Randomised controlled field trial to assess the safety and efficacy of a killed autologous vaccine against Streptococcus dysgalactiae subspecies dysgalactiae in a sheep flock

Introduction Joint‐ill in neonatal lambs, primarily caused by Streptococcus dysgalactiae subspecies dysgalactiae (SDSD), results in increased mortality and morbidity. An effective vaccine is needed to prevent the disease. Methods A blinded randomised controlled trial was carried out using an autologous SDSD vaccine in a commercial sheep flock in the UK. Two doses of vaccine were administered to 481 pregnant ewes, with 509 ewes left unvaccinated. The SDSD antibody titres of the ewes and their lambs were then measured. Any adverse effects or cases of joint‐ill were also reported. Results Ten cases of joint‐ill occurred, with five of these lambs born to vaccinated ewes and five to unvaccinated ewes. Serum from 85 vaccinated ewes and 88 control ewes was analysed using an SDSD antibody ELISA, with higher titres found in vaccinated ewes. Higher SDSD titres were observed in lambs from vaccinated ewes ( n = 87) than in lambs from unvaccinated ewes ( n = 91). No differences were detected between colostrum samples from vaccinated and unvaccinated ewes. No vaccination‐associated adverse effects were detected. Limitations Randomisation was effective; however, ELISA data were limited to mainly crossbred ewes, reducing the power of the breed comparisons. Conclusions The vaccine had no effect in reducing the number of lambs with joint‐ill born to vaccinated ewes compared with unvaccinated ewes. However, vaccinating ewes did have a positive effect on their serum SDSD antibody concentrations and those of their lambs.