Mixed infection of Culicoides variipennis with bluetongue virus serotypes 10 and 17: evidence for high frequency reassortment in the vector.

The primary vector species for bluetongue virus (BTV) in the United States, Culicoides variipennis, was orally infected with BTV serotype 10, BTV serotype 17, or a mixture of the two viruses. The recovery of virus from the infected flies was low following a period of extrinsic incubation. Electrophoretic analysis of progeny virus from singly infected flies revealed that only the parental electropherotype could be isolated from those flies. In contrast, electrophoretic analysis of virus from mixedly infected flies revealed that eight of the 11 virus-positive flies produced virus progeny with reassortant electropherotypes. The proportion of reassortant progeny varied from 7 to 78% (mean 42%), depending on the individual fly. Analysis of segregation of the parental origin of genome segments in the reassortant progeny virus suggested that, while reassortment of most segments was random, selection for genome segment 8 from the type 17 parent may have occurred. Analysis of segregation in individual mixedly infected flies showed that each fly yielded a relatively unique set of reassortants, but that specific electropherotypes were isolated repeatedly from individual flies. These data indicated that the vector species C. variipennis was a permissive host for high frequency reassortment of genome segments of BTV.