Abstract
The ability to reduce clinical signs, induce neutralizing antibodies, and perhaps most importantly, to prevent or reduce viraemia (and therefore virus-transmission), represent primary criteria for assessment of bluetongue virus (BTV) vaccine efficacy. Identification of BTV challenge-strains that reliably induce viraemia and clinical signs comparable to those in naturally infected animals, is therefore important for vaccine evaluation. Texel cross-breed and Dorset Poll sheep vaccinated with inactivated BTV-8 vaccine ('Bovilis(®) BTV8' from MSD Animal Health), were challenged with low-passage BTV-8 (Northern European strain) grown in either insect (Culicoides) or mammalian cell-cultures. The severity of clinical signs was recorded (using a modified numerical scoring-system, which is described) along with viraemia and serum neutralizing (SN) antibody levels. Low level SN-antibodies were detected at the time of challenge (three weeks after vaccination). All unvaccinated control animals became infected after challenge, developing high SN-antibody titres by 21 days post challenge (dpc). Vaccinees showed faster increases in SN-antibody titres ('booster' response), with significantly higher titres at 6 dpc than unvaccinated controls. Although only limited clinical-signs could be attributed to BTV in younger animals infected with the mammalian-cell-culture derived virus, both BTV-8 challenge preparations induced severe clinical signs comparable to 'bluetongue' observed during natural outbreaks in older unvaccinated animals. Challenge with BTV-8 grown in Culicoides cell-cultures seemed to induce greater severity of clinical-scores and 'post-mortem lesions' than the mammalian-derived BTV-8 strain. Vaccination reduced clinical signs, fever, and viraemia equally well after challenge with either virus preparation.