West Nile a Threat to Horses, Too
Infected Mosquitos Don’t Discriminate

By Jim Evans

West Nile virus is a disease that has gained notoriety in the past several years — especially in California, where, according to the federal Centers for Disease Control, more than 779 cases have been reported this year. That’s more than any other state and almost twice as many as in second-place Arizona.

Considered a seasonal epidemic by most experts, with most cases reported in summer and fall, West Nile virus is actually exacerbated by the year-round temperature in California and other temperate states. The warmer climate helps extend its “season.”

Spread by the bite of mosquitos primarily infected from feeding on infected birds, West Nile virus is spread by those mosquitos to humans and animals and can cause serious illness or death. Although it is reported primarily as a human disease, the virus has caused illness and death among horses. Small pets, such as dogs and cats, and small mammals, such as squirrels, seem to be relatively unaffected, but horses are definitely another matter.

CDC data indicates that most horses infected with West Nile virus recover, but it has been responsible for some deaths among the equine population in the United States, and more than 21,000 cases in horses have been reported since 1999.

The infected mosquitos feed on the horse, and the virus is injected into the animal’s bloodstream, interfering with the central nervous system and eventually causing inflammation of the brain. Although there is no evidence to date that West Nile virus can be transmitted from an infected horse to other animals — or to their human owners — the CDC recommends that horses suspected to carry the virus be isolated from other horses and control precautions be followed when caring for a diseased horse.

According to the Ohio State University College of Food, Agricultural and Environmental Sciences, West Nile virus infects the central nervous system of horses and causes symptoms of encephalitis, such as loss of appetite; depression; fever; weakness or paralysis of hind limbs; muscle twitching; impaired vision; poor coordination; head pressing; aimless wandering; convulsions; inability to swallow; circling; hyperexcitability; or coma.

Traditional vaccinations against Eastern equine encephalitis, Western equine encephalitis and Venezuelan equine encephalitis do not protect against West Nile virus. However, a new vaccine against the virus in horses has been developed in a collaboration between the CDC and Fort Dodge Animal Health, and licensed by the U.S. Department of Agriculture.

The vaccine is “is truly an exciting innovation and an incredible scientific breakthrough that has potential benefits far beyond preventing (West Nile virus) in horses,” Dr. Julie Gerberding, CDC director, said in July.

“This science will allow for the development of safer and more effective human and animal vaccines more quickly.”

The CDC and Fort Dodge Animal Health have been developing and testing the vaccine since 2001, and it was officially licensed by the USDA on July 8. The vaccine is expected to be commercially available to veterinarians through Fort Dodge Animal Health in early 2006.

In the meantime, horse owners should do everything possible to prevent their horses from being bitten by potentially infected mosquitos. Because mosquitos lay eggs in standing water, watering troughs should be changed frequently to provide fresh water for horses and eliminate potential breeding grounds, and any other standing water in the proximity of stalls and corrals should be drained or eliminated. A mosquito can lay eggs in as little as 1 inch of water.

Some horse owners treat livestock water tanks with BTI, a bacteria that kills larvae but is safe for animals. BTI, aka mosquito dunks or pellets, is usually available at most home and garden stores.

Reporting dead birds to state and local health officials also will help control any potential spread of West Nile virus. More than 130 species of birds have been known to be infected with the virus, but not all infected birds die from it. And of course, birds die from other causes, so a dead bird is not necessarily cause for alarm. Dead birds should not be touched, because state and local agencies will have their own policies for collecting and testing them.

Some insect repellents for humans, such as DEET and Picaridin, are not usually recommended for horses or household pets because they are poisonous when ingested; however, certain extracts of Neem oil are deemed safe and effective for humans and animals, according to www.neemamerica.org. The extracts can be used to ward off more than 200 species of insect pests, including mosquitos, gnats and flies.

The most important thing is to prevent your horses from becoming bitten in the first place — and you, too! An ounce of prevention is definitely worth a pound of cure, because West Nile virus is nothing to mess with, but the risk that your horse is going to be bitten by a single infected mosquito is low, and the risk that your horse will die from the virus— even if infected — is similarly low.

Still, better safe than sorry, and your horse will appreciate your caution.