Kansas City, Mo. ? For both humans and horses, there’s evidence that this year’s West Nile virus outbreak may be less severe than last summer.

There are two reasons, researchers say: Horses are getting immunized in greater numbers, and humans may be building up resistance and taking prudent precautions to avoid getting bit by mosquitos, which carry the virus.

Human cases of West Nile virus soared in Kansas last year compared with the year before, but horse cases began their downward trend in 2003.

“In 2002, about 800 horses got the disease,” said Ludek Zurek, a professor of entomology at Kansas State University. “In 2003, only about 80 got it.”

Health officials emphasize the importance of preventing the disease, which has killed more than 500 people nationwide — including seven in Kansas last year — since the first U.S. case turned up in New York in 1999.

“The horse vaccine works,” Zurek said, and farmers and ranchers have gotten the message.

In Garden City, veterinarian Rodney York at The Animal Hospital said no horses in his practice showed signs of the disease last year.

In 2002, when the disease first appeared, the hospital treated 25 to 30 horses, seven or eight of which died, he said.

York said exposure to the disease in 2002 also reduced its incidence last year because horses, like humans, develop resistance.

“Only one out of 10 or 11 show clinical signs,” he said. “The rest would recover and then be immune.”

No vaccine is available for humans, Zurek said, though several companies are trying to develop one.

Instead, humans must take other preventive measures, said Sharon Watson, spokeswoman for the Kansas Department of Health and Environment.

“The best thing is to use insect repellent with DEET,” she said. “Also, go beyond personal protection and look at the environment around us and remove standing water. Those two things together are most effective.”

Standing water is prime breeding ground for mosquitoes.

If standing water can’t be removed, “Using larvicide is a good option,” Watson said, “and it’s available at local hardware stores.”

West Nile first appeared in Kansas in 2002, with 22 cases and no deaths reported. Victims can develop encephalitis, a swelling of the brain, or meningitis, a swelling of the lining around the brain and spinal cord.

In 2003, KDHE confirmed 90 human cases at the state lab, including seven deaths, and another 723 were reported by commercial laboratories, Watson said.

A surveillance program to monitor the virus begins later this month. Mosquitoes and dead birds will be tested, Zurek said.

The program is a cooperative arrangement involving KDHE and the university’s entomology department and veterinary school, Zurek said. It includes a telephone hot line to report birds that may have died from the virus.

It’s difficult to predict how severe the disease is likely to be this year, Watson said.

“The trend in other states shows a decrease in the third year,” she said. “The (Centers for Disease Control) research shows people build up resistance, and there is increased public awareness. If people got it last year and didn’t realize it, they would still be immune.”