Immunisation against ovine caseous lymphadenitis: correlation between Corynebacterium pseudotuberculosis toxoid content and protective efficacy in combined clostridial‐corynebacterial vaccines

Groups of sheep were dosed with vaccines containing Corynebacterium pseudotuberculosis toxoid combined in varying amounts with 5 clostridial antigens. Resistance of the sheep to infection with C pseudotuberculosis was tested at 1, 6 and 12 months after vaccination by infection with pus from ovine lymph glands actively infected with C pseudotuberculosis. The outcome was assessed 3 months after challenge by slaughter and inspection of the sheep for caseous lymphadenitis lesions. Protection was demonstrated by a significant reduction in the proportion of immunised sheep exhibiting lesions compared with control sheep, and by fewer abscesses in affected immunised sheep than in affected control sheep. A positive correlation was found between amount of C pseudotuberculosis toxoid administered and degree of protection obtained. Chromatographically-purified toxoid induced essentially the same protection, suggesting that anti-toxic immunity is the major factor in protection.