Monitoring of biting midges (Culicoides spp.), the potential vectors of the bluetongue virus, in the 12 climatic regions of Switzerland

Midges of the genus Culicoides are the only known biological vectors of the bluetongue disease virus (BTV). Their occurrence at altitudes below 900 meters above sea level (m a.s.l.) is monitored in Switzerland by the Swiss Federal Veterinary Office, to determine vector-free periods around winter. In this study, data about the number of midges caught at stations representing the 12 climatic regions of Switzerland are shown. The tiny midges of 1-3 mill in size were caught once per week with UV light traps and grouped under the stereomicroscope into Obsoletus complex, Pulicaris complex and other Culicoides spp. Midges were caught at all stations, albeit in very different numbers. The highest monthly average was 10'000 midges per night (Dit-tingen/BL); the third highest average of all 12 stations was recorded for the highest-located station (Juf/GR, 2130 m a.s.l.). At stations below 1500 m a.s.l., midges of the Obsoletus complex (98% in Dittingen), which in Central Europe are most likely considered to be responsible for the transmission of BTV, were predominant. With increasing altitude, midges of the Pulicaris complex prevailed (91% in Juf). Hence, there are no regions of the populated areas in Switzerland which are free of midges, but the vector competence regarding BTV of the various midges needs to be urgently clarified.