Stephanofilariasis in Cattle

Stephanofilaria stilesi is a small, filarial parasite that causes circumscribed dermatitis along the ventral midline of cattle (see cutaneous stephanofilariasis image). The parasite has been reported throughout the US but is more common in the West and Southwest.

Cutaneous stephanofilariasis, cow

Image

Courtesy of Dr. Sameeh M. Abutarbush.

S stilesi adults 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 into third-stage infective larvae in 2–3 weeks. Infective larvae are introduced into skin as the horn fly feeds.

Dermatitis in cases of stephanofilariasis develops along the ventral midline, usually between the sternum and navel. With repeated exposure, the lesion spreads and often involves the skin caudal to the navel. Active lesions are covered with blood or serous exudate; chronic lesions are smooth, dry, and alopecic. Hyperkeratosis and parakeratosis develop in the epidermis of the parasitized area.

Diagnosis of stephanofilariasis involves macerating tissue from deep skin scrapings in isotonic saline solution and examining them microscopically for adults or microfilariae. S stilesi microfilariae must be differentiated from microfilariae of Onchocerca lienalis, Onchocerca gutturosa, and Setaria spp, which are much larger (200–250 mcm), and those of Rhabditis (formerly Pelodera) strongyloides, a small free-living nematode occasionally responsible for moist, superficial dermatitis. The rhabditiform esophagus of R strongyloides is not found in filarial nematodes.

No approved treatment is available for stephanofilariasis; however, topically applied organophosphates (trichlorfon 6–10%, every 24–48 hours for 7 days) are effective against other Stephanofilaria spp. Ivermectin (200 mcg/kg, SC, once) is effective against microfilariae of Stephanofilaria zaheeri.