Because West Nile virus kills one in three infected horses and there are more than 1 million horses in California, UC Davis equine experts today made new recommendations for vaccinations and mosquito-control measures.

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.

"California's horsemen should heed the lessons learned by other states," said Gregory Ferraro, director of the UC Davis Center for Equine Health. "West Nile virus is a potent pathogen that has the capacity to infect large numbers of horses in a short period of time. One-third of those infected can be expected to die and many of the rest can be left with long-term and even permanent disabilities.

"However, intensive research at UC Davis and elsewhere has led to methods for the development of a preventive vaccine that is safe and effective against the West Nile virus. We are encouraging horse owners to talk to their veterinarians about getting the vaccine now, so their animals will be protected this summer."

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

Since the first U.S. case of West Nile virus was reported in birds in New York in 1999, the virus has spread westward rapidly. Last summer the first West Coast cases were reported in humans, horses and birds.

The infection is far more dangerous to horses than it is to people. About one in three horses that are infected with West Nile virus will die. 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 that first became available last year under conditional approval by the U.S. Department of Agriculture now has been given unconditional approval, Ferraro said. It is effective, and the reported incidence of vaccine reactions is lower than would normally be expected with any vaccine.

Manufactured by Fort Dodge Animal Health, a division of Wyeth Laboratories, in Overland Park, Kan., the vaccine is named "West Nile -- Innovator." It is available only from licensed veterinarians.

In humans, West Nile infections usually cause only mild illnesses; according to the U.S. Centers for Disease Control and Prevention, of the 4,071 people known to have been infected in the United States in 2002, 274 died -- a death rate of about six in 100. Elderly people are at greatest risk of severe infection. Immunocompromised people are at increased risk.

In humans, symptoms of mild infections -- West Nile fever -- are fever, headache and body aches, with occasionally a skin rash on the trunk of the body and swollen lymph glands. The symptoms of severe infection -- West Nile encephalitis or meningitis -- include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness and paralysis.

There is no vaccine yet for humans.

The Center for Equine Health at UC Davis maintains a herd of 250 horses. Its mission is to advance the health, well-being, performance and veterinary care of horses through research and education.

The Center for Equine Health experts made the following general recommendations for preventive vaccinations in horses. (Specific medical decisions for individual horses always should be made by the owner and the horse's regular veterinarian.)

Unvaccinated horses

• Adult horses: Vaccinate all previously unvaccinated adult horses in March or April 2003 with two doses, three to four weeks apart. This is called a "primary series."
• Pregnant mares: Avoid vaccinating pregnant mares in the first trimester of gestation. Talk to the veterinarian about vaccinating pregnant mares after the first trimester.
• Horses under 1 year old: Vaccinate all horses less than 1 year old in March or April 2003 with a three-dose primary series. Allow three to four weeks between doses 1 and 2, and allow six to eight weeks between doses 2 and 3. (Exceptions: If the foal was born to an unvaccinated mare or a mare that was not booster-vaccinated four to six weeks before foaling, begin the vaccination series at 3 months of age. If the foal was born to a mare that was fully vaccinated or booster-vaccinated within two months of foaling, then begin the vaccination series at 4 to 6 months of age.)

Vaccinated horses

• If the horse was fully vaccinated in 2002 or if the primary vaccination series was completed in 2003, follow up with a booster every six months.
• When the first case of West Nile virus in a horse is identified in California, booster-vaccinate all horses that have not been vaccinated within the previous six to eight weeks. Then follow with boosters every six months from the date of the last vaccination.

UC Davis mosquito-control experts say it is too early to tell if mosquito populations this year will be normal or not. In any case, they suggest these measures to reduce the number of mosquitoes around horses (and homes):

Reduce mosquitoes and their breeding grounds:

• Eliminate any unnecessary standing water (tires, wheelbarrows, old buckets, etc.).
• Check around faucets and air conditioner units and repair any leaks and remove puddles.
• Make certain roof gutters drain properly and remove any standing water, especially from flat roofs.
• Clean out waterers, birdbaths, plant saucers, etc., at least weekly.
• Keep swimming pools clean and free of water on covers.
• Schedule pasture irrigation to minimize standing water.
• Keep ponds aerated and/or stocked with fish; consider stocking livestock water tanks with fish that consume mosquito larvae.

Reduce horses' exposure to mosquitoes:

• Stable horses inside during active mosquito feeding times (dawn and dusk).
• Use fans, barrier cloths, screens, flysheets, repellent sprays (permethrin, DEET) and insecticide misting systems.

More information:

• UC Davis Center for Equine Health information on West Nile virus: http://www.vetmed.ucdavis.edu/ceh/wnv_info.html
• UC Davis expert sources on West Nile virus: http://www-pubcomm.ucdavis.edu/search/news_detail.lasso?id=6315
• U.S. CDC web site on West Nile virus: http://www.cdc.gov/ncidod/dvbid/westnile/index.htm
• A graphic showing how West Nile virus cycles between mosquitoes and animals: http://www.cdc.gov/ncidod/dvbid/westnile/cycle.htm
• A map showing the spread over time of West Nile cases across the United States: http://www.cdc.gov/ncidod/dvbid/westnile/surv&control.htm
• Betacam B-roll is available showing horses with typical symptoms of West Nile infection. Contact Sylvia Wright, below.