Clinical disease in sheep caused by bluetongue virus serotype 8, and prevention by an inactivated vaccine

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 (R) 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.