Neonatal joint-ill in lambs, predominantly caused by Streptococcus dysgalactiae subspecies dysgalactiae (SDSD), leads to elevated rates of morbidity and mortality, highlighting the need for an effective preventive vaccine. A blinded, randomized controlled trial was conducted on a commercial sheep flock in the UK to evaluate an autologous SDSD vaccine. A total of 481 pregnant ewes received two doses of the vaccine, while 509 ewes remained unvaccinated. SDSD-specific antibody titres were measured in both ewes and their lambs, and any adverse reactions or occurrences of joint-ill were recorded. Ten lambs developed joint-ill, evenly split between those born to vaccinated and unvaccinated ewes. Serum samples from 85 vaccinated and 88 control ewes were analyzed using an SDSD antibody ELISA, revealing higher titres in vaccinated ewes. Lambs from vaccinated ewes (n = 87) also showed higher antibody titres than those from unvaccinated ewes (n = 91). Colostrum antibody levels did not differ between groups, and no adverse effects related to vaccination were observed. Although randomization was successful, ELISA data were primarily obtained from crossbred ewes, limiting the power to assess breed-specific differences. Vaccination of ewes did not reduce the incidence of joint-ill in lambs compared to unvaccinated ewes, but it significantly increased SDSD-specific antibody levels in both ewes and their offspring.
