Overwintering of Bluetongue virus in temperate zones.

Within Northern California, Culicoides sonorensis is the major vector of Bluetongue virus (BTV) and annual infection of livestock is distinctly seasonal (typically July­November). Our recent studies compare the population dynamics of C. sonorensis midges with occurrence of BTV infection of C. sonorensis and sentinel dairy cattle throughout both the seasonal and interseasonal ('overwintering') periods of BTV activity. Spring emergence and seasonal abundance of adult C. sonorensis on the sampled farms coincided with rising vernal temperature. Intensive surveillance confirmed widespread infection of both sentinel cattle and vector midges during the August­November period of seasonal BTV transmission. Bluetongue virus infection of parous female midges captured in traps set during daylight hours was also detected during the interseasonal period of virus activity, whereas there was no concurrent active infection of sentinel cattle during the overwintering period. The finding of BTV­infected vector midges during mid­Winter suggests that BTV can overwinter in Northern California by infection of long­lived female C. sonorensis midges that were infected during the prior seasonal period of virus transmission and which, then, entered a quiescence in the fall (Autumn) and re­emerged sporadically during the overwintering period. Notably, vertical transmission of BTV was not detected among progeny of midges infected in the laboratory nor in field­collected larvae. In addition to defining the mechanism of BTV over­wintering in a temperate region, the studies reviewed in this article also provide precise documentation of temporal changes in the annual abundance, dispersal and dynamics of BTV infection of Culicoides midges. Collectively these findings are critical to the creation of accurate predictive models of BTV infection in livestock and to development of sound abatement strategies.