Summary
Bluetongue virus (BTV) is transmitted between its mammalian hosts almost exclusively via bites from the adults of certain species of Culicoides biting midges. Theoretically, the spread of BTV into the more northerly areas of Europe should therefore be terminated by the harsh winters experienced in these regions, when adult midges disappear for extended periods of time. However, it has been shown that BTV can survive for periods as long as 9 to 12 months in such locations in the absence of adult insect vectors, with no detectable cases of viraemia, overt disease or seroconversion in the host species. Virus survival in this manner throughout the winter is called ‘overwintering’ but the mechanism involved has not been satisfactorily explained. With knowledge currently available and results from a series of preliminary experiments, the authors discuss a possible overwintering mechanism.

Keywords
African horse sickness, Bluetongue virus, Culicoides, Growth arrest, Orbivirus, Overwintering, Persistent infection, Gamma delta T-cells.