California Riding Magazine • August, 2008

West Nile Strikes
in Southern California
First reported cases of equine West Nile in 2008.

by Rebecca Sparenberg

California’s first 2008 case of equine West Nile was reported in Riverside County’s Corona on June 23. The horse was a 15-year-old, Quarter Horse that died of complications related to the virus. California Department of Food and Agriculture (CDFA) spokesman, Steve Lyle says it isn’t known when exactly the horse died.

A second West Nile related death was reported on July 7 in San Diego County’s Blossom Valley. Dr. Greg Smith of East County Large Animal Practice in Blossom Valley says the horse, a 30-year-old gelding, was found having a seizure in his paddock and was humanely euthanized. The virus was detected in a postmortem exam.
The first cases of California acquired equine WNV were confirmed during June and July 2004 in San Bernardino and Riverside counties. Since it’s detection in California, the disease has expanded to the north. In 2007, 28 cases of equine West Nile were confirmed in California, with a 50 percent mortality rate.

“We’ve had positive horses for several years now,” explains county veterinarian Dr. Nikos Gurfield. “Usually it’s been later on in the season. Horses are very susceptible to the virus ... It is extremely important that horse owners properly vaccinate their horses against this deadly disease. We recommend that horses receive booster shots every six months.”

The CDFA believes that the virus peaked in 2004, with cases having dropped dramatically in the past four years. California had 540 equine West Nile cases in 2004, and in 2006 the number of confirmed equine cases slowed to 58. This is consistent with WNV becoming more endemic in California and Lyle expects only a small number of cases this year. “The belief is that any horse that hasn’t been vaccinated that is going to be exposed to the disease, has been (exposed). The real peak years have passed us by, but that’s no reason not to be ever vigilant about vaccination.”

West Nile virus is primarily a bird disease that can be transmitted to humans and horses by infected mosquitoes. Many horses infected with WNV will not develop any illness and clinical symptoms range from mild, “flu-like” signs to encephalitis (inflammation of the brain) including stumbling, staggering, wobbling, weakness, muscle twitching and inability to stand. Currently, there is no specific treatment.

Bite Back

The CDFA urges horse owners to take additional steps (beyond vaccinations) to protect themselves and their horses from West Nile. Dr. Gurfield says stabling horses during the peak mosquito feeding hours, dusk to dawn, can help decrease the chances of horses being exposed to WNV by minimizing their exposure to mosquitoes. Ensure that stable doors are closed and screens are well-maintained to keep mosquitoes from entering the barn. Utilize fans to keep air circulating and mosquitoes from settling in the area.

Applying insect repellent and covering horses with fly sheets also effectively reduces the number of mosquito bites.

Mosquitoes lay their eggs in standing water. The eggs develop into thousands of mosquitoes in 7-10 days. Drain any unnecessary standing water (wheelbarrows, tires, etc.) and keep rain gutter clear to reduce mosquito-breeding sites. Cover or turn upside down all buckets, wheelbarrows and feeders so they can’t hold water. Be sure to change water troughs at least twice a week and consider stocking ponds and/or water tanks with fish that consume mosquito larvae.

For more information on the equine WNV and tips on how to minimize the risk of outbreak in horses, visit www.cdfa.ca.gov/ahfss/Animal_Health. Dead birds should be reported immediately to the CDFA though their toll free number, 1-877-WNV-BIRD.