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SUMMARY: The protective effect of 2 Moraxella bovis pili vaccines against infectious bovine keratoconjunctivitis (IBK) experimentally induced by homologous or heterologous strain challenge with virulent, haemolytic M. bovis strain, Dal 2d, was measured in trials using weaned calves aged 3 to 7 months. Purified pili vaccines were prepared from haemolytic strain Dal 2d, (pilus serogroup IV), and haemolytic strain Epp 63, (pilus serogroup 111). Calves were challenged by conjunctival instillation of 1 × 109 colony forming units of virulent M-bovis strain Dal 2d 14 days after the second of 2 subcutaneous doses of vaccine. Each consisted of 200 pg of pili in alum-oil adjuvant administered at an interval of 21 days.

In trial 1 the level of protection against challenge with the homologous strain was 46.7% (p < 0.01). Small, rapidly resolving lesions of IBK occurred in some vaccinates compared with a larger proportion of severe lesions that required treatment in non-vaccinated calves (p < 0.025). In trial 2, the level of protection against IBK after exposure of vaccinates to the homologous Dal 2d strain was 72.7%, but no significant level of protection or reduction in the size and duration of lesions was apparent in similarly challenged calves vaccinated with Epp 63 pili when contrasted with susceptible, non-vaccinated controls. No marked reduction in the duration of infection with M. bovis Dal 2d following challenge resulted from vaccination with pili of either 01 the serogroups III or IV. Rising homologous serum IgG antibody titres to serogroups III and IV pili were recorded in response to vaccination with each antigen. Serum and lachrymal IgG titres showed significant positive correlation (p < 0.05) and lachrymal IgG and IgA responses to homologous or heterologous serogroup antigens were also positively correlated (p < 0.001). Mean cross-reactivity of serum IgG, lachrymal IgG and IgA titres was found to be 8%, 9% and 18% respectively. The post-booster, pre-challenge IgG antibody titres of trial 1 calves which were vaccinated with Dal 2d pili and which showed some susceptibility to IBK, were significantly lower (p < 0.05) than those of the totallyresistant calves in trial 2 that had been vaccinated with this antigen.