California Riding Magazine • March, 2013

Prevention Pays
West Nile Virus is here to stay and vaccinations are the best defense.

by Dr. Jessica Stokes, DVM, MS in Immunology

Summer is coming and that means mosquitoes, which should by now trigger an alert for every horse owner: West Nile Virus. It's not going away.

The numbers for 2012 are in. Cases of the mosquito-transmitted disease in horses in California were the highest since 2008. During last year 22 horses were found to have a confirmed case of WNV in California1. Nationwide, 618 horses were confirmed to have WNV 2 and 5,387 people were confirmed to have the disease3. Why the discrepancy in numbers of cases between horses and humans? Part of the explanation is that currently there is no vaccine approved for use in humans to provide defense against the virus. Widespread vaccination of horses has decreased the number of cases that develop signs of disease that require treatment.

When Norma Arceo, with the California Department of Public Health, was asked, "What is the most important thing horse owners should know regarding West Nile Virus?" Her response was, "Horses should be immunized." Experts agree that vaccination is the best way to protect your horse.

Think of vaccination as self-defense training for your horse's immune system. Vaccines prepare the body to recognize and mount an attack against a foreign invader. A 2006 study looked at the efficacies of three commonly used vaccines in the face of a known West Nile exposure4. It showed that, in the horses vaccinated and then exposed to West Nile, the survival rate was 100 percent. The vaccinated horses were protected from the onset of WNV encephalitis and viremia. However, the unvaccinated group all showed neurological signs, viremia and histopathologic lesions in the brain and spinal cord and were euthanized for humane reasons.

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 (such as corticosteroids) or have a disease that depresses the immune system (such as Cushings). If a horse is immunologically compromised it is especially important to vaccinate them as they are at a higher risk of contracting the disease if exposed. Horses with disease caused by WNV can show many different signs linked to the involvement of the brain. These commonly include fever, depression, appetite loss, muscle weakness, tremors and incoordination.

Environmental Measures

Along with vaccination, vector control measures should be instituted to further decrease chances of exposure. WNV is transmitted between infected birds and mosquitoes. Other animals, including horses and humans, are dead-end hosts. This means that an infected horse cannot pass WNV to a mosquito when bitten and is thus not dangerous or infectious to other horses5.

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.

Because mosquitoes acquire WNV from infected birds, efforts should be made to decrease bird populations around the stable environment. WNV has been found in birds of at least 326 species6. 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 California and find a dead bird (or squirrel) that you would like to report call 1-877-968-2473.

West Nile virus is here to stay. 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 received her masters degree in immunology from UC Davis and her veterinary degree from Oklahoma State University.  After practicing in New Mexico and Texas she returned to her home town of San Diego. In 2010 she started Exact Equine, Inc. with her partner, Dr. Max Wilcox, and enjoys working on horses in an ambulatory setting.

1 California West Nile Virus Website:
2 USDA: Animal Health Monitoring & Surveillance Data: 2012 Equine Case Reports of West Nile Virus reported to the ArboNET reporting system as of 12/11/2012
3 CDC: Statistics, Surveillance, and Control: West Nile Virus (WNV) Human Infections Reported to ArboNET, by State, United States, 2012 (as of December 11, 2012)
4 Seino, et al. Clin Vaccine Immunol. 2007 November; 14(11): 1465–1471. Comparative Efficacies of Three Commercially Available Vaccines against West Nile Virus (WNV) in a Short-Duration Challenge Trial Involving an Equine WNV Encephalitis Model
5 Filette, et al. Vet Res. 2012; 43(1): 16. Recent progress in West Nile virus diagnosis and vaccination
6 The CDC Database.