A virus that causes a serious disease in cows and sheep is able to survive the winter by reproducing in the biting bugs that transmit it.
The discovery solves a century-old mystery about bluetongue virus—and is particularly significant as climate change brings more moderate winter temperatures. In the United States alone, the disease costs the cattle and sheep industry an estimate $125 million annually.
“By conducting this epidemiological study on a commercial dairy farm in Northern California, we were able to demonstrate that the virus overwinters in female midges that had fed on an infected animal during the previous season,” says lead author Christie Mayo, a veterinarian and postdoctoral researcher in the School of Veterinary Medicine at University of California, Davis.
“This discovery has important ramifications for predicting the occurrence of bluetongue in livestock and, we hope, for eventually developing controls for the disease,” says coauthor James MacLachlan, veterinary professor and viral disease expert.
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, and deer and other wild ruminants. In the US, 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 doesn’t 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.
In California, bluetongue is most prevalent when midges are abundant in late summer and fall, but there has been speculation over how the virus survives through the winter. When temperatures turn cold and the biting-midge populations plummet, transmission appears to cease for more than six months, but the virus reappears when temperatures warm the following season.
Long-lived female midges
For the new study, published in PLOS ONE, 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 that were 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 that 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 bluetongue virus may also have other yet-to-be discovered modes of overwintering in temperate regions, the researchers say.
Other researchers from UC Davis, UC Riverside, University of Florida, Gainesville, and the Atlantic Veterinary College, Charlottetown, Prince Edward Island, Canada contributed to the study.
The US Department of Agriculture’s National Institute of Food and Agriculture, and the UC Davis School of Veterinary Medicine’s Center for Food Animal Health provided funding.
Source: UC Davis