Summary
The overwintering mechanism of bluetongue virus (BTV) has eluded researchers for many years. It was recently proposed that ovine gamma delta T-cells may become persistently infected with BTV, and serve as a reservoir for infection of naive vectors in the next transmission season. Since cattle are more numerous than sheep in the western United States (where BTV is endemic), this hypothesis was tested in bovines. In the winter of 2002-2003, 54 cattle from an endemic site in northern Colorado were age-selected to ensure that possible BTV exposure must have occurred in the summer of 2002. These cattle were tested for the presence of anti-BTV antibody by ELISA; 53 were seropositive, and one was seronegative. Naive Culicoides sonorensis colony insects were fed on skin sites of four seropositive and one seronegative cattle at day 1 (135 days after the first frost), then sequentially on separate sites for three days. Virus isolation and/or reverse transcriptase-nested polymerase chain reaction from engorged insects and 6 mm skin biopsy samples were performed for detection of viable BTV or BTV nucleic acid; all were negative. These data suggest that cattle are not a reservoir host for BTV overwintering in the western United States. The role of sheep in the trans-seasonality of BTV still remains to be determined.

Keywords
Bluetongue, Cattle, Culicoides sonorensis, Overwintering, United States of America, Virus.