SAN DIEGO -- County officials said Friday that two horses, including one that had to be euthanized, have been diagnosed with West Nile virus -- the first two horses to be stricken with the disease this year.

West Nile is mainly a bird disease, but it can be passed on to, and kill, people, horses and other animals by bites from mosquitos that have fed on infected birds.

Five people around the state have died from West Nile virus this year, and Gov. Arnold Schwarzenegger has declared a state of emergency in three counties: Kern, Colusa and San Joaquin. However, no persons in San Diego County have been diagnosed with the virus this year.

County officials said that a horse from Lakeside with the virus was euthanized, but a horse from La Mesa that tested positive was still alive.

There is no vaccine for people, but there is one for horses. Officials said horse owners should contact their veterinarian immediately if they show any symptoms, which include stumbling, staggering, wobbling, weakness, muscle twitching or an inability to stand.

Officials say the best ways for people to protect themselves are to wear insect repellant containing the chemical DEET -- especially around mosquito feeding times of dawn and dusk -- to make sure doors and windows have tight-fitting screens, and to eliminate pools of standing water around their home.