West Nile Virus in Horses

West Nile virus (WNV) has become endemic in the United States and is here to stay. Since its first occurrence in New York in 1999, migrating birds have spread the virus throughout the United States and Canada.
In the United States there were 15,257 equine cases and 4,156 human cases in 2002, 4,636 equine cases and 9,306 human cases in 2003, and 1,314 equine cases and 2,470 human cases in 2004.
In Michigan there were 341 equine cases and 644 human cases (51 deaths) in 2002, 9 equine cases and 19 human cases (2 deaths) in 2003, and 21 equine cases and 16 human cases in 2004.
The equine mortality rate continues to be about 30 percent. The fatality rate in people is reported to be around 10-15 percent. For all species, those at greatest risk are the young, the old, and those with compromised immune systems.
Typically, horses and people are considered ‘dead-end hosts,’ i.e., they do not act as a source of infection for others; however, transmission of the virus in humans through blood transfusions, organ transplants, and across the placenta has occurred. These routes, especially blood transfusions, should be considered with horses.
Most other species develop an immune response without becoming ill, except for rare cases where the animal’s immune system is compromised.
Although the disease has been reported in a variety of other species, such as dogs, wolves, llamas, and squirrels, it has been found that most of the animals that contracted WNV already had health issues.
However, WNV has been found to be a huge problem with otherwise healthy alligators in the southern United States.
It is important to remember that most horses (and people) bitten by WNV-infected mosquitoes don’t become ill.
Those horses that do are most likely to exhibit clinical signs that are neurological in nature. Muscle fasciculations (muscle twitching) seem to be one of the most common signs. Additional signs that may be seen are incoordination, muscle weakness, fever, somnolence (sleepiness), inability to eat and drink, recumbency, and seizures.
Unlike Eastern equine encephalitis and Western equine encephalitis which have a very low survival rate, horses clinically affected by WNV appear to have a 65 percent chance of recovery and return to normal function when treated.
However, a recent study in Minnesota has shown that it may take several months for some horses to recover and that some horses may have persistent neurological deficits.
Once a horse has been infected with the WNV and survived, it is protected from development of clinical disease for an extended period of time.
There is no specific treatment. Horses that are clinically infected are treated by supportive measures, such as IV fluids, control of fever, and safe, quiet housing. Prevention remains the best practice to follow.
The horses that remain able to stand or are able to stand with assistance usually make a complete recovery over several weeks to months. Those horses that become recumbent have a guarded prognosis for survival.
Management of recumbent horses is very difficult and costly. Often horses are required to be euthanized for humane reasons even with an owner committed to treatment.
Vaccination is recommended for all equids--horses, donkeys, and mules. There is no evidence that vaccination is related to pregnancy loss or birth defects.
Fort Dodge has two products available. The WNV vaccine and a combination vaccine where WNV is combined with EEE, WEE, and tetanus.
Vaccination recommendations for previously unvaccinated horses are to give two doses of vaccine three to six weeks apart. It takes about four weeks after the second dose (i.e. the booster vaccination) for maximum protection to occur.
A new DNA vaccine for WNV from Merial is also available.
If the vaccinations are started after an outbreak, it is advised to give the booster after only two weeks time. If horses are exposed to mosquitoes for more than six months per year, most veterinarians are recommending a booster every six months.
In addition, Fort Dodge has recommended that previously vaccinated horses that receive a booster in early spring get another booster in July. The vast majority of cases across the country occur in August and September.
Research shows the efficacy of both vaccines to be about 95 percent; a few horses that were vaccinated according to manufacturer recommendations prior to WNV exposure have developed WNV.
Also, research by the vaccine manufacturer has demonstrated that the majority of properly vaccinated horses do not appear to develop a viremia (the vaccine prevents the virus from replicating in the animal’s body).
At present, recommendations for reducing the risk of your horse developing WNV 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, masks, and leg wraps; turn barn 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.