For more than a century, a livestock disease mystery has stumped producers and veterinarians alike—how does bluetongue disease overwinter?

The answer is the virus survives the winter by reproducing in the tiny midges that transmit it, report veterinary scientists at the University of California, Davis.

The disease cycle was confusing—bluetongue is most prevalent when midges are abundant in late summer and fall on the West Coast. When cold temperatures cause the biting-midge populations to plummet, transmission appears to cease for more than six months, but the virus reappears when temperatures warm the following season.

Solving the mystery is a significant step forward in helping livestock producers manage the health of their herds. The bluetongue virus causes a serious disease that costs the cattle and sheep industries in the U.S. an estimated $125 million annually.

Bluetongue disease, first identified during the 1800s in southern Africa, is transmitted by the Culicoides biting midge, a tiny gnat sometimes referred to as a "no-seeum."

The disease mostly sickens sheep but also infects cattle and goats, as well as deer and other wild ruminants. In the U.S., the virus' greatest economic impact is in the cattle industry, because it is bigger than the domestic sheep industry and most adversely impacted by international trade barriers related to bluetongue. The disease does not pose a threat to human health.

The name bluetongue derives from the swollen lips and tongue of affected sheep, which may turn blue in the late stages of the disease. The virus that causes bluetongue was first isolated and identified in the Western Hemisphere in the early 1950s at the UC Davis School of Veterinary Medicine.

Findings from a California dairy

The researchers monitored cows and midges on a Northern California dairy farm for more than a year. They documented, for the first time, the presence of genetic material for the bluetongue virus in female midges collected during two consecutive winter seasons.

The bluetongue virus was widespread in both the dairy cows and the midges from August to November. Surprisingly, however, the researchers discovered the virus was also present in female midges captured in February of both 2013 and 2014. There was no sign of infection in the dairy cattle being studied.

The researchers concluded that those long-lived female midges had been infected with the bluetongue virus during the previous warm-weather season. They were carrying the virus through the winter months and would later in the season once again transmit it to cows on the dairy.

The research team notes that the bluetongue virus may also have additional, yet-to-be discovered, modes of overwintering in temperate regions.

Other members of the research team were William K. Reisen and Cameron J. Osborne, both of UC Davis; E. Paul J. Gibbs of the University of Florida, Gainesville; Bradley A. Mullens of UC Riverside; and Ian A. Gardner of Atlantic Veterinary College, Charlottetown, Prince Edward Island, Canada.

Burt Rutherford  Sept 15, 2014