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UC Davis Solves Bluetongue Mystery

The bluetongue virus reproduces and overwinters in female midges, researchers find.

The bluetongue mortality rate can be as high as 90 percent in susceptible breeds of sheep.

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The UC Davis School of Veterinary Medicine, which isolated and identified bluetongue virus in the Western Hemisphere in the 1950s, has discovered how the ruminant disease manages to withstand the winter cold and reappear the next spring.

The virus reproduces in female biting midges, the insect long known to transmit the disease, according to University of California, Davis, researchers. Their study, published Sept. 12 in the online journal PLOS ONE, for the first time explains why bluetongue returns years after year and why more animals could become infected during global climate change.

“This discovery has important ramifications for predicting the occurrence of bluetongue in livestock and, we hope, for eventually developing controls for the disease,” said UC Davis professor and co-author James MacLachlan, DVM, Ph.D.

Bluetongue virus is noncontagious and sometimes fatal. It mainly infects sheep, producing lesions on the lips and gums, but cattle and goats may be stricken as well.

Bluetongue refers to the swollen lips and tongue of affected sheep, which may turn blue in the late stages of the disease, UC Davis reported.

The cost to the U.S. sheep and cattle industries is estimated at $125 million a year.

The UC Davis researchers went to a Northern California dairy farm, where they monitored cows and midges for more than a year.

“We were able to demonstrate that the virus overwinters in female midges that had fed on an infected animal during the previous season,” said lead author Christie Mayo, DVM, a postdoctoral researcher.

The midges, a gnat known scientifically as Culicoides, “had been infected with the bluetongue virus during the previous warm-weather season,” the university reported.

“They were carrying the virus through the winter months and would later in the season once again transmit it to cows on the dairy,” UC Davis added.

More moderate winter temperatures tied to climate change could bring bluetongue to nontraditional regions, the university stated.

The disease is commonly found in latitudes ranging from 35 degrees south to 40 degrees north, according to The Merck Veterinary Manual, but the virus has moved as far as 50 degrees north since the 1990s.

Vaccines may be effective against bluetongue, depending on the virus serotype. Rest and soft food are recommended for afflicted animals.

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