The bluetongue virus, which causes a serious disease that costs the cattle and sheep industries in the United States an estimated $125 million annually, manages to survive the winter by reproducing in the insect that transmits it, a study from veterinary scientists at the University of California, Davis has found.
The findings solve a century-old mystery and are particularly significant as global climate change brings more moderate winter temperatures around the world. The new study appears Sept. 12 in the journal PLOS ONE.
The researchers focused their study on a commercial dairy farm in Northern California, where they monitored both cows and the Culicoides biting midge, a tiny gnat sometimes referred to as a "no-seeum."
Cases of bluetongue disease, and populations of the midge – transmitters of the virus – are abundant in late summer and fall. 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.
Female midges, captured in February 2013 and 2014, were found to carry the disease, though 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.
"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," said lead author Christie Mayo, a veterinarian and postdoctoral researcher in the UC Davis School of Veterinary Medicine.
"This discovery has important ramifications for predicting the occurrence of bluetongue in livestock and, we hope, for eventually developing controls for the disease," said co-author James MacLachlan, a UC Davis veterinary professor and viral disease expert.
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.
Funding for the study was provided by the U.S. Department of Agriculture's National Institute of Food and Agriculture, and the UC Davis School of Veterinary Medicine's Center for Food Animal Health.
Source: UC Davis