Cattle and Culicoides biting midges as possible overwintering hosts of bluetongue virus

Abstract

Light trap catches over 7 years at the Veterinary Research Institute, Onderstepoort showed that Culicoides adults are active on occasional winter nights and in some years remain active almost throughout the winter. Culicoides were also found to survive refrigerator temperature for up to 53 days and to survive outdoors during winter for up to 51 days. Moreover, day-time temperatures during winter were high enough to allow continued development throughout this season. Bluetongue virus (BTV) may therefore be able to survive short winters in infected Culicoides midges while warmer nights in some years may allow midges to fly and feed and so reinfect new hosts. By testing large numbers of midges BTV has been shown to be present in Culicoides adults at the beginning of October. Apparently BTV does not disappear completely in spring but is present in so few midges as to be difficult to detect. BTV could only be detected in late November or December in sample groups of five cattle at Onderstepoort. If a high percentage of the cattle population could be tested BTV will almost certainly be isolated at an earlier date and even perhaps in winter. The results of these investigations support the theory that the biological cycle of BTV can continue in Culicoides and/or cattle throughout the winter in the Onderstepoort area.