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Protect Your Horse from Diseases Carried by Insect Pests

Horse owners tend to think of flies and mosquitoes as annoying nuisances. That’s certainly true, but they can also be downright dangerous as both can spread serious and even life-threatening diseases.
mosquito and fly

Mosquitoes transmit West Nile Virus (WNV), Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE). Flies are responsible for spreading Pigeon Fever, Equine Infectious Anemia (EIA) and play a role in summer sores.

Let’s take a brief look at these diseases and what you can do to protect your horse.

Equine Encephalomyelitis

Also known as “sleeping sickness,” Eastern and Western Equine Encephalomyelitis (EEE, WEE) are life-threatening viral diseases that attack the central nervous system. Both EEE and WEE are spread by infected mosquitoes that pick up the virus by feeding on an infected bird. There is no cure.

“EEE is a very serious neurologic disease, with a mortality rate in affected horses of 90 to 95%,” says Martha Mallicote, DVM, a large animal internal medicine specialist at the University of Florida in Gainesville.

“Although we think of these diseases as being primarily a problem in the southern U.S., a number of EEE cases were reported in New England and other northern regions in 2013,” she notes. “It is essential to vaccinate strategically to ensure your horse’s immunity to disease is at its best during mosquito season.”

West Nile Virus

This disease originated in Africa and the first cases were found in the U.S. in 1999. Since then, it has been reported in almost every state and most Canadian provinces. WNV is less likely to result in death than encephalomyelitis, but it can require extensive veterinary treatment and may result in long-lasting neurologic deficits in affected horses.

“Horses can be protected from these viruses with a routine vaccination program. Vaccines are very effective if given correctly and are a core part of equine vaccine recommendations,” says Mallicote.

Talk with your veterinarian about a vaccination schedule that fits your particular horse and where you live. Recommendations may vary depending on your horse’s age, use and whether or not you travel to shows and events.

Mallicote recommends reducing mosquito exposure by eliminating their breeding grounds.

“Water troughs should be routinely dumped and cleaned and any standing water around the farm should be drained,” she advises. “Insect repellents (labeled for use on horses) will make your horse less attractive to the mosquitoes and fans can prevent them from landing, due to creating air turbulence around the horse.”

Equine Infectious Anemia

When your veterinarian pulls blood for a “Coggins” test, he/she is testing for the viral disease known as Equine Infectious Anemia (EIA). Horse flies spread EIA when they bite an infected horse and then bite a healthy horse. There is no vaccine for EIA.

Talk with your veterinarian about a vaccination schedule that fits your horse and where you live.

EIA once killed thousands of horses every year, but fortunately, this is no longer the case. Since EIA testing was instituted in the early 1970s, the incidence of EIA in the U.S. has been reduced to almost zero.

Summer Sores

Some horses are more susceptible than others to cutaneous habronemiasis, more commonly known as “summer sores,” those unsightly, weeping skin lesions caused by the infective larvae of stomach worms.

House and stable flies are part of the process because their larvae ingest the larvae of stomach worms (Habronema muscae and Draschia megastoma) found in the horse’s manure. Once adult flies emerge from the pupa, they carry the infective nematode larvae. When these flies feed on a horse’s lips and nostrils, or on wounds on their body, they leave larvae behind. The skin lesions are a reaction to migration of larval stages of the worms. When the horse licks or swallows the larvae, they mature into worms in the horse’s stomach and the cycle continues. Horses can also ingest the larvae if infected flies fall into water or feed tubs.

Pigeon Fever

Also known as “dryland distemper” and Corynebacterium pseudotuberculosis, Pigeon Fever was first reported in horses in California in 1915. Although historically considered most common in that state, in recent years Pigeon Fever has been reported throughout the U.S., including several Eastern states.

The bacterium that causes Pigeon Fever survives and thrives in soil contaminated by fecal matter. The infection is spread by exposure to contaminated soil, by horn flies, stable flies and house flies, and may also be spread by horse-to-horse contact. High temperatures and drought conditions often precede outbreaks.

“Most infected horses develop deep muscle abscesses that may resemble Strangles abscesses,” says Mallicote. “The name Pigeon Fever stems from the fact that many abscesses are found in the pectoral region and result in an appearance similar to a ‘puffed-out pigeon.’ It is difficult to truly quarantine affected animals, due to the spread of bacteria by flies in their environment. It is most essential that when abscesses are drained, the purulent material that drains out is cleaned up and the area disinfected carefully. Good fly control is also important to prevent the spread of the bacteria.”

To fight against fly-borne disease, it is important to practice good sanitation methods and fly control. Be diligent about removing manure from horse areas and keep the manure pile as far from the barn as possible. Ideally, you should utilize a program in which manure is deposited in a dumpster and removed regularly.

Protect your horse with fly repellents and insecticides during fly season. Read labels and apply products carefully following all recommendations. The use of fly masks, sheets and fly boots may also be helpful.

Your veterinarian may also recommend feed-through fly control products. These are added to the horse’s feed and contain insect growth regulators that kill the larvae in the horse’s manure, interrupting the fly life cycle. Such products don’t control adult flies, but can aid in the overall battle against these pests.

By Cynthia McFarland


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