Nasal bots are often thought of as a type of worm that occurs in the nose, and causing symptoms such as continuous sneezing and a runny nasal discharge.

Lees dit in Afrikaans

In reality, however, it is the larvae of the nasal bot fly (Oestrus ovis) that shed their skin in the nasal cavities, nasal passages and head cavities during maturation, so as to complete its lifecycle. The fly is found across the world and is a source of irritation in sheep, goats and some game species.

In 2016 even a dog was diagnosed with nasal bots in Italy. Cases have also been recorded of ophthalmomyiasis, conjunctivitis and eye catarrh in people, because of the presence of the fly larvae in the eye.

Initially, people were only affected if they lived and worked on farms, but recent cases have also been observed in city dwellers. Once the larvae have been removed, the symptoms clear up quickly.

In South Africa, nasal bots are found in 73% or more of sheep and goat herds. Infected sheep usually have an average of 15 bots, while goats have around four. However, there have been cases where up to 105 larvae were found in a single sheep. It would seem that goats tend to avoid the nasal bot fly more successfully, meaning that fewer larvae are deposited in their nasal cavities.

Impact on production

The impact of nasal bot infestation on production is often ignored, yet studies have shown that the presence of ten larvae over a period of six months can lead to weight loss of 2kg per sheep.

Mature flies cause irritation in sheep and they tend to keep noses against the soil or bury it in the wool of other sheep. They also stomp their feet, all to ward off the flies. This irritation causes sheep to ingest less feed, which in turn leads to a deterioration in their condition. Affected sheep are also more susceptible to secondary bacterial infections of the upper respiratory tract.

As its sense of smell becomes affected, rams also tend to lose their ability to identify ewes in heat. This has a direct influence on the duration of the mating season and the pregnancy percentage. In turn, the weakened sense of smell among ewes can result in more bottle-fed lambs.

Runny nose with discharge

Animals infested by nasal bot usually exhibit a slimy, purulent nasal discharge. The mucous membranes of the nose are swollen, and erosion lesions are visible.

The presence of larvae causes pressure on the mucous membranes and the larvae secrete enzymes, enabling them to move upwards in the nasal passages.

There is proof that sheep develop hypersensitivity when larvae are present. This leads to an excessive immune response that further damages their tissue. Sometimes the larvae migrate through the nasal passage to the brain, lesions occur in the cerebral cortex and neurons are broken down. Neurological signs are noticed and are typical of the signs seen in so-called gid or turning disease.

Hatching and infestation

Nasal bots usually hatch in October, after which they mate. Each female fly can produce 500 larvae that are located in the nasal cavities of sheep and goats. The larvae migrate through the nasal passages, shed their skin twice and then migrate to the frontal sinuses.

The mature larvae move through the nasal passages to the outside and are usually sneezed out by the sheep onto the ground, where the flies hatch again after three to six weeks in summer.

Three or four generations can complete their lifecycles by the end of summer. Infestations usually occur from October and reach a peak in May. Larvae can overwinter in sheep and develop further in spring.

Tips for treatment

To control spring and summer infestations, it is recommended that a treatment is administered at the end of October and again in March. Ewes need to be treated before lambing and rams before the mating season, so as to ensure a strong sense of smell. Lambs must be treated to prevent weight loss.

Injectable products containing ivermectin (e.g. Ecomectin 1% G2275) are effective in killing nasal bot larvae. Oral treatment of sheep and goats with products containing rafoxanide (e.g. Tramisol Plus G1830), closantel, ivermectin (e.g. Ecomectin Sheep Drench G2630) or moxidectin (e.g. Moxziben G4040) can also be administered.

It is important to keep in mind that an improvement will only be seen three days after treatment. It can take even longer, as the dead larvae need to be secreted and the inflammation in the mucous membranes must subside.

In summer, nasal bots can be fended off by treating sheep with a deltamethrin-containing dip (e.g. Clout G1447, Decaspot G3433, Decatix 3 G1348, or Triatix plus G3434) on their heads. – Dr Hanré Bredenkamp Ferreira, Afrivet

For source references or more information, contact an Afrivet representative or download the Afrivet app. Alternatively, phone 012 817 9060 or 0860 VEEARTS, or