Issue 33, April 2008
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Spotlight on Bluetongue Virus

Bluetongue virus (BTV) infection only rarely results in the swollen, bluish mouth tissue for which it was named, but its other symptoms—such as fever, swelling, and salivation—can cause significant discomfort for the animals it affects. The virus targets ruminants such as cattle, sheep, goats and deer.  Sheep are particularly susceptible to BTV and may have mortality rates above 10 percent.

Biting midges known as Culicoides spread the virus, which has 24 different serotypes. Although it's not always fatal, BTV infection reduces production efficiency, resulting in significant economic losses. There is no known cure, but precautions can be taken to prevent its spread.

In 2006 and 2007, emerging exotic strains of the virus spread disease through sheep and cattle in northern Europe, which had never seen bluetongue before. Although these strains have not yet crossed the Atlantic, American farmers are concerned by the potential for them to appear on U.S. farms. As farmers on both sides of the Atlantic gear up for warmer weather and higher Culicoides numbers, scientists are studying the virus's origin and development—a vital step in countering the disease.

To learn more about the disease, Agricultural Research Service (ARS) scientists at the Arthropod-Borne Animal Disease Research Laboratory (ABADRL) in Laramie, Wyoming, are investigating the interplay between the virus, its vectors, and its animal hosts.

Microbiologist Linda McHolland has been instrumental in the development of Culicoides cell lines that researchers around the world are using to isolate the virus and to study virus-vector interaction.

Sheep are among the ruminants affected by bluetongue virus.

"If you're using these lines for BTV research purposes, they'll give you a more accurate picture of how the virus and vector interact than if you'd used a cell line from another insect," McHolland says.

Two new Culicoides cell lines, known as ABADRL-Cs-W3 and ABADRL-Cs-W8A—or W3 and W8—have been developed from field-collected insects. 

The virus replicates much faster in the W3 cell line, making it a better choice for some studies, McHolland says.

McHolland and microbiologist James Mecham have developed techniques for detecting and quantifying BTV in these Culicoides cell lines.

"Traditionally, the problem with using insect cells for BTV studies is that they don't show any adverse effects when they're infected with the virus," Mecham says.

As a result, researchers had to add an extra step: exposing mammalian cells—which do respond physically to infection—to insect cells believed to be infected with BTV. By staining the mammalian cells, scientists could determine whether the cells had been killed by the virus and quantify the amount of virus present in the sample.

This tiny biting midge, Culicoides sonorensi, is a vector for bluetongue virus. Scanning electron micrograph (magnification about 150x) by Richard Nunamaker.

"This research means it's possible to directly detect BTV in the insect cells without wasting time on extra steps," Mecham says.

Mecham and McHolland are currently working with their ABADRL colleagues to investigate the mechanism that allows the virus to overwinter in areas where it reappears every year. A better understanding of the vector's lifestyle will help scientists develop better disease controls in the future.

This research also has implications for other important animal diseases. In addition to vectoring BTV, biting midges are suspected of vectoring additional hemorrhagic viruses, including West Nile and vesicular stomatitis.

ABADRL scientists are also applying their expertise in diagnostics, entomology and virology to investigate a recent BTV outbreak in Wyoming and exotic BTV introductions in Florida and Louisiana, and to assess the risk of introducing into the U.S. a BTV strain that is causing severe clinical disease in Europe.

For more information about ARS research on arthropod-borne diseases, contact James Mecham or Cyril Gay, Leader of ARS National Program #103: Animal Health.