The 30-year-old horse was found on the ground in Blossom Valley suffering from seizures and later died from the mosquito-borne disease.

“Horses are very susceptible to the virus,” said county veterinarian Dr. Nikos Gurfield. “... It is extremely important that horse owners properly vaccinate their horses against this deadly disease.”

Gurfield said that about 30 percent of horses infected with the virus who develop symptoms will die. The state Department of Food and Agriculture recommends that horses receive booster shots every six months.

Another 11 dead birds – 10 American Crows and one red-tailed hawk – found in the county this past week also have tested positive for West Nile virus, bringing the total number this year to 72, county officials said.

West Nile virus is primarily a bird disease, but it can be transmitted to humans and horses by infected mosquitos. There have been no human cases of West Nile virus reported in San Diego County so far this year.

Signs of the disease in horses include stumbling, staggering, wobbling, weakness, muscle twitching or inability to stand, and a veterinarian should be contacted immediately.

For more information on the virus, call county Vector Control at (888) 551-4636. Dead birds can be reported on the county's West Nile virus Web site, www.SDFightTheBite.com.

 Breaking News Team: (619) 293-1010; breaking@uniontrib.com