Kvet Forum
Well-known member
Infestation of sheep and goats with larvae of the nasal bot fly (Oestrus ovis) has a serious effect on the productivity and welfare of both sheep and goats. Adult fly activity induces stress responses and significant behavioral change while larval infestation induces moderate to severe pathology that reduces productivity.
Similar flies are known to affect horses, donkeys, and mules (Rhinoestrus spp.) in the Mediterranean region and camels (Cephenemyia titillator) in Africa and Australia. Wild ungulates are also affected by nasal bots (e.g. Cephenemyia spp.).
Aetiology
The sheep nose bot affects sheep and goats across most regions. The larvae inhabit the nasal passages and sinuses, eventually being expelled through the nares.
Goats are less dramatically affected than sheep. The slightly dorsoventrally flattened, segmented larvae are light cream in color, but as they reach maturity dark bands appear on each segment.
Life Cycle & Epidemiology
The adult fly is stout, mottled gray in color, and about 1cm long. Its mouthparts are rudimentary, and it does not feed. Adult flies mate, and the females begin larviposition activities approximately 2 to 3 weeks later.
Adult flies attempting to deposit larvae on the nares annoy the sheep and cause them to bunch or seek shelter. Stamping of the feet and shaking of the head are common. Sheep may bunch together and press their heads into the fleece of others.
Larval development takes place in in the dorsal turbinates and frontal sinuses. The period of development can vary 3 weeks to several months, after which they migrate to the nostrils. Larvae feed on the mucosal secretions and cells eroded from the mucosal epithelium.
The larvae are thick, yellowish-white in color, and when mature there is a dark dorsal band on each segment. The ventral surface has rows of small spines on each segment.
Mature larvae exit the host, usually during a bout of sneezing, and actively burrow beneath the upper layers of soil and ground litter. Pupation occurs at these locations and development of the adults requires 4 to 5 weeks but this may take longer at low temperatures.
O. ovis are an important zoonosis because the females may larviposit in the eye, in the nose, or on the lips of humans. In some countries ophthalmomyiasis or infection of the upper respiratory tract is a common occurrence.
Pathogenesis
The stress of the larviposition attacks can be significant with reduced grazing time and overheating as animals bunch together. Herdsmen find the animals are more nervous and difficult to handle during the fly activity periods.
Larvae induce a gradually increasing rhinitis and sinusitis as the infestation persists. There is destruction of the nasal epithelial tissues and loss of the ciliary layer. These changes are a result of both mechanical activity of the larval spines and mouthhooks and the effect of proteolytic enzymes excreted or secreted.
Varying degrees of mucous discharge is observed in the later stages of the infestation. This can lead to occlusion of the nostrils by adherent straw and dust as the animals feed.
Clinical Findings
Early in the infestation there is a distinct rhinitis accompanied by a mucoid to mucopurulent discharge.
Later as larvae mature a sinusitis is evident. Presence of mature larvae in nasal cavities may induce excessive sneezing, which assists larval exit.
Activity of the larvae in the nasal cavities, and the changes they induce lead to an increase in the incidence of secondary pathology. The number and severity of lung abcess are more significant in nose bot-infested sheep. The presence of bots is also correlated with increased carcinomas and may lead to reactivation of latent “orf” symptoms.
Diagnosis
The behavioral changes during fly activity, including bunching and burying of noses in neighbors’ fleeces, is a reliable indicator of fly attack. Nasal discharge and excessive sneezing are highly suggestive but not definitive.
Treatment
Closantel 5 mg/kg and ivermectin 0.2 mg/kg, in addition to other macrocyclic lactones, are effective.
Control
Treatment should be applied once or twice a year. This is not absolutely necessary but it will increase both endurance and well-being of the animals. Population control of the flies is probably not likely.