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Nature of the disease
Bluetongue is a vector-borne disease of ruminants caused by a virus of the Reoviridae family and biologically transmitted by 5 species of Culicoides mosquitoe. The disease is characterised by inflammation of mucous membranes, congestion, swelling and haemorrhages. Sheep are generally the worst affected, with cattle having milder disease. In some parts of the world, infection without clinical disease is recognised.
OIE List A disease
Susceptible species
Sheep, goats, cattle, buffaloes and deer.
Distribution of Bluetongue is strongly related to the distribution of the Culicoides vector.

Bluetongue occurs as a clinical disease in Africa, the Middle East, Indian subcontinent, China, the USA, Mexico, south Europe.

Serological evidences have been also found in South East Asia, northern South America, northern Australia, Solomon Islands and Papua New Guinea.

Clinical signs 
In sheep, while the disease can be quite variable, the following signs are commonly seen:
  • Fever (42 °C)
  • Loss of condition and emaciation
  • Inflammation, ulcers and necrosis in and around the mouth (gums, cheeks and tongue)
  • In a small percentage of cases the tongue is cyanotic and appears purplish-blue.
  • Reddening and haemorrhages of the coronary band (above the hoof) causing lameness
  • Abortions and congenital malformations can also occur.
  • Sometimes pneumonia
  • Mortality is variable, up to 30%

Infection is generally sub-clinical in cattle. Cattle can remain a source of infection for sheep for some time. In about 5% of cases, fever, salivation, congestion and swelling and ulcers inside the mouth may occur.



Post-mortem findings
In sheep, most deaths occur as the result of secondary pneumonia. Hence severe, bilateral pneumonia is a common finding. Other findings may include:
  • Haemorrhages in the heart
  • Swelling and necrosis of muscles
  • Enlarged lymph nodes
  • Swelling and congestion of the spleen and liver
Differential diagnosis
Differential diagnosis:
Specimens required for diagnosis
Serum can be used for competitive ELISA which is the OIE reference test or Agar Gel Immunodiffusion, serum samples should be paired to demonstrate a rising antibody titre.

For virus isolation — fresh whole blood collected with heparin anticoagulant, fresh tissue from recently dead animals (spleen, lymph node, liver, heart blood, bone marrow) or abortion material. The samples should be kept at 4°C and not frozen.

Bluetongue is spread by small biting midges from the Culicoides genus. It is not transmitted by direct or indirect contact between animals in the absence of the insects. The vector competence of all the different species of Culicoides has not yet been totally explored however new species are regularly found to be potential vectors.

Rarely virus may be excreted in the semen when males are viraemic. Contaminated semen may infect recipient cows but would be unlikely to establish in an area unless abundant vectors were present. Blood is also an infective material

Risk of introduction
Bluetongue can be introduced to new regions by:
  • Introduction of infected animals
  • Insects on aeroplanes,
  • Wind-borne movement of insects
  • Semen from infected animals.

While bluetongue can be introduced to new regions by the movement of infected animals, it will not survive unless competent vectors are present and sufficient susceptible hosts are available.

A recent survey in Vanuatu found Culicoides brevitarsus a known vector of bluetongue in high numbers on all islands sampled. Culicoides spp. have also been found in Fiji, New Caledonia, French Polynesia, Tonga, Samoa, Guam and Palau.

There is no reason to believe that bluetongue would not establish if it were introduced hence quarantine and vector control in aircraft are of primary importance.

Control / vaccines
There is no treatment for Bluetongue disease.

Attenuated vaccines are widely and effectively used in southern Africa and the USA, but have a number of disadvantages. Vaccination of pregnant ewes should be avoided because of the risk of foetal abnormalities. Inactivated vaccines are not used in endemically infected countries, as effective ones are yet to be developed.

The best control is based on vector control methods,

  • Bluetongue, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 520-521
  • Bluetongue disease in Southeast Asia and the Pacific, proceeding of the first Southeast Asia and Pacific Regional Bluetongue Symposium, Greenlake Hotel, P.R. China, 22-24 August 1995, ACIAR proceedings N° 66
  • BOORMAN JPT, HAGAN DV (modified and updated after 1996), A Name List of World Culicoides (Diptera: Ceratopogonidae), International Journal of Dipterological Research 7(3): 161-192.
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Lardeux, communication on Ceratopogonidae Information Exchange No. 61- May 1998 May 1998: http://www.belmont.edu/Science/Biology/cienews/CIE.v61.M98.html
  • Office International des Epizooties, 2002