Stephanofilaria stilesi is a small, filarial parasite that causes a circumscribed dermatitis along the ventral midline of cattle. It has been reported throughout the USA but is more common in the west and southwest. Adult worms are 3–6 mm long and usually are found in the dermis, just beneath the epidermal layer. Microfilariae are 50 mcm long and are enclosed in a spherical, semirigid vitelline membrane. The intermediate host for S stilesi is the female horn fly, Haematobia irritans. Horn flies feeding on the lesion ingest microfilariae that develop to third-stage infective larvae in 2–3 weeks. The infective larvae are introduced into the skin as the horn fly feeds.
The dermatitis develops along the ventral midline, usually between the brisket and navel. With repeated exposure, the lesion spreads and often involves the skin posterior to the navel. Active lesions are covered with blood or serous exudate, whereas chronic lesions are smooth, dry, and devoid of hair. Hyperkeratosis and parakeratosis develop in the epidermis of the parasitized area.
Deep skin scrapings are macerated in isotonic saline solution and examined microscopically for adults or microfilariae. The microfilariae must be differentiated from microfilariae of Onchocerca lienalis, O gutturosa, and Setaria spp, which are much larger (200–250 mcm), and Pelodera strongyloides, a small free-living nematode occasionally responsible for a moist, superficial dermatitis. The rhabditiform esophagus of P strongyloides is not found in filarial nematodes.
No approved treatment is available for S stilesi, but topically applied organophosphates (trichlorfon 6%–10%, daily or on alternate days for 7 days) have proved effective against other species of Stephanofilaria. Ivermectin is reported to be effective against microfilariae of S zaheeri.