Merck Manual

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Professional Version

Sheep Nasal Bot Myiasis


Evelyn MacKay

, DVM, Texas A&M University

Reviewed/Revised Sep 2022 | Modified Oct 2022
Topic Resources

Larvae of the sheep nasal botfly (Oestrus ovis) develop in the nasal sinuses of sheep. Signs of infestation include nasal discharge and sneezing. Rarely, more serious disease develops. Antiparasitic treatment with ivermectin is effective.

The sheep nasal bot, the larval stage of the botfly Oestrus ovis, is a parasite that inhabits the nasal passages and sinuses of sheep and goats. O ovis has a worldwide geographic distribution.

The adult fly is grayish brown and ~12 mm long. Gravid females deposit larvae in and around the nostrils of sheep without landing. 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 months. 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 weeks, depending on the environmental conditions, after which the fly emerges from the pupal case and pushes its way to the surface. The adult flies mate soon after emergence, with the life cycle continuing following larviposition by gravid females.

Clinical Findings of Sheep Nasal Botfly Myiasis

After the O oestrus larvae begin to move around in the nasal passages, a profuse discharge develops. At first the discharge is clear and mucoid; later it is mucopurulent and frequently tinged with fine streaks of blood from minute hemorrhages produced by the hooks and spines of the larvae. Paroxysms of sneezing accompany migrations of the larger larvae. Larvae in the sinuses are sometimes unable to escape; they die and may gradually become calcified or lead to septic sinusitis. However, the principal effects are irritation to the animal, with a resulting decrease 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, keep 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.

Treatment of Sheep Nasal Botfly Myiasis

Ivermectin (0.2 mg/kg, PO or SC, once; repeated as necessary when reexposure to parasites occurs) is highly effective against all stages of the larvae in nasal botfly infestations.

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