Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Flies Lead to Summer Sores on Horses

Pet Column for the week of August 1, 2005

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Kim Marie Labak
Information Specialist

Summer is the season for flies, and with flies can come Habronema, a genus of nematode worm that can infect the skin, eyes, stomach, and lungs of your horse. Dr. Karen Campbell, veterinary dermatologist at the University of Illinois Veterinary Teaching Hospital in Urbana, explains that Habronema infection (known as habronemiasis) commonly manifests as "summer sores," ulcerations of moist areas of horses' skin, especially around the eyes, mouth, and prepuce, or foreskin.

Habronema nematodes inhabit the stomachs of horses, asses, and mules. Female worms produce eggs in the horse's stomach, and then larvae may hatch from these eggs further down the gastrointestinal tract in the intestines. These larvae go out in the horse's feces where maggots of stable flies or house flies can pick up the larvae. When the maggots mature into adult flies, Habronema larvae migrate to the mouthparts of the fly from where they can be spread to other horses.

Dr. Campbell explains that Habronema larvae can be passed to a horse when a fly feeds around moist areas, such as the mouth, lips, eyes, wounds, and prepuce. "If the larvae move into the mouth, they can enter the gastrointestinal system and happily complete their life cycle, maturing into adults in the stomach." Severe infection can cause gastric inflammation or ulcers.

"However, if the larvae migrate somewhere else, they will not mature, but their migration can still cause irritation," she says. They may migrate through the nose and end up in the lungs, where they can get trapped and form cysts, which are usually harmless, or they may migrate through the skin causing cutaneous irritation and ulceration.

If the larvae get deposited in the skin, they are most likely to thrive on the moist areas near the mouth, nose, and eyes, or the prepuce of geldings that may remain damp with urine.

The larval migration can cause inflammation and open sores and may even produce sores that are messy and moist with pus. "Unfortunately, this moisture attracts more flies, perpetuating the cycle of infection and irritation," Dr. Campbell says.

Habronema larvae can be seen through a microscope, so a skin biopsy is a good method for diagnosing cutaneous habronemiasis. A biopsy can also differentiate habronema infection from squamous cell carcinomas, which are also common around the eyes, mouth, and prepuce.

Treatment for cutaneous habronemiasis usually involves an orally administered de-worming medication as well as oral and/or topical steroids to reduce inflammation and irritation.

The ideal but often difficult method for preventing summer sores is fly control. Some fly repellants are made specifically for horses in the forms of sprays or "spot-on" products that are dabbed on and spread through the skin. Around the face, fly masks can protect the sensitive and moist areas around the mouth, nose, and eyes.

While any age of animal can get summer sores, Dr. Campbell points out that some animals may be more susceptible than others. Since the inflammatory reaction to Habronema larvae may be an allergic reaction, some animals in a herd may develop an inflammatory reaction with sores, while other animals may not.

Overall, cutaneous habronemiasis cannot spread far from initial sites of infection, since the larvae dont survive long in the skin. However, the resulting ulceration can predispose an animal to other types of infection that can spread and become very serious, so summer sores should be diagnosed and treated promptly.

For more information about summer sores and other forms of habronemiasis, consult your equine veterinarian or a veterinary dermatologist.