Antibody responses to lactic acid producing bacteria in sheep immunized with experimental acidosis vaccines

Acidosis is an economically important disease of ruminant livestock, involving both reduced productivity and serious animal welfare implications. Major lactic acid producing bacteria in sheep were identified by Al Jassim and Rowe (1999) and it is likely that the proliferation of these bacteria, under certain feeding regimes, leads to acidosis. Antibiotic feed additives are currently used in management of acidosis but there are serious concerns about this approach and, consequently, vaccination against acidosis-producing bacteria as a means of preventing the condition is under active investigation in our laboratory. The aim of this trial was to measure antibody responses to a range of acidosis-producing bacteria, following administration of experimental vaccines. Forty-nine healthy adult merino wethers were used for this study and run together as a single flock throughout the experiment. Animals were randomly allocated into 7 groups, to be immunized with one of six different bacterial isolates, or non-immunized as controls. Vaccines were prepared from pure bacterial isolates (formalin killed), emulsified in Freund's adjuvant, and administered intramuscularly at weeks 0 and 4. Freund's complete adjuvant was used for the primary immunization, and Freund's incomplete adjuvant was used for the secondary immunization. Samples of venous blood were collected before immunisation and every two weeks for ten week. Concentrations of IgG antibodies in serum were determined by ELISA. Antibody levels produced in response to immunizations with each of the 6 isolates are shown in Table 1. Strong antibody responses were produced by the three streptococcal isolates, with peak antibody concentrations at week 6. There was a 12-fold increase produced by the two 'S. bovis' strains, whereas a 57-fold increase was produced by the 'S. equinus'. Antibody responses were also produced by 'Selenomonas ruminantium', with antibody levels reaching a peak at week 10. There was a relatively weak antibody response to 'Lactobacillus vitulinus'.