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EQUINE HEALTH
Summer sores or cutaneous habronemiasis, also called granular dermatitis and jack sores, are the result of a detour in the life cycle of certain equine stomach worms. These worms of the Habronema and Draschia species are not very dangerous internal parasites in a typical scenario. As adults they live in the horse’s stomach, feeding on the stomach wall and rarely causing serious damage. Their larvae can some- times cause problems, however, if they can’t get into the stomach. Summer sores are caused by larvae of these stomach worms trying to develop in the horse’s skin instead of the stomach after being brought to the skin by flies.
The adult worms in the horse’s stomach
lay eggs that are passed in manure and quickly hatch. The tiny larvae that emerge from the eggs must get back into a horse to complete their life cycle, and they need help to do that. Their help comes from fly maggots in the manure. These maggots are larvae of house flies, face flies and stable flies that live in manure after hatching from eggs laid in manure by the adult flies. The young maggots ingest the worm eggs and larvae as they feed, and the worm larvae develop inside the maggots as they develop into adult flies. Then the tiny worms are inside the adult fly which serve as both incubators and carriers for the worms, allowing the stomach worm eggs to develop into more advanced larval stages inside the flies that can transport them back to a horse.
All of these species of flies are attracted
to secretions around the horse’s mouth, eyes, nostrils, wounds and other openings because the flies feed on these secretions. When they land
to feed, the tiny worm larvae sense the moisture and leave the fly to get into the moisture. When
these infected flies land near a horse’s lips, for instance, the larvae are released and swallowed by the horse, completing their life cycle.
The lucky ones find themselves near the mouth and can migrate into the mouth to be swallowed by the horse and get into the stom- ach where they grow and mature into adult worms and start the life cycle all over again, laying eggs to pass out with the horse’s manure.
Problems arise when the tiny worm larvae are deposited by the fly in other areas of the horse’s body instead of near the mouth. The larvae
may end up in a wound or on moist membranes around the eyes, the sheath or the vulva. When infected flies feed on a horse’s skin wounds, eye secretions or other moist tissue such as the sheath, worm larvae are transmitted to the skin, but this is a dead end for them because they can’t get to the horse’s stomach. They keep trying, however, and migrate around through the tissue at the spot where they were deposited by the fly. They can survive as long as they have moisture, causing local inflammation and intense itching.
The horse usually bites or rubs the area where the larvae were deposited, but this trauma to the tissues just makes the problem worse. The result is usually a raw, swollen area that oozes bloody fluid. At first it looks like an infected wound, but it doesn’t heal. The area enlarges as granulation tissue develops, like the “proud flesh” created when a wound repairs itself, protruding from the skin surface when the skin can’t properly close over a wound. There is a mass of reddish-brown, sometimes bloody, abnormal tissue that is often covered with coagulated wound drainage. There may be white or yellowish granules of firm mate- rial sprinkled through the affected tissue.
by Heather Smith Thomas
It is important to have a proper diagnosis before you try to treat this raw area. Horse owners should not assume a bloody bulg-
ing lesion is a summer sore just because of its appearance or because of their experience with summer sores. Granulation tissue from several causes can look like a summer sore but may be the result of a different type of infection or skin problem. Thus, it is crucial to contact a veteri- narian at the first sign of a potential summer sore before any treatment is administered.
Summer sores are most prevalent in warm climates. These skin lesions occur in summer after infected flies deposit the worms, but the worm larvae cannot mature and finish their life cycle in the skin. They can, however, live up to 2 years in the skin and underlying tis- sue, causing ongoing problems for the horse.
Summer sores can appear on any part of the body but most often on the lower legs (where wounds are common), at the corner of the eye, and on the sheath. After the summer sore wound is 2-3 weeks old, it becomes circular and may have areas of dead tissue within it. There is usu- ally drainage and severe itching. Often, the first clue is that a wound is growing larger instead of healing. If a wound is untreated in summer, it may develop into a summer sore and won’t heal until cold weather when the worm larvae are
less active. Thus, these skin lesions usually heal in the fall but often reappear the next spring because the larvae are still present and become active again with warmer weather.
Some horses develop a hypersensitive reaction or exaggerated immune response to the worm larvae, resulting in swelling, hives, or even asthma-like respiratory symptoms.
SUMMER SORES IN HORSES
“. . . these skin lesions usually heal in the fall but often reappear the next spring because the larvae are still present
and become active again with warmer weather.”
60 SPEEDHORSE July 2021