The sheep nose bot fly, Oestrus ovis, is a cosmopolitan parasite that, in its larval stages, inhabits the nasal passages and sinuses of sheep and goats. Its geographic distribution is worldwide.
The adult fly is grayish brown and ~12 mm long. The female deposits larvae in and about the nostrils of sheep without alighting. These small, clear-white larvae (initially <2 mm long) migrate into the nasal cavity; many spend at least some time in the paranasal sinuses. The larval period, which is usually shortest in young animals, lasts 1–10 mo. When mature, the larvae leave the nasal passages, drop to the ground, burrow down a few inches, and pupate. The pupal period lasts 3–9 wk, depending on the environmental conditions, after which the fly emerges from the pupal case and pushes its way to the surface. Mating soon occurs, and the female begins to deposit larvae.
Once the larvae begin to move about in the nasal passages, a profuse discharge occurs, at first clear and mucoid, but later mucopurulent and frequently tinged with fine streaks of blood emanating from minute hemorrhages produced by the hooks and spines of the larvae. Paroxysms of sneezing accompany migrations of the larger larvae. Larvae present in the sinuses are sometimes unable to escape; they die and may gradually become calcified or lead to a septic sinusitis. However, the principal effects are annoyance, with a resulting reduction in grazing time, and loss of condition. Usually only 4–15 larvae are found, although many more may be present.
To avoid the fly’s attempts at larval deposition, a sheep may run from place to place, keeping its nose close to the ground, sneeze and stamp its feet, or shake its head. Commonly, especially during the warmer hours of the day when the flies are most active, small groups of sheep gather and face the center of a circle, heads down and close together.