California Riding Magazine • November, 2014

Health Alert
San Diego County horse tests positive for West Nile Virus.

by Dr. Jessica Stokes DVM, MS in Immunology

An 18-year-old Tennessee Walking Horse gelding has had a confirmed positive diagnosis for the West Nile Virus (WNV) in San Diego County. The diagnosis was confirmed in early September. This follows the first confirmed case of West Nile in a human in this region since 2012. Multiple birds have also tested positive across the county, including a dead crow found on a ranch currently housing over 100 horses.

Common signs of West Nile in horses can vary widely and include: fever, lethargy and depression, colic, lameness with or without incoordination, refusal to eat and tremors of the face and neck. However West Nile does not always follow the rules. The horse that was positive did not have a fever and only showed very slight signs of lethargy, incoordination and twitching of his nose. An infected horse is not infectious to other horses and cannot transmit the virus to mosquitoes.

Protecting Your Horse

Experts agree that vaccination is the best way to protect your horse. Widespread vaccination of horses has decreased the number of cases that develop signs of disease which require treatment. However, vaccinated horses can still succumb to the disease. This is because vaccine effectiveness is dependent on the immunological status of the horse. Some horses may not mount an adequate immune response to vaccination. This group can include, but is not limited to, horses that are in bad health, are on medications that suppress the immune response or have a disease that depresses the immune system. This is why the bird and mosquito population of the environment must also be managed.

Mosquito control measures can include turning off lights around barns (mosquitoes are attracted to lights), keeping horses indoors during peak mosquito hours (dawn and dusk), fans to produce a breeze around stables (mosquitoes are weak fliers), mechanical or chemical deterrents (sprays or fly sheets), and draining or drying up mosquito breeding areas.

Mosquitoes acquire WNV from infected birds. Efforts should be made to control bird populations around the stable environment. Chickens and turkeys are resistant to the disease and do not act as a reservoir of infection. However, other birds, such as crows and blue jays generally succumb to the disease and can transmit WNV to mosquitoes. If you are in San Diego and find dead birds, squirrels or green swimming pools report them to the San Diego County Vector Control Program at (858) 694-2888.

West Nile virus is a threat to both human and horse health. Proper vaccination and vector control measures can go a long way in decreasing exposure to West Nile. Our horses rely on our ability to prevent and, if needed, recognize signs of disease. Informed horse owners are aware that the old saying regarding an ounce of prevention being worth a pound of cure holds true in the fight against West Nile virus. Ask your veterinarian if you have any questions regarding ways that you can further protect your horses.

Author Dr. Jessica Stokes and Dr. Max Wilcox operate Exact Equine, a mobile equine veterinary service serving San Diego County. For more information, visit