The bluetongue virus (BTV) causes significant economic damage in cattle and sheep, but is much about the disease remains somewhat mysterious. Researchers in California recently published results of a study that could help clear up one question – that of how the virus overwinters in temperate regions.

The virus is known to be spread between livestock and wildlife though insect vectors, and outbreaks typically are seasonally concentrated during the insect-friendly summer months. The researchers, from the University of California – Davis, found monitored cattle and a midge vector, Culicoides sonorensis (C. sonorensis), year-round at a Northern California dairy.

The researchers found widespread BTV in the sentinel herd and in C. sonorensis midges from August to November. They also, however, found traces of the virus in midges they captured during February in 2013 and 2014. At that time there was no sign of BTV in the cattle.

The finding of BTV-infected vector midges during mid-winter, the researchers conclude, suggests that BTV may overwinter in northern California by infection of long-lived female C. sonorensis midges that were infected during the prior seasonal period of virus transmission, and reemerged sporadically during the overwintering period. They note, however the data do not definitively preclude other potential mechanisms of BTV overwintering.

The new study appears Sept. 12 in the journal PLOS ONE.

Much about BTV, its transmission, virology and control remains unclear. To seek answers, the USDA and the United States Department of Interior conducted a “gap analysis workshop,” on BTV and other orbiviruses in Manhattan, Kan., on May 14–16, 2013. Recently, USDA released “Orbiviruses, Bluetongue and Epizootic Hemmorhagic Disease: Gap Analysis Workshop Report,” outlining the results of the workshop. Read about that report in “USDA reports on orbivirus diseases, research gaps.”