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. It also has been seen in bighorn sheep (Ovis canadensis) and European ibex (Capra ibex) and in uncharacteristic hosts such as people and dogs. 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. As the larvae (bots) mature, they become cream-colored, then darken, and finally show a dark or black band on the dorsal surface of each segment. 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. Continuing activity of the larvae, particularly if they are numerous, causes a thickening of the nasal mucosa that, together with the mucopurulent discharge, impairs respiration. 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. The purulent inflammation produced in the sinuses occasionally may spread to the brain with fatal results. 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.
Ivermectin at 200 μg/kg, PO or SC, is highly effective against all stages of the larvae. Other drugs reported to be effective include eprinomectin (0.5 mg/kg, pour-on) applied after shearing and doramectin (200 μg/kg, IM). In the USA, these drugs or recommended routes of administration may constitute extra-label use, requiring a valid veterinary-client relationship and an appropriate withdrawal time for slaughter purposes.
Last full review/revision March 2012 by Michelle Kopcha, DVM, MS