Immunogenicity of experimental Streptococcus equisimilis vaccines in swine

The protective and complement-fixing antibody immune responses to Streptococcus equisimilis vaccines were evaluated in young, surgically derived, colostrum-deprived swine. Comparable levels of protection against live S equisimilis challenge exposure developed in response to sonic-extract, acid-extract, and whole-cell vaccines combined with incomplete Freund adjuvant. The extract vaccines induced higher levels of complement-fixing antibody than did killed, whole cells. Protection, as well as complement-fixing antibody immune responses, were comparable when swine were given doses of vaccine beginning at 3 or 8 weeks of age.