Oestrus ovis

Oestrus ovis, the sheep nasal bot, can be found in sheep farming areas worldwide, including in Canada. It infects primarily sheep and goats but has occasionally been reported in ibex, camel and people. The adult fly is approximately 12 mm long and a grey-brown color with small black spots on the stomach and is covered with short brown hair. The larva are about 30 mm long and pale yellow in colour with dark transverse stripes on the dorsal surface.

The life cycle of O. ovis is direct. The female adult deposits larvae into the nostril of the sheep while flying past by squirting liquid containing approximately 25 first-stage larvae into the nose. The first-stage larvae migrate through the nasal passage to the frontal sinus where they develop into third-stage larva then migrate back to the nostrils and are expelled into the environment through the host sneezing. In Canada O. ovis has an annual life cycle.  Larval development in the nasal passages and frontal sinuses takes up to 9 weeks. First and second-stage larva become dormant and remain in the nasal passages through the winter and only continue their development and migrate to the frontal sinuses in the spring. The larva pupates in the environment, taking 3-9 weeks to become an adult. Female adults only survive 2 weeks. In warmer climates the life cycle can be completed more quickly and it is possible to have 2 or 3 generations per year. 

Most infections with O. ovis are light and only mild clinical signs such as nasal discharge, sneezing and irritated nasal tissue are found. In rare, heavier infections involving larvae dying within the frontal sinus secondary bacterial infections can result, and these can extend into the brain causing neurological signs, including ataxia or circling. Larvae and the mucosal changes they induce can result in impaired breathing, hypersensitivity reactions and interstitial pneumonia due to chronic antigenic stimulation. The persistent attacks of the adult flies cause sheep to gather together and press their noses into the fleece of other sheep resulting in reduced food intake and increased stress which can cause weight loss.

Diagnosis can be made through the associated clinical signs, although other conditions have similar symptoms. Occasionally larvae can be seen on the ground after a severe sneezing attack, and this can be diagnostic, but often diagnosis is made only at necropsy. Treatment of light infestations is typically not economic. Heavy infestations can be treated with oral or injectable ivermectin.

Human infection with O. ovis is rare. In these cases the larvae are usually deposited near the eyes or lips, and  can result in catarrhal conjunctivitis or stomatitis. Larvae do not fully develop in human hosts.