The female sheep nose bot fly, Oestrus ovis, deposits living larvae (maggots) in or around the nostrils of the sheep during the spring and summer months. When flies are attacking sheep, the animals bunch together and keep their noses to the ground in an effort to avoid the strikes. The larvae migrate through the nostrils into the head sinuses, bronchi, or cavities in the horns or bones of the jaw or nose, where they feed on the internal secretions. Migration of the larvae irritates the nasal membranes and is often followed by secondary infections. Infested sheep shake their heads, stamp their feet, or hold their noses to the ground. Sneezing and labored breathing can be common among infested sheep. Blood flecks in the nasal discharge, and sheep banging their heads against feed bunks, fences, or the ground indicate the presence of nose bots. Severely infested, older, or weak sheep may die as a result of the bots. The larvae develop during the winter; the following spring they are sneezed out or drop out to the ground, where they pupate and become adults.
A systemic oral drench treatment is currently available. Frequent change of pastures when bot flies are active may be of some help in reducing infestations, since the flies are short-lived and not capable of long flights.