The Associated Press

SIOUX FALLS - Cases of West Nile virus are expected to soar soon, according to Jim Wilson, pesticide education coordinator at South Dakota State University.

"I think we are going to be seeing a rapid increase in incidents over the next few weeks," Wilson said. "We also are suggesting that if the community has a function planned, whether it be a rodeo or something similar, that they use a residual treatment" to help protect the public.

"We don't know how it is going to play out ultimately this year," said Lon Kightlinger, state epidemiologist in South Dakota. "We do know West Nile is here and it's making people sick."

"West Nile virus is probably the most important mosquito-borne illness in the United States and is the most common," said Marc Fisher, a medical epidemiologist at the Centers for Disease Control and Prevention.

So far this year, 26 human cases of West Nile virus and one fatality have been reported to the CDC throughout the nation.

Unofficially, state health departments show more than 30 human infections nationwide.

In North Dakota, West Nile cases swelled from two confirmed human infections a week ago to nine as of Friday.

"It is significant to have that number of cases right now," said Michelle Feist, North Dakota's West Nile virus surveillance coordinator. "But then again our mosquito counts are high, so that may definitely have something to do with the number of cases that we're seeing."

South Dakota has had nine human infections this season.

The West Nile virus peaked in 2003, when almost 10,000 people were infected nationally. Since then, numbers have fallen throughout the United States, but the threat is still present.

"We're just tracking and continuing to follow all cases of West Nile virus in humans and seeing how those compare to previous years," Fisher said. "We're not expecting cases necessarily in new states or new places, but it varies year by year in specific communities how much disease they have."

Kightlinger said nine mosquito pools have been detected and tested positive for West Nile in South Dakota. He recommended communities take a comprehensive approach by monitoring mosquito larvae as they become adults and educating the public on how to be safe.

"Even though all human and mosquito detections are on the eastern half of the state, we just consider all of South Dakota to be really at risk right now," he said.

"We're urging people to take personal responsibility and personal precautions."

People should modify their lifestyles enough to lower the risk of infection, Kightlinger said.

"During the day you might not see as many (mosquitoes) as you would, but they'll hide in tall grass or shrubbery," Feist said. "We definitely want to encourage people to still enjoy their time outside, but just take the necessary precautions."

Annette Bredthauer, Nebraska's public health veterinarian, said animals, particularly horses and cattle, are at great risk of being infected as well. "Our number one domestic animal that has problems with West Nile virus is horses," she said.

Many horses can develop an immunity to the virus, but it takes time to build up and is often hard to detect.

"If people are worried about West Nile in their horses, they should get them vaccinated as quickly as possible," she said. Vaccinations should have taken place around April to give the horse time to build up immunity.