from the Vet Corner Archives

Part of Horse Previews Magazine website. Posted on 08/05/2001; 2:00:00PM.

Veterinary Corner 08/01: Summer Sores (Habronemiasis)

by Frosty Franklin, DVM
Edgecliff Equine Hospital
S. 1322 Park Road, Spokane, WA 99212 * 509/924-6069

Summer sores occur when the ambient temperature remains above 70 degrees and the face and stable fly vectors are prevalent. Usually there is a history of a wound, however, a lesion may develop at mucous membranes of body openings like the eye or the urethra.

Usually the lesions appear as slow healing wounds that develop exuberant granulation tissue (proud flesh). Itching may be mild to severe. Many times the granular lesions are caseated, calcified, and have very small yellow calcified granules that are highly suggestive of Habronema.

Habronema is the equine stomach worm. The adult worms live in the nodular lesions of the stomach wall of the horse and produce larvae that are passed in the manure. Larvae of the house fly, face fly, and stable fly ingest the stomach worm larvae. When the fly pupates (matures) the stomach worm larvae mature to their infective stage and the fly while feeding deposits the infective stomach worm larvae around the horse's mouth, the larvae are swallowed and the life cycle is complete. The infective larvae are also deposited in skin wounds and moist places such as the eye, sheath, and penis as the fly is feeding. In horses that are susceptible, indicating a hypersensitivity or immunodeficiency, because pasture mates will remain unaffected, an itchy ulcerated sore or marked mucopurulent discharge from the eye results.

The time of year, the appearance of the wound and the small yellow sulphur-like granules are characteristic. However, these lesions can be confused with sarcoids, fibrogranulomas caused by fungal or bacterial infections, or other skin cancers. Biopsies can be taken for histologic examination.

Many treatments have been used in attending summer sores. Removing the large ulcerated lesions surgically usually produces a better response than medical therapy alone. Formulations of trichlorfon, dexamethasone, and DMSO have worked well for me in the past. Ocular habronemiasis needs careful evaluation to rule out corneal ulcers before treating the eyes with cortisone/antibiotic ophthalamic ointments. Deworming with Ivermectin has been effective and may need to be repeated in animals with severe lesions. Deworming all the horses on the premises with Ivermectin makes sense to me. One would have to assume that there are horses on the premises that have adult habronema in their stomachs that are acting as the source of the infective larvae. Fly control on the premises is a must as well. Fly predators, fly spray systems and topical treatment of the individual horse, and feed supplements containing organophosphates may be used in preventing fly larvae from surviving in the manure are all effective with proper management.

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