Nasal bots are the maggots or larvae of the sheep nasal bot fly, Oestrus ovis, which is present throughout the world wherever there are sheep and goats. It was introduced into Australia in sheep in the early 1900s and was first recorded in Western Australia in 1919. They may now be less common in areas where the macrocyclic lactone (ML) anthelmintics, such as ivermectin, have been frequently used.
Nasal bots usually cause little harm to host animals and owners are rarely aware of the infestations unless expelled bots are seen. Individual sheep may show signs of nasal irritation or (rarely) breathing difficulties and flocks may show signs of disturbance due to bot fly attacks.
Life cycle of the nasal bot fly
Fly activity is seasonal and generally peaks in spring and late summer when temperatures exceed 20 degrees Celsius, but the pattern of fly activity varies between regions.
The female fly has a life span of approximately two days in summer and four weeks in cold weather. In cold climates, such as Russia or Siberia, there are only one or two generations each year, but in hotter places such as much of Australia, there may be five or six generations of flies.
The sheep nasal bot fly deposits larvae, not eggs, on its host, unlike the related bot fly of horses (Gasterophilus species) which attaches eggs to horse hair. Once larvae have been deposited at the sheep’s nostrils, they move and grow within the nasal cavity and the frontal sinuses. They develop through three larval stages and when mature (about 3cm long) the larvae are sneezed out. This is the only stage likely to be observed, typically when sheep are in a shed or yards.
The rate of development of larvae within the sheep’s head is highly variable and can take as long as ten months or as little as six weeks. Expelled bots form pupae in the soil and flies emerge from these after a few weeks. The duration of each stage of the life cycle is highly variable, mainly dependent on temperature.
Effects of nasal bots
Most infestations pass unnoticed, but sheep may show disturbed behaviour when bot flies are present. This includes snorting, stamping the front feet, running in short bursts and burying their noses into the fleeces of other sheep. Sheep may congregate in shaded places where the flies are less active. Infested sheep may have a discharge from the nostrils, difficulties in breathing and may sneeze or cough.
The economic impact of nasal bots is debatable. In Australia they are generally not considered to be of economic importance. In some countries nasal bots are believed to cause reduced body and fleece growth, mostly due to the disturbance of flocks. They have also been suspected of suppressing normal immunity, possibly predisposing animals to secondary bacterial infections including chronic sinusitis. Nasal bots do not affect the ability of rams to find, by smell, ewes in oestrus.
Bot flies occasionally target humans, dogs and cats. In these species bots may be found in the throat or eye, where they can cause severe irritation. Sometimes (rarely) they are found in the nasal passages where they can cause breathing difficulty. Bots do not develop to maturity in species other than sheep and goats.
There is no commercially available test which will identify infested sheep. The flies themselves are a little smaller than the common blowfly but are rarely seen. The peculiar behaviour of sheep when the bot flies are active may indicate that some sheep in the flock are likely to become infested. A nasal discharge, with or without coughing and sneezing, would arouse suspicion but is not diagnostic for nasal bot infestation.
Sometimes a diagnosis is made incidentally when, while shearing or drenching a sheep, a bot is expelled from the sheep’s nostrils onto the handler. Similarly, expelled bots are sometimes found in water or feed troughs. Bots may be more common in horned breeds and sometimes when sheep are dehorned bots may emerge from the horn core.
The signs in sheep could resemble some other diseases that affect sheep behaviour. Possible alternative diagnoses should always be considered and excluded before making a diagnosis of nasal bot infestation. If in doubt, seek veterinary advice.
Goats rarely show any signs of infestation.
Treatment with an effective product can be used as an indirect method of diagnosis. If the signs disappear following treatment, it is reasonable to make a retrospective diagnosis of nasal bot infestation.
Treatment with an anthelminthic solely for nasal bots is rarely warranted unless it is believed the health of animals is being affected by these parasites. Three macrocyclic lactone (ML) drenches (ivermectin, abamectin, moxidectin) and closantel are registered for treatment against nasal bot in sheep, whereas only abamectin is registered for use in goats.
However, treatment, other than where significant ill effects from nasal bots were suspected, would be contrary to current recommended worm control practices and could contribute to the already high level of worm resistance to the ML chemicals.