The sheep nasal botfly, Oestrus ovis, is a widespread parasite, but of almost no significance as a cause of disease.
The female fly lays batches of about 50 newly hatched larvae in or on the nostrils of the sheep. The larvae then crawl onto the mucous membrane of the nasal passage, where they will remain for at least two weeks.
Development of the first instar larvae 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 botfly 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 (membranes covering the brain and spinal cord).
Treatment is rarely warranted, but ivermectin, abamectin, moxidectin and closantel are registered for treatment against nasal bot.