Contamination in bluetongue virus challenge experiments

Five cattle and five sheep that had never been exposed to bluetongue virus (BTV), as well as ten animals that had been experimentally infected with BTV-8, were inoculated with BTV-1. Previous exposure to BTV-8 did not prevent a second infection with another serotype. After the experiment, the BTV-1 preparation was found to be contaminated with BTV-15. The inoculum and blood samples taken during the experiment were analysed by serotype-specific real-time RT-PCR. There was 100-fold less BTV-15 than BTV-1 in the inoculum. Unexpectedly, BTV-15 dominated the infection in cattle that had previously been exposed to BTV-8. In sheep of both groups, on the other hand, BTV-1 prevailed over the contaminant. Regardless of the outcome, the incident demonstrates the need for a thorough contamination screening of virus preparations. For this purpose, two type-specific RT-PCR primer sets for each of the 24 established BTV serotypes as well as Toggenburg Orbivirus were designed.