The Idaho Department of Fish and Game is warning to prepare for West Nile virus, the mosquito-transmitted disease that's killed 24 Idahoans in the past two years.

Health experts say it's difficult to guess how hard the virus will strike the area this summer. In 2006, officials reported more than 1,000 cases, including 23 fatalities; last year the number was down to 132, with just one death. But gauging the virus's effects is sure to be more difficult.

The state won't ask hospitals to report potential West Nile fevers this year because the tests for the virus are too expensive, said Tom Machala, South Central Public Health District's director of communicable disease.

Labs started charging a test fee late last summer in response to high demand for the tests since 2006, when Idaho reported three times as many cases as any other state.

Fevers are the main indicator of West Nile infections, showing up in 120 of last year's reported cases. The other 12 contracted encephalitis, an inflammation of the brain that usually occurs in 20 percent of cases.

Officials are reluctant to say there may be fewer occurrences this year, but numbers are typically lower over time after an initial outbreak, according to trends in other states.

"It is never easy to predict what's going to happen," Machala said.

Preventive measures can help curb the disease, says Fish and Game. Residents should rid their properties of standing water, repair broken screens and avoid outdoor activities at dawn and dusk.

It's also important to vaccinate horses, according to the department, since the virus kills one-third of the horses it infects.

Twin Falls County voters may have the chance to continue to fund a mosquito abatement district - where the county sprays to keep mosquito populations down - later this year.

The emergency district, which county officials and some residents credit for West Nile's relatively minor impact in 2007, was established after county commissioners declared an insect emergency in May. Their decision opened up $90,000 from a temporary tax district and as much as $200,000 to be spent this spring and summer.

Under a new state law, residents could vote to make the district permanent when the temporary funding schedule runs out later this year.