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.
The journals have been scanned in colour with a HP 5590 scanner; 600 dpi. Adobe Acrobat v.11 was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.