Serologic response of vaccinated cattle to strains of Moraxella bovis isolated during epizootics of keratoconjunctivitis.

In studies to determine whether there were antigenic differences between strains (isolates) of Moraxella bovis, the sera from vaccinated calves were tested with isolates of M bovis while the calves were experiencing epizootics of infectious bovine keratoconjunctivitis (IBK). Before the epizootics of IBK, the calves were intramuscularly vaccinated with a formalin-killed autogenous M bovis bacterin. During the epizootics, the eyes were examined by cultural technique, and isolates which were obtained were categorized by catalase activity, source (diseased or nondiseased eyes), and reactivity with the various sera. The serum reactivity of the isolates was compared with that of the vaccinal strain. The vaccinal strain and 8 of the 1 5 selected isolates obtained during the 1974 epizootic were catalase negative. Seven of the 15 isolates from the 1974 epizootic and all of the selected isolates from the 1975 epizootic were catalase positive. A significantly higher (P less than 0.01) percentage of calf sera were serologically reactive with the vaccinal strain and other catalase-negative isolates (45.0%) than with catalase-positive isolates (34.8%). The results, although not definitive, suggest that there may be antigenic differences among strains of M bovis. These differences should be considered when cattle are vaccinated against IBK under natural conditions of exposure.