Wyoming residents cannot let their guard down against the West Nile virus (search), which caused few problems in the state last year, a state health official said.

"I'm afraid that people will look at last year and say 'Oh, it's gone away,'" said Terry Creekmore, the Wyoming Department of Health's West Nile virus coordinator. "But it hasn't. It's just waiting for the right conditions to come back."

In 2003, 393 Wyoming residents were sickened and nine died from West Nile. But last year only 10 Wyoming residents contracted West Nile, and no one died.

Those differences are largely attributable to the weather, Creekmore said.

"Last year was it was exceptionally cold, and most mosquitoes infected with West Nile died before they could transmit the disease," he said. "Conversely, 2003 was an exceptionally hot year, and we had a lot of West Nile virus in horses, humans and birds."

Creekmore predicts more West Nile cases this year than last, but fewer than in 2003.

Health officials remind residents that a fish pond, a gutter clogged with leaves, an old tire sitting in the sun, a flower pot, a Coke can or even a horse's footprint in the mud — anything that can hold water for at least seven days — can become a mosquito breeder reactor.

"One horse hoofprint will breed 30-40 mosquitoes," said Bob Lee, director of environmental management for the Cheyenne Weed and Pest Department. "It does not take much for mosquitoes to breed."

West Nile virus arrived in New York City from Asia in 1999. Since then, it has spread swiftly across the country and reached Wyoming in August 2002.

Though most people who have died from the virus have been 70 or older, all people are susceptible. About 80 percent of infected people experience no symptoms. Less than 1 percent experience the severe form of West Nile, which manifests as encephalitis (search) or meningitis (search) that can prove permanently debilitating or lethal.