California's horse owners should act now to prevent West Nile Virus, according to UC Davis equine experts.

Speaking at a campus scientific seminar on West Nile in horses, the specialists said a new preventive vaccine against West Nile is effective and should be administered now, before the peak mosquito season. West Nile virus kills one in three infected horses, and there are more than 1 million horses in California.

"We are encouraging horse owners to talk to their veterinarians about getting the vaccine now, so their animals will be protected this summer," said Gregory Ferraro, director of the UC Davis Center for Equine Health.

West Nile is a virus transmitted by mosquitoes that can cause inflammation of the brain, or encephalitis, in people, horses and birds.

The infection is far more dangerous to horses than it is to people. Signs of infection in horses vary markedly but can include twitching or flaccidity of the lips, listlessness, stumbling and uncoordination, leg weakness, and a tendency to startle easily. Fever is not usually present.

A vaccine for horses manufactured by Fort Dodge Animal Health, a division of Wyeth Laboratories, is available from licensed veterinarians.

In humans, West Nile infections usually cause only mild illnesses. Elderly and immunocompromised people are at increased risk of severe infection. There is no vaccine yet for humans.

UC Davis mosquito-control experts also recommend controlling mosquitoes around horses (and homes) by removing unnecessary standing water, repairing leaking faucets and air conditioners, aerating ponds or stocking them with fish, stabling horses at dawn and dusk and using fans, screens and repellents such as DEET and permethrin.