Failure of an Actinomyces pyogenes vaccine to protect sheep against an intravenous challenge.

The immunity conferred by an A. pyogenes bacterin-toxoid was evaluated in sheep, using an intravenous challenge system. Three sheep were vaccinated and 3 served as controls. The vaccinated sheep were not protected against pyogenic conditions. High antitoxin levels were induced by vaccination but could not be associated with protection against infection. Antibacterial antibody levels elicited with initial vaccination dropped progressively with the 2nd and 3rd vaccinations. Nevertheless, these antibodies did not seem to be necessary for protection against A. pyogenes conditions.