Cutaneous Habronemiasis in Animals

Cutaneous habronemiasis is a skin disease of Equidae caused in part by the larvae of the spirurid stomach worms in the genera Draschia and Habronema (see Gastrointestinal Parasites of Horses). When larvae emerge from flies feeding on preexisting wounds or on moisture of the genitalia or eyes, they migrate into and irritate the tissue, causing a granulomatous reaction. The lesion becomes chronic, and healing is protracted.

Diagnosis is based on finding nonhealing, ulcerated skin granulomas that contain yellow, calcified material the size of rice grains (see habronemiasis image). Lesions are most commonly located at the oral commissures, the medial canthus of the eye, and the penis (often at the tip of the urethra). Larvae, recognized by spiny knobs on their tails, can sometimes be demonstrated in scrapings of the lesions.

Habronemiasis, oral commissure, horse

Image

Courtesy of Dr. Stephen D. White.

Many different treatments for habronemiasis have been tried—most with poor results. Insect repellents can be of benefit, and organophosphates applied topically to the abraded surface can kill larvae. Surgical removal or cauterization of excessive granulation tissue could be necessary.

Treatment with ivermectin (200 mcg/kg, PO, once) has been effective against habronemiasis, and although lesions can be temporarily exacerbated (presumably in reaction to the dying larvae), spontaneous healing can be expected. Moxidectin (400 mcg/kg, PO, once) also appears to be active against gastric Habronema spp.

Control of fly hosts of Habronema spp and regular collection and stacking of manure, together with anthelmintic treatment, can decrease the incidence of habronemiasis. In addition, use of fly masks and attention to horses with excessive periocular moisture can decrease fly feeding in the periocular tissues.