Summer and fall are the most important times to have your horse protected against West Nile virus. Dr. Tammy Beckham from the Texas A&M Veterinary Diagnostic Laboratory indicates the death rate among horses from West Nile is from 30-40 percent, and of those that recover, 40 percent may still exhibit some symptoms six months or more after recovering from it. Although West Nile is not in the news as much as it was in the past, the diagnostic lab indicates the percentage of horses they tested for West Nile that were positive has increased significantly over the past few years. The virus uses birds as hosts and mosquitoes to transmit the virus. West Nile is now considered to be endemic in the U.S., and transmission to humans and horses is possible.
Clinical signs of West Nile include neurological signs such as depression, wobbling, weakness of the legs, lameness, partial paralysis, muscle twitching, and the inability to stand. There are many nervous system diseases that can cause similar symptoms so blood testing is required to confirm the diagnosis. The appropriate test is required because some tests are affected by vaccination and will provide a false positive due to vaccination. Prevention of West Nile infection centers around vaccination as it is difficult to keep mosquitoes off of your horse. Two vaccines are required initially within a 30-day period and then the vaccine is usually administered once or twice yearly, depending on the recommendation of your veterinarian and the horse’s chance of exposure. Using normal methods of decreasing mosquito population, such as eliminating standing water, will help decrease numbers of West Nile cases in both humans and horses.