The virogenesis of bluetongue virus in Culicoides variipennis.

: A direct immunofluorescence (IF) technique was developed to detect bluetongue virus (BTV) antigen in Culicoides variipennis. After approximately 10 days extrinsic incubation (EI), the IF test was as sensitive as virus isolation for detection of infected flies. The IF technique was used to determine the virogenesis of BTV in the fly. After 6 days EI, virus antigen was detected in most midguts; by 10 days, most head squash preparations contained antigen. Other secondary target organs, such as ovaries, abdominal ganglia and thoracic muscle, only infrequently contained detectable BTV antigen.