Infectious bovine keratoconjunctivitis: effects of vaccination on Moraxella bovis carrier state in cattle.

A study was conducted to determine whether vaccination of cattle while they were undergoing an acute episode of infectious bovine keratoconjunctivitis (IBK), would cause vaccinated cattle to abort Moraxella bovis infection sooner than nonvaccinated cattle. Fourteen calves were allotted into two groups of seven calves each, and the eyes of each calf were exposed to a virulent culture of M bovis. Twenty days after calves were infected and showing signs of IBK, seven calves were vaccinated with M bovis pilus vaccine made from the exposure strain. Ocular and nasal discharges were collected and examined for M bovis for 64 days. Most calves developed signs of IBK after exposure and all but one calf (nonvaccinated) developed ocular infection with M bovis. The mean number of days (33 and 33.3, respectively) of ocular infection was not significantly (P = 0.05) different in the vaccinated calves than in the nonvaccinated calves. The eyes of the calf that did not become infected with M bovis were infected with nearly a pure culture of Escherichia coli. This calf failed to develop ocular infection, but had M bovis in its nasal discharge throughout the study (64 days). Moraxella bovis was isolated from the nasal discharge of other calves only when there were concurrent ocular infections.