Experimental infectious bovine keratoconjunctivitis: efficacy of a vaccine prepared from nonhemolytic strains of Moraxella bovis.

Calves vaccinated with a bacterin prepared from 4 nonhemolytic strains of Moraxella bovis were challenge exposed with a virulent hemolytic strain of M bovis. Ten calves were given 2 vaccinal inoculations 21 days apart. Twenty-one days after the last vaccinal inoculation, the eyes of the 10 vaccinated and the eyes of the 12 non-vaccinated calves were exposed to a hemolytic heterologous strain of M bovis. The percentage of eyes with infectious bovine keratoconjunctivitis was lower, the lesions were less severe, and the duration of infection was shorter in vaccinated calves than in nonvaccinated calves. Seemingly, the resistance induced by nonhemolytic strains of M bovis was at least comparable with the induced by hemolytic virulent strains of that organism. The nature of the immune response in vaccinated calves is discussed.