Temperature and the persistence of viruses in Culicoides spp. during adverse conditions.

The average daily maximum temperature for the coldest month during the winters of 1977-1978 and 1978-1979, when bluetongue (BT) virus overwintered in a cycle between Culicoides spp. and cattle in western Turkey (Aydin and Izmir provinces), was found to be 12.5 degrees C. This temperature agreed with the lowest temperatures found for Culicoides activity and for flight over long distances. Overwintering of BT virus in western Turkey and Lesbos (1979-1980), African horse sickness (AHS) virus in Spain (1987-1990) and BT and epizootic haemorrhagic disease of deer (EHD) viruses in British Columbia (1987-1988) was assessed using the following conditions for survival: maximum temperatures > or = 13 degrees C on 45% of days or more, a weighted "degree day" value of > or = 1.35 of 6 and an average daily maximum temperature of > or = 12.5 degrees C (all for the coldest month) together with a total of 40 days or fewer with maximum temperatures < 13 degrees C and < or = 10 consecutive days with maximum temperatures < 13 degrees C. Using these criteria, overwintering is unlikely to have occurred in Culicoides in Lesbos, Madrid, Toledo, Ciudad Real or British Columbia, but could have taken place in western Turkey (Aydin and Izmir) and southern Spain (Guadalquivir, Guadiana and Tagus Valleys). Isotherms of average daily maximum temperatures of 12.5 degrees C and 18 degrees C for the coldest month were drawn on maps covering various regions of the world, to indicate areas where virus and Culicoides activity could continue during the winter and to compare these areas with the known distribution of BT, EHD, AHS and Akabane infection.