But it's still risky for older people and it won't go away," Ingram said. "We're seeing fewer cases now, but we want people to remain vigilant because West Nile virus is here to stay."

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E-mail tbonfield@enquirer.com