Disease in sheep that farmers miss to spot

Wanjiku Waimeri [Kibata Kihu, Standard]

A few weeks ago, a friend invited me to his ranch in Isinya where he keeps 100 sheep. He supplies high-end hotels in Nairobi and he seems to be doing well. We walked through his ranch and enjoyed the scene of sheep grazing gracefully until I noticed some disturbance.

Disturbed behaviour

Though the visit was purely social, the vet in me was concerned. I was convinced that his sheep were not fine though my friend had not noticed. For the two hours I was on the ranch I noticed that the sheep exhibited disturbed behaviour. Some snorted, others stamped their front feet, while still others ran in short bursts. I saw two burying their noses into the fleeces of other sheep. On a closer look, I noticed a discharge from the nostrils of quite a number that also sneezed or coughed. All was not well.

My Diagnosis – Nasal Bots

Under the shade of an acacia tree; I told my friend all was not well with his sheep. 

According to my friend, all the clinical signs I was talking about are normal signs – nothing to worry about. Interestingly, many farmers assume that it is normal for sheep to have nasal discharge, sneeze or cough, and bursts of short runs.

Diagnosis of nasal bots is based on the clinical signs but still, these signs are shared with other conditions. Some vets have had the chance to see freshly coughed out larvae on pasture or onto themselves during drenching.

Due to difficulty in diagnosis of this condition, a mass deworming exercise that clears coughing, nasal discharge, and disturbance is taken as a diagnosis. That is what we did on my friend’s ranch. Within a week, all the clinical signs had disappeared. Most of these larvae can also be identified at slaughter in the nasal sinuses.

What are nasal bots

The adult nasal bot fly is grayish brown and about 12mm long. Nasal bots are larval stages of the sheep nasal bot fly (Oestrus ovis). These larval stages infest the nostrils of sheep and goats. Goats will rarely show any clinical signs. In most cases, bot flies apart from the disturbance do not cause much harm and pass unnoticed.

The adult female nasal bot fly will deposit larvae (whitish and about 2mm in length) at the sheep’s nostril from where they can move into the inner parts of the nostril and even further into the sinuses. The larvae upon maturity will be sneezed out. The time from deposit to maturity of the larvae can be as little as six weeks or as long as 10 weeks. Once sneezed out the larvae turn into pupae, burrow into the soil and emerge as a fly after a few weeks.


Injectable anthelmintics like ivermectin control nasal bots. Isolation of affected sheep will reduce the spread of the parasite. While clinical signs may go unnoticed, the economic effects of nasal bot flies may include slow growth rate and poor fleece. Infestation with nasal bot flies will also predispose the sheep to other secondary bacterial infections that result in sinusitis. On a rare occasion, bot flies may target dogs and cats, or humans. Here, nasal bots may be found in other organs like the eye or throat.

[Dr Othieno is a veterinary surgeon and the head of communications at the Food and Agriculture Organization of the United Nations (FAO) Kenya. The views expressed here are not necessarily those of FAO]