Development of Stephanofilaria stilesi in the Horn Fly

Abstract

Stephanofilaria stilesi Chitwood, 1934, uses the horn fly, Haematobia irritans (L.), as an intermediate host. Filarioid larvae found in 455 of 3,707 female flies and six of 2,234 male flies collected from cattle having stephanofilariasis were morphologically identical to the immature forms of S. stilesi recovered from stephanofilarial lesions. When laboratory-reared horn flies were allowed to feed on stephanofilarial lesions of naturally infected cattle, 13 of 183 females and one of 201 males became infected with filarioid larvae morphologically identical to those found in naturally infected flies. Four Holstein calves exposed to naturally infected horn flies developed lesions typical of stephanofilariasis, while three exposed to uninfected flies did not. When laboratory-reared horn flies were exposed to one of the infected calves, nine of 62 females became infected with filarioid larvae identical to those found in naturally and experimentally infected horn flies; none of 122 males developed such larvae. At necropsy, adult S. stilesi were found in the skin of each infected calf, and none were found in the controls. Stephanofilaria stilesi Chitwood, 1934, is a small filarioid nematode responsible for a disease of cattle known as stephanofilariasis, a circumscribed dermatitis along the midventral line of the body. This disease occurs throughout the continental United States (Lucker, 1956), but is especially prevalent among beef cattle on western and southwestern rangelands. When S. stilesi becomes established in western beef herds, 80 to 90% of the mature animals may be infected (Maddy, 1955). S. stilesi was once thought to be restricted to the North American continent, but reports of its occurrence in Hawaii (Alicata, 1947) and the Soviet Union (Gnedina, 1950) indicate that it may occur throughout the world. The horn fly, Haematobia irritans (L.), is a prevalent and relatively host-specific pest of cattle that is found commonly on stephanofilarial lesions. Because of this relation, Dikmans (1934) suggested it was the intermediate host. Observations of a preliminary nature, initiated independently in the Soviet Union and the United States, have strengthened Dikmans' suggestion that horn files are involved. Ivashkin, Khromova, and Shmytova (1963) Received for publication 24 June 1965. This work was carried out in cooperation with the New Mexico Agricultural Experiment Station and is a contribution to the Western Regional Project W-35-Nematode Parasites of Ruminants. *Present address: Department of Pathology, Colorado State University, Fort Collins, Colorado. reported that larvae found in a European species of horn fly, Lyperosia titillans (Bezzi), were morphologically identical to infective stages of S. stilesi recovered from the skin of cattle, and Hibler (1964) recorded similar findings for the American species of horn fly, H. irritans. In investigations on other species, Rahman (1957) concluded that transmission of Stephanofilaria assamensis Pande, 1936, was mechanical and contaminative, but Srivastava and Dutt (1963) found that seven of 787 Musca conducens Walker, collected from cattle with Humpsore, were infected with filarioid larvae similar to the immature forms of S. assamensis recovered from lesions. MATERIALS AND METHODS The incidence of stephanofilariasis, together with information regarding the clinical appearance and history of the condition, was obtained by examination of cattle in abattoirs, on range, in pasture, and in drylots in south central New Mexico. Lesions, and healthy tissues adjacent to the lesions, were examined by maceration, digestion, and histological section to determine number and location of adults and microfilariae of S. stilesi. Ten ml of blood from the jugular vein was added to 40 ml of 0.85% physiological saline containing 2% sodium citrate and 2% saponin (w/v). After hemolysis occurred, the blood was centrifuged and the sediment examined for microfilariae. All Diptera collected from infected cattle were examined to determine their role in the dissemination of S. stilesi. Collections were made at weekly intervals throughout the fly season over a period of 2 years. First-generation horn flies were obtained by placing eggs on fresh, insect-free manure. The