The sheep nasal botfly Oestrus ovis is a widespread parasite. The female fly lays batches of about 50 newly hatched larvae in or on the nostrils of the sheep.

When they are deposited in the nostril of a sheep, the larva crawls onto the mucous membrane of the nasal passage, where it will remain for at least two weeks.

Development of the 1^st instar larva may be delayed for one to nine months. This plays a role in the over wintering cycle. The pupal stage lasts one to two months depending upon temperature.

Sneezing and a mucopurulent nasal discharge (snotty nose) may be a result of the bot fly larvae. The main effects for the host are persistent annoyance and associated debility. Very rarely secondary bacterial infection spreads from the olfactory mucosa to the meninges.

Ivermectin, abamectin, moxidectin and closantel are registered for use against nasal bot.