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Bluetongue disease a growing threat

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03.02.2004

Research into the reasons for the recent world-wide spread of the devastating animal disease, bluetongue, could have major implications for the long-term future of Australia’s sheep industry.

According to a senior epidemiologist at CSIRO Livestock Industries’ Australian Animal Health Laboratory in Geelong, Dr Peter Daniels, bluetongue disease is spreading rapidly in Europe and new strains of bluetongue virus have been detected in Australia.

"While Australian sheep are currently free of bluetongue we must learn more about how the virus spreads so we can ensure that the national flock remains free of this destructive disease," Dr Daniels says.

Sheep infected with bluetongue develop a high fever and swelling and/or lesions around the face and feet. Lesions can also develop in the animal’s mouth and, in severe cases, sheep die within two weeks of being infected.

Bluetongue infections are common in sheep, cattle and other ruminants in tropical and subtropical areas around the world but recently began to spread to previously unaffected regions.

"Bluetongue has turned up in parts of Europe where it was previously unknown," Dr Daniels says. "This emergence seems to be linked to global warming which may enable the virus’ insect carriers to survive where they previously could not."

In Australia, the disease does not occur in commercial sheep flocks. The insects that spread the virus do not occur in the same areas as the major sheep populations and strains of bluetongue virus are non-virulent (non disease-causing).

However, bluetongue is spreading in Australia, moving south of the Kimberley into the Pilbara. In recent years, three new South East Asian strains of the virus have entered the Northern Territory, but these have not spread beyond the Top End.

"The reasons for the spread are complex, but partly relate to changing patterns of land use, with cattle replacing sheep in some areas. Some of the insects that spread bluetongue feed more readily on cattle than sheep," Dr Daniels says.

The disease movements were detected by the National Arbovirus Monitoring Program (NAMP), managed by Animal Health Australia. Disease information is collected by monitoring young cattle in sentinel herds throughout the country. AAHL acts as a reference laboratory for the program.

Dr Daniels says scientists want to understand whether the spread of virulent bluetongue viruses in Australia is only a matter of time, or whether the Top End of the Northern Territory is a separate ecosystem for bluetongue viruses, with different characteristics that would operate to contain the spread of more virulent strains.

"Such an ecosystem would depend on the relationships between the viral strains and insect carriers present as well as the habitat requirements of those insect carriers," he says.

"If the virulence of the viruses is discovered to be linked to the species of insect by which it is spread, surveillance programs could be fine-tuned and the risk of severe bluetongue disease in Australia more accurately assessed."

More information:
Dr Peter Daniels, CSIRO Livestock Industries, 03 5227 5272

Media assistance:
Emma Homes, CSIRO Livestock Industries 03 5227 5123

Background information:

Bluetongue is an insect-borne viral disease of ruminants. Sheep, cattle, goats, buffaloes, antelopes and deer can be infected. Of the domestic species, sheep are the most severely affected.

Bluetongue virus cannot be transmitted by direct contact between animals. It is spread by small, blood-feeding, biting midges (Culicoides species). The virus multiplies in the salivary glands of the midges and is transferred to the blood of an animal host when bitten by the midge.

The virulence of different strains of the virus varies considerably. For example, India possesses extremely infectious strains and suffers severe bluetongue disease whereas,in eastern Australia, only non-virulent strains are present. Other factors may also influence the severity of the disease in sheep, including breed (British breeds are the most susceptible, followed by Merinos) and exposure of the animals to sunlight and stress.

Bill Stephens | Source: CSIRO
Further information: www.csiro.au/index.asp?type=mediaRelease&id=prbluetongue

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