Nasal Bots (Oestrus ovis)

Causative Agent/Disease Transmission: The adult nasal bot fly is a wide-bodied, mottled yellowish to gray-brown, hairy insect that grows up to 5/8ths of an inch long. From spring until fall, the female bot fly deposits her larvae around the sheep or goat's nostrils. These larvae (bots) migrate up the sheep or goat's nasal passage, develop into a second stage, then invade the sinuses. In the sinuses they mature for pupation. Larvae can grow up to 1.25 inches long, and feed on mucosal tissue in the nasal passages and the frontal sinuses of the host animal. As the larvae mature, they become cream-colored, then darken and develop a dark or black band on the back surface of each segment. Once fully grown (about 2 to 10 weeks), the larvae exit the nostrils (through sneezing on the host's part) to pupate in debris or soil. Some larvae will overwinter in the pupal stage and hatch in the spring. Occasionally, the larvae will grow too large to exit the host's sinuses, causing severe irritation. The female fly will sometimes deposit her larvae near the eyes of humans. This can result in painful conjunctival ophthalmomyiasis.

Clinical Signs: Infestation of the nasal bot (Oestrus ovis) in sheep and goats causes great irritation to the host animal. The irritation is usually milder in goats than in sheep. Until the larvae begin to grow, there are no clinical signs unless the sheep or goat develops a secondary bacterial infection and/or sinusitis (inflammation of the sinus). Sometimes the larvae are discovered during dehorning. Extensive nasal discharge, without the signs of pneumonia, is also a good indication of nasal bot infestation. Once the grubs gain a hold in the host, the irritation can cause the animal to shake its head, rub its nose, sneeze violently, and stomp its feet. Airflow may decrease and the animal may develop rhinitis (inflammation of the membranes in the nose). If the host is affected by generations of bots, it may develop a hypersensitivity and produce excessive clinical signs. Sometimes the sheep or goat will develop secondary interstitial pneumonia. Rarely, a bot will migrate into the sheep or goat's brain (false gid), causing ataxia (incoordination). Death has even occurred in severe cases.

Treatment: If the infestation is minor, many sheep and goat owners ignore the condition. However, when it interferes with feed consumption or the general health of the animal, treatment is necessary. Ivermectin is highly effective against all stages of nose bots. The oral solutions (Ivomec sheep drench) work well. It can also be administered subcutaneously at the standard dose of 0.9 mg/10 lbs, except to animals producing milk for human consumption.

Prevention: Treatment of affected animals with Ivermectin in the late summer can prevent the buildup of heavy infestations.

* Additional details about nasal bots can be found on page B620.