West Nile Encephalitis in Horses

EPM and Eastern EEE Western WEE and Venezuelan VEE equine encephalitis are examples of other diseases with neurological signs that may be confused with West Nile encephalitis. A positive diagnosis of West Nile encephalitis can be made by testing blood from an infected horse. Determining the actual number of horses exposed to the West Nile virus is difficult as some horses exposed to the virus may not show any clinical signs or perhaps may exhibit only vague signs of the disease.
Even when clinically ill horses recover from West Nile encephalitis some horses may have residual effects such as gait abnormalities and a change in their usual temperament or behavior. These effects may be significant and affect the owners use or enjoyment of the horse. Keep the veterinarian involved in reassessing the horses neurological condition beyond the apparent resolution of the initial signs. Continued veterinary involvement is very important since some of these effects may impact a persons safety when handling affected horses and impact their future use and management.
Treatment for West Nile Encephalitis
Consult with your veterinarian for recommendations for current treatment methods. Affected horses should be treated based on the presenting signs with emphasis on reducing the severity of the disease. Weakened and impaired animals should be protected from injuring themselves. Fluid and nutritional supportive therapy also may be required either by intravenous methods or by stomach tube.
West Nile Virus Vaccine Information
Several vaccines are available that have met the USDA requirements for safety. When using most current vaccines the first time the horse receives the vaccine it is very important that a series of two doses be given about 3 to 6 weeks apart. It appears that horses will not develop high presumably protective antibody levels until they receive the booster or second vaccination in the series. Optimal protection occurs

2 weeks after this second dose. In addition this second dose should be given at least 4 weeks before mosquitoes are likely to bite and infect the horse. These vaccines require an annual booster. Depending on the time of year the horse is vaccinated and the regional presence of the disease veterinarians might recommend boosters be given twice yearly. Horses vaccinated against Eastern Western andor Venezuelan equine encephalitis will not be protected against West Nile virus.
Vaccines are generally effective and recommended to prevent West Nile encephalitis. Some research has shown that some horses do not respond well to vaccinations. Be sure to consult with your veterinarian for current vaccination recommendations schedules and management practices to prevent West Nile encephalitis.
At the current time none of the vaccines are labeled for use in pregnant mares in the United States. However a vaccine has been licensed in Europe for use in pregnant mares. Furthermore a Journal of the American Veterinary Medical Association article by Vest et al. concluded that Vaccination of pregnant mares during any period of gestation was not associated with increased incidence of pregnancy loss. Again consult with your veterinarian for the most uptodate information and research on available vaccines and recommendations for use. In addition discuss vaccination schedules for mares and foals with your veterinarian as the schedules may be different for various vaccines.
Owners should keep records of all vaccinations or treatments which may affect international shipment of horses. Owners are advised to contact the U S. Department of Agriculture or their state Department of Agriculture for current regulations involving interstate and international travel of horses.
Prevention and Control
Horse owners can reduce the likelihood of exposure to mosquitoes by implementing the following pest management practices
Keep horses stabled during peak periods of mosquito activity dusk and dawn.
Fans may reduce the ability of