West Nile Virus and Horses
West Nile virus (WNV) has become endemic in the United States. In Michigan in 2002, there were 341 equine cases and 577 human cases (51 deaths) of West Nile Virus.
The equine mortality rate for West Nile Virus in unvaccinated horses is about 30 percent. The fatality rate in people is reported to be around 10-15 percent. Those at greatest risk are the young and old and those with compromised immune systems.
Horses and people are considered 'dead-end hosts,' i.e., they do not act as a source of infection for others. Most other species develop an immune response without becoming ill, except for rare cases where the animal's immune system is compromised.
It is important to remember that most horses (and people) bitten by West Nile Virus-infected mosquitoes don't become ill. Those horses that do become ill are most likely to exhibit clinical signs that are neurological in nature. Muscle twitching seems to be one of the most common signs. Additional signs that may be seen are incoordination, muscle weakness, somnolence (sleepiness), inability to eat and drink, and recumbency (laying down). Many infected horses walk with their hindquarters twisted toward one side, usually the horses' right. The horses that remain able to stand or are able to stand with assistance usually make a complete recovery over several weeks. Those horses that become recumbent have a guarded prognosis for survival.
The Fort Dodge vaccine against equine West Nile Virus has received full licensure and is available through your veterinarian. Vaccination recommendations are: give 2-3 doses of vaccine initially 3-6 weeks apart. It takes about four weeks after the booster vaccination for maximum protection to occur. This means that you must give the vaccine at least four weeks before the start of mosquito season.
If the vaccinations are started after an outbreak, it is advised to give the booster after only 2 weeks time. If horses are exposed to mosquitoes for greater than 6 months per year, some veterinarians are recommending a booster every 6 months. In addition, Fort Dodge is recommending that previously vaccinated horses receive a booster the next spring, followed by another booster in July. It appears that the vast majority of cases across the country are in August and September.
At present, recommendations for reducing the risk of your horse developing West Nile Virus include proper vaccination, minimizing your horse's exposure to mosquitoes and mosquito control.
To reduce mosquito exposure: stable horses at dusk and dawn, use fly blankets, etc., turn lights off, use fans to move air, and use mosquito repellents. Mosquito control programs include reduction of standing water, encouraging natural predators (fish, birds, etc.), and use of chemicals for larval and adult mosquito control. Regular dumping of water tanks and buckets (every 2 to 3 days) will also prevent mosquito larval growth without chemicals.
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