Immunization with Streptococcus bovis protects against lactic acidosis in sheep

Lactic acidosis is a gastrointestinal disorder resulting from the rapid ove rgrowth of lactic acid-producing bacteria when ruminants are suddenly intro duced to grain feed. The present study has investigated the ability of live and killed bacterial vaccines to reduce lactic acidosis in sheep, via a st imulation of specific antibody production against lactic acid-producing bac teria. Forage-fed sheep were immunized with live or killed Streptococcus bo vis Sb-5 vaccine, with or without adjuvant, via intramuscular injection. Af ter the primary immunization, three boosters were given at 2-4 week interva ls. Sheep were subsequently challenged by a sudden switch to a grain-based diet. Following challenge, vaccinated sheep maintained significantly higher feed intake, and had higher rumen pH, lower L-lactate concentrations, and less severe diarrhoea scores than nonvaccinated control sheep. Higher rumen pH, lower mortality and less severe diarrhoea were found in the animals im munized with live vaccine compared to the animals immunized with killed vac cines. Significant increases in mucosal and systemic antibody responses wer e observed after boosting; the S. bovis-specific antibody concentrations we re significantly higher in samples of saliva, rumen fluid and serum from sh eep immunized with live vaccine than with killed vaccines. These results de monstrate that lactic acidosis can be reduced by immunization against S. bo vis, and that live Sb-5 vaccine is effective in invoking mucosal as well as systemic antibody responses. (C) 2000 Elsevier Science Ltd. All rights res erved.