ELK GROVE, Calif. - Every six months, Pam Murphy brings her five horses from her rolling property in the Sierra foothills town of Plymouth to suburban Sacramento for vaccinations against the West Nile virus.

Like other horse owners, Murphy is bracing for another outbreak of the virus, a potentially fatal disease that afflicts horses more than any other mammal.

"It's definitely here because people have lost horses throughout the Sacramento Valley," Murphy said as she readied her 3-year-old horse Whoopie for its vaccination.

West Nile virus has killed 430 horses in California during the last two years, according to the state Department of Food and Agriculture. Veterinarians fear a wet spring coupled with warm weather could further spread the disease among an estimated 20 percent of horses in the state that have yet to be vaccinated.

The record rains that have saturated Northern California since the first of March have created ripe breeding grounds for mosquitoes, the primary conduit of the virus to horses and humans.

"If there's water standing, we're going to have lots of mosquitoes," said Gregory Lanzaro, director of the Mosquito Research Program at the University of California, Davis. "And right now, we're soaked."

Female mosquitoes lay their eggs in pools of standing water, where the young insects spend the first half of their lives.

Rice fields, swollen drainage ditches and stock ponds in livestock pastures are prime breeding grounds in rural areas. In more populated areas, mosquitoes breed in storm sewers, discarded containers that fill with rain, bird baths and dirty swimming pools.

"The more water you have, the more breeding grounds you have for mosquitoes," said Vicki Kramer, who heads the vector disease section of the state Department of Health Services.

Most of California has had water in abundance this year. The National Weather Service has reported record rainfall since March in the Central Valley, causing flooding along the San Joaquin River and its tributaries. The forecast for the rest of April is brighter, with temperatures beginning to rise with the approach of summer.

So far this year, 11 dead birds have tested positive for the West Nile virus in Sacramento, Santa Clara, San Diego and Orange counties, state health officials said.

Health experts are on watch in each of the state's 58 counties. They are especially vigilant in Sacramento County, where last year more than one in 10 mosquitoes carried West Nile.

Statewide, 1,765 people have been infected during the past two years, and 47 have died, according to the state health department. Many of the deaths occur in the elderly or people with compromised immune systems.

Among domestic animals, horses are affected most.

About 44 percent of horses in 43 counties that were infected with West Nile have died in the last few years, according to the California Department of Food and Agriculture.

Others that have contracted the virus could suffer long-term effects such as changes in disposition, muscle tremors and sensitivity to light and touch. The more seriously affected horses suffer permanent neurological impairment, rendering them unable to ride or show, said Dr. W. David Wilson, a professor of equine medicine and a director at the Veterinary Medical Teaching Hospital at UC Davis.

"It's a definite threat. Horses in California will die this year," Wilson said. "We know there are still horses that have not been vaccinated. If they get vaccinated, the number will go down."

He said it remains a mystery why horses are more susceptible to the disease than other domestic animals and livestock.

A $26 vaccine administered every six months can shield horses from the virus. In February, state veterinarian Dr. Richard Breitmeyer issued an alert and urged horse owners to vaccinate their animals in anticipation of a high-infection year.