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Bluetongue (BTV)

Bluetongue is an insect-borne, viral disease primarily of sheep, occasionally goats and deer and, very rarely, cattle. The disease is non-contagious and is only transmitted by insect vectors. The disease is caused by a virus belonging to the family Reoviridae.

Species affected

Primarily a disease of sheep but other species such as goats, cattle, buffaloes, camels, antelopes and deer can be infected. Humans are not affected.

Distribution

The virus is present in most countries of Africa, the Middle East, India, China, the United States, and Mexico. Bluetongue virus infection, without associated clinical disease, is present in Southeast Asia, Papua New Guinea, northern South America and northern Australia. A strain of bluetongue virus was first identified in Australia in 1975 from trapped insects but despite its long-term presence, it has not caused any clinical disease.

Key signs

The disease is characterised by fever, widespread haemorrhages of the oral and nasal tissue, excessive salivation, and nasal discharge. In acute cases the lips and tongue become swollen and this swelling may extend below the lower jaw. Lameness, due to swelling of the cuticle above the hoofs and emaciation, due to reduced feed consumption because of painful inflamed mouths, may also be symptoms of this disease. The blue tongue that gives the disease its name occurs only in small number of cases. Convalescence of surviving sheep is slow. The high fever in sheep results in wool breaks, which adds to production losses.

Spread

The virus cannot be transmitted between susceptible animals without the presence of the insect carriers. The incidence and geographical distribution of bluetongue depends on seasonal conditions, the presence of insect vectors, and the availability of the susceptible species of animals. The insect carriers, biting midges, prefer warm, moist conditions and are in their greatest numbers and most active after rains.

Persistence of the virus

Bluetongue virus does not survive outside the insect vectors or susceptible hosts. Animal carcases and products such as meat and wool are not a method of spread. Survival of the virus within a location is dependent on whether the vector can overwinter in that area.

Control strategy

The strategy is to contain the outbreak and minimise trade impact by:
  • using a combination of quarantine and movement controls to prevent spread
  • treatments and husbandry procedures to control vectors, reduce transmission and protect susceptible animals
  • tracing and surveillance to determine the extent of virus and vector distribution
  • zoning to define infected and disease-free areas.
There is no justification for stamping out but some animals may need to be destroyed for welfare reasons. It is not possible to eradicate the bluetongue vectors.

Note: The Exotic Animal Disease Response Agreement only apply to bluetongue disease in its classical virulent form.

© The State of Queensland (Department of Primary Industries and Fisheries) 1995 - 2007.

Post-Mortem Blue Tongue Pictures

Bluetongue is an insect-borne viral disease to which all species of ruminants are susceptible. The virus is transmitted by a small biting midge of the Culicoides genus rather than from animal to animal. The virus does not affect humans. Bluetongue is a notifiable disease and, where suspected, should be reported to Animal Health.

VLA is involved in the surveillance for the disease and was responsible for the detailed post mortem examinations of the first cases in the UK. The images below illustrate the pathology associated with this disease as shown by the first 3 cattle cases in East Anglia.

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Cow 1 bluetongue case – clinically affected Highland cow. Extensive superficial erosion of the muzzle with a mucopurulent nasal discharge

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Cow 1 bluetongue case – clinically affected Highland cow. Diffuse reddening of the dental pad with multifocal haemorrhages on the upper lip and dental pad



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Cow 1 bluetongue case – clinically affected Highland cow. Ocular discharge visible at the medial canthus (wrinkling of the cornea reflects post mortem change)

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Cow 1 bluetongue case – clinically affected Highland cow. 'Coronitis’: skin reddening around and above the coronary band intensifying distally with serous crusting at the coronary band



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Cow 1 bluetongue case – clinically affected Highland cow. Multifocal haemorrhages with oedema visible on the cut surface of the submandibular lymph node

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Cow 1 bluetongue case – clinically affected Highland cow. Extensive ecchymotic subepicardial haemorrhages



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Cow 1 bluetongue case – clinically affected Highland cow. Extensive subendocardial haemorrhages in the left ventricle





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Cow 2 bluetongue case – Gloucester cow. Identified as PCR positive. Multiple pinpoint haemorrhages in the skin around the bases of the teats. There were also scattered petechial haemorrhages on the skin of the ventral abdomen. No other lesions distinctive of bluetongue were seen in this cow



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Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Focally extensive necrosis at the mucocutaneous junction of the nares.




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Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Focally extensive area of haemorrhage (with associated ulceration not clearly visible in this image) on the ventral surface of the tongue. There are multifocal pinpoint haemorrhages covering most of the ventral surface of the tongue



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Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Focally extensive haemorrhage involving the bulbar conjunctiva with associated subconjunctival petechiation

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Cow 3 bluetongue case – clinically affected Friesian/Holstein cow. Generalised lymph node enlargement with oedema




© 2003 Crown Copyright.

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