Association of virulent and avirulent strains of bluetongue virus serotype 11 with premature births of late-term bovine fetuses.

Two strains of bluetongue virus serotype 11 (BTV 11), UC-2 avirulent and UC-8 neurovirulent in newborn mice, were inoculated into late-term bovine fetuses to investigate whether infection with these two BTV strains in late gestation would produce congenital infection and pathological changes. Fetuses were inoculated by intramuscular injection through the uterine wall at 243 days gestation and recovered after spontaneous delivery. In calves inoculated with UC-8, births occurred 15 to 27 days before expected parturition, resulting in small, weak calves. These calves had a mild encephalitis and were unthrifty at birth. Calves inoculated with UC-2 appeared healthy when born 7 to 11 days prior to expected parturition. No lesions were found in these calves at necropsy. All calves seroconverted by the time of birth. Viraemia was present in the calves inoculated with UC-8 and in one calf inoculated with UC-2. Plasma cortisol concentrations were prematurely elevated, particularly in the calves inoculated with UC-8, indicating that they were stressed by the infection. The elevated cortisol, associated with an active congenital infection caused by bluetongue virus serotype 11 strain UC-8, is capable of causing premature delivery of low birth-weight, weak calves.