Stephanofilaria stilesi

Nematodes of the genus Stephanofilaria infect cattle in many parts of the world, including Canada.  The adult parasites live in the skin, especially around the ventral mid line, where the females produce first-stage larvae that remain in the skin.  The life cycle is indirect and requires a horn fly (Haematobia irritans) intermediate host in which infective third-stage larvae develop.  Infection of the cattle occurs when these are deposited on the skin.  Pathology associated with Stephanofilaria is restricted to the skin, usually along the ventral mid line, and is characterized by hair loss, exudation and the development of granulomatous lesions.

Phylum: Nematoda
Class: Rhabditea
Subclass: Rhabditia
Order: Spirurida
Suborder: Spirurina
Superfamily: Thelazoidea
Family: Thelaziidae

The closest relatives of S. stilesi of importance in veterinary medicine are Setaria, adults of which live in the peritoneal cavity of a range of hosts, and Dipetalonema, the various species of which are found in a variety of locations in several hosts.

Adult S. stilesi are approximately 5 mm in length and are usually identified on the basis of their location in the host.

Stephanofilaria stilesi occurs in cattle in North America, including Canada, and in Russia and surrounding areas. Several other species of Stephanofilaria are found in the skin of cattle and buffalo, particularly in Asia.

Adult S. stilesi live in cyst-like structures at the bases of hair follicles, especially in the ventral mid-line, but also on the flanks, udder, teats, face and neck. Females produce first-stage larvae, which stay in the skin. The life cycle continues when these larvae are ingested by a horn fly (Haematobia irritans) during feeding. Infective third-stage larvae develop in the flies and can infect the cattle during subsequent feeding.

Life Cycle: Stephanofilaria stilesi

In other than very light infections, S. stilesi (perhaps together with the flies) may cause papules, crusts, ulcers and alopecia, especially in the ventral mid-line. There may also be hyperkeratosis with thickening of the skin. Pruritus is variable. Clinical sign are non-seasonal, but they may be worse in the summer.

Other than history and clinical signs, microscopic examination of a small skin biopsy that is macerated and incubated in saline at 37C for 24 hours should allow detection of eosinophils, microfilariae and perhaps also adults.

Ivermectin (VARIOUS) is believed to be affective against S. stilesi, and doramectin (DECTOMAX) and moxidectin (CYDECTIN) may be also.  These are extralabel uses for these products.

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Stephanofilaria stilesi is not known to be zoonotic.