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Question of the Week: What
Is Bluetongue And How Is It Spread?
This question comes from Teton County
Nature of the disease. Bluetongue is an insect-spread disease of ruminants characterised by inflammation of mucous membranes, congestion, swelling and haemorrhages. The disease is variable in severity. Sheep are generally the worst affected, with cattle having milder disease. In some parts of the world, infection without clinical disease is recognised.
Susceptible species. Sheep, goats, cattle, buffaloes and deer.
Where it occurs. Bluetongue occurs as a clinical disease in Africa, the Middle East, Indian subcontinent, China, the USA and Mexico. Virus strains, without associated disease, have been found in South East Asia, northern South America, northern Australia and Papua New Guinea.
Clinical signs. In sheep, while the disease can be quite variable, the following signs are commonly seen:
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:
Specimens required for diagnosis. For virus isolation — fresh whole blood collected with EDTA or heparin anticoagulant from animals wit high temperatures; fresh tissue from recently dead animals (spleen, lymph node, liver, heart blood, bone marrow).
Sera, preferably paired to demonstrate a rising antibody titre. Sera from convalescent sheep, or from in-contact sheep or cattle.
Transmission. Bluetongue is spread by small biting midges. It is not transmitted by direct or indirect contact between animals in the absence of the insects.
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.
Risk of introduction. Bluetongue can be introduced to new regions by:
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.
Control / vaccines. 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 fetal abnormalities. Inactivated vaccines are not used in endemically infected countries, as effective ones are yet to be developed.
New Tests Quickly Diagnose Bluetongue, Related Diseases
New laboratory tests will allow scientists, regulators and livestock producers to quickly identify animals with bluetongue or epizootic hemorrhagic disease (EHD).
ARS scientists in Laramie, Wyo., have developed the first single genetic test that distinguishes all five types of the virus that causes bluetongue in the U.S. They’ve also developed rapid tests that distinguish bluetongue from EHD.
Bluetongue, so named because it can cause a loss of oxygen and a blue tinge to the tongue, affects sheep, goats, deer, elk and antelope. Cattle can carry the virus, usually without becoming ill. Worldwide, there are 24 strains of bluetongue virus. Countries without bluetongue strictly regulate import and export of livestock and related products, costing U.S. producers about $125 million annually.
Previous tests were not always definitive, requiring additional testing. The new test, developed by ARS microbiologist William C. Wilson, reduces the time it takes to identify the virus type from several days to a single day. All official bluetongue testing in the U.S. is performed at the USDA’s National Veterinary Service Laboratory in Ames, Iowa, and the lab has incorporated Wilson’s test into its procedures.
Wilson and ARS microbiologist James O. Mecham also developed tests that identify the two types of EHD present in the U.S. and distinguish them from bluetongue viruses. Both scientists work at ARS’ Arthropod-Borne Animal Diseases Research Laboratory in Laramie.
EHD can cause a bluetongue-like disease in cattle and is often fatal to white-tailed deer. Correctly identifying which virus an animal harbors is important for trade purposes. There is no cure for either bluetongue or EHD.
ARS is USDA's chief scientific research agency. A detailed story on the research appears on the Internet at: http://www.ars.usda.gov/is/AR/archive/jul99/blue0799.htm
Sources of Information:
Bluetongue, so named because it can cause a loss of oxygen and a blue tinge to the tongue, affects sheep, goats, deer, elk and antelope.
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