Factors affecting the distribution of Culicoides spp. (Diptera: Ceratopogonidae) vectors of bluetongue virus (BTV) in Australia

This study examined environmental and geographical factors affecting the distribution of the bluetongue virus vectors Culicoides brevitarsis Kieffer, C. actoni Smith, C. fulvus Sen and Das Gupta and C. wadai Kitaoka in Australia using graphical analysis. Calculations and mapping were carried out in the R programming environment using freely available data. Average low temperatures of 13.5 degrees C in June and in September were estimated as thresholds and expressed as contours at which C. brevitarsis activity ceases and then recommences during and after winter, respectively. The June threshold also designated areas where populations would be unable to survive the winter. There was substantial variability in the contours between years, around which the activity of C. brevitarsis would be correspondingly dynamic. Culicoides wadai, C. actoni and C. fulvus appeared to be less tolerant of low temperatures. A high temperature limit was proposed at a maximum 35 degrees C during summer. Culicoides brevitarsis was found in areas above and below this temperature, while C. actoni, C. fulvus and C. wadai occurred mainly in coastal environments at maximum temperatures of less than 35 degrees C. Possible effects of global warming were considered with 1 degrees C or 2 degrees C rises in temperature. Changes to low temperature thresholds could expose greater areas of southern Australia to vector activity, while temperature rises in the north could increase the size of areas in which temperatures are lethal to the vectors. Rainfall had no apparent effect on C. brevitarsis during the northern wet and dry seasons. Distributions of C. brevitarsis and cattle overlapped until both ended with the climatic change to desert, which formed a physical barrier preventing C. brevitarsis movement to the south. Temperature restricted associations between C. brevitarsis and sheep. The other Culicoides spp. were found outside major cattle and sheep production areas. Apart from seasonal and dynamic activity around the contours, cattle and sheep in areas below a June average temperature of 13.5 degrees C should remain vector free at current climatic temperatures.