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 the larvae emerge from flies feeding on preexisting wounds or on moisture of the genitalia or eyes, they migrate into and irritate the tissue, which causes a granulomatous reaction. The lesion becomes chronic, and healing is protracted. Diagnosis is based on finding nonhealing, reddish brown, greasy skin granulomas that contain yellow, calcified material the size of rice grains. Larvae, recognized by spiny knobs on their tails, can sometimes be demonstrated in scrapings of the lesions.
Many different treatments have been tried, most with poor results. Symptomatic treatment, including use of insect repellents, may be of benefit, and organophosphates applied topically to the abraded surface may kill the larvae. Surgical removal or cauterization of the excessive granulation tissue may be necessary. Treatment with ivermectin (200 mcg/kg) has been effective, and although there may be temporary exacerbation of the lesions (presumably in reaction to the dying larvae), spontaneous healing may be expected. Moxidectin at 400 mcg/kg also appears to be active against Habronema spp in the stomach. Control of the fly hosts and regular collection and stacking of manure, together with anthelmintic therapy, may reduce the incidence.