First occurrence of Culicoides obsoletus-transmitted Bluetongue virus epidemic in Central Europe

In August 2006, Bluetongue virus disease (BTD) was detected for the first time in the Netherlands, Belgium, Germany and Northern France. Serological tests as well as reverse transcriptase polymerase chain reaction (RT-PCR) proved the occurrence of Bluetongue virus (BTV) in diseased sheep and cattle, and the virus was identified as serotype 8. Therefore, the search for possible vectors was immediately initiated in the outbreak region in Germany. Traps with automatically regulated ultraviolet light lamps were placed at two different farms with sero-positive cattle, and insect monitoring was done from August 2006 until January 2007. The caught arthropods were weekly determined, and it could be observed that midges of the dipteran family Ceratopogonidae occurred in large numbers, sometimes representing up to 40% of all individuals. The microscopical analysis of the wing morphology showed that the species (complex) Culicoides obsoletus was most abundant covering about 97% of the analysed midges. On the second place ranged C. pulicaris, while C. nubeculosus and C. festivipennis were found only as single individuals. Fed and unfed females were separated, sent to the National Reference Laboratory for Bluetongue disease (Friedrich-Loeffler-Institut, Isle of Riems, Germany) and investigated with a BTV-8-specific real-time RT-PCR. It could be demonstrated that at both farms both fed and unfed C. obsoletus were tested positive for BTV-8 genomes, while none of the other species scored positive. This finding strongly supports that the BTD-epidemic, which reached in the meantime wide regions of North Rhine-Westphalia in Germany and of the neighbouring countries with several hundreds of affected farms, is initiated by virus transmission during the blood meal of midges of the C. obsoletus complex. Since they were captured still at the 21st of December close to cattle with clinical signs, it must be feared that BTV-8 is now established in Central Europe, where it had been absent until now.