Duration of viraemia infectious to Culicoides sonorensis in bluetongue virus-infected cattle and sheep.

The duration of viraemia infectious to Culicoides sonorensis (C. sonorensis) was evaluated in bluetongue virus (BTV)-infected sheep and cattle by feeding laboratory-reared C. sonorensis directly on the skin of ruminants that previously were infected with BTV by insect inoculation. The intervals after infection when infectious BTV and BTV nucleic acids were present in the blood of infected cattle and sheep, respectively, were determined by virus isolation and reverse transcriptase (RT)-nested polymerase chain reaction (PCR) assays. The presence of BTV in vector insects that fed on the BTV-infected cattle and sheep at 7, 21, and 49 days post-infection (p.i.) was also determined by virus isolation and RT-PCR assays. BTV was isolated from the blood of infected cattle for up to 49 days p.i., whereas it was not isolated from the BTV-infected sheep after 11 days p.i. In contrast, BTV nucleic acids were detected in the blood of infected ruminants for 111-222 days p.i. The maximal duration of viraemia that was infectious to C. sonorensis was 21 days p.i. of both cattle and sheep and, with the notable exception of one sheep at 21 days p.i., only ruminants whose blood contained BTV as determined by virus isolation were able to infect C. sonorensis after oral feeding. Data from this and previous studies indicates that viraemia is transient in BTV-infected ruminants, and that the RT-nested PCR assay provides a very sensitive and conservative test for the screening of cattle and sheep for the presence of BTV.