Immunization of mice with a potassium thiocyanate extract of type A (bovine strain) imparted complete protection against an experimental challenge infection with 1.6 × 10 colony-forming units of the homologous strain. Formalin-treated vaccine (one or two doses) was much less effective. Significantly higher bactericidal antibody titres were observed in the sera of mice immunized with the extract compared with those given the vaccine. The bactericidal activity was localized in the IgM and IgG fractions. The extract consisted of protein(s) and carbohydrate(s) and on immunoelectrophoresis displayed at least three antigenically distinct components.


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