Seasonal transmission of bluetongeue virus by Culicoides sonorensis (Diptera : Ceratopogonidae) at a southern California dairy and evaluation of vectorial capacity as a predictor of bluetongue virus transmission

Vectorial capacity of Culicoides sonorensis Wirth & Jones for the transmiss ion of bluetongue (BLU) virus was examined at a southern California dairy f rom January 1995 to December 1997. Insects were collected one to two times per week in five CDC-type suction traps (without light) baited with CO2 at a constant release rate of 1,000 ml/min. BLU virus was detected in midges c ollected from May through December with an estimated overall infection rate of 0.08%. The BLU virus infection rate of field-captured midges was not co rrelated with sentinel calf seroconversions to BLU virus. Sentinel calf ser oconversions were highly seasonal, occurring from August through November w ith most calves seroconverting during September and October. Vector compete nce of field-collected nulliparous flies fed a locally acquired serotype of BLU virus in the laboratory was stable among years (17-23%). Vectorial cap acity was strongly correlated with BLU virus transmission (measured by sent inel calf seroconversions) during 1995 and 1996, but not during 1997. Host biting rate estimated for traps nearest to the sentinel calves was the inde x best correlated with BLU virus transmission for all study years and was m ost highly correlated with sentinel seroconversions 4 wk later. The utility of vectorial capacity and its component variables is discussed for this sy stem.