Effectiveness of two commercial infectious bovine keratoconjunctivitis vaccines

Two commercially available infectious bovine keratoconjunctivitis (IBK) vaccines were evaluated for their effectiveness in protecting cattle from disease caused by experimental challenge exposure and natural transmission of Moraxella bovis infections. The study was conducted as 2 experiments, using a total of 81 cattle that were culture-negative for M bovis prior to vaccination. In each experiment, young adult cattle were randomly allotted to 4 groups. Each calf in groups 1 and 2 was vaccinated according to the vaccine manufacturer's directions. Groups 3 and 4 were unvaccinated controls. Three weeks after the last vaccination, each calf in groups 1 and 3 was experimentally challenge exposed by dropping a suspension of viable cells of a virulent strain of M bovis directly onto the corneal surface of each eye. Calves in all 4 groups were then commingled in open pastures so that calves in groups 2 and 4 could be naturally exposed to the calves with experimentally induced infections. Each calf was examined for signs of ocular disease on a regular basis by 2 experienced clinicians who scored each eye for severity of disease on the basis of a prearranged scale. Neither clinician was aware of the vaccination or exposure status of the calf nor to which experimental group they belonged. Lacrimal secretions were collected regularly to determine the number of eyes in which the virulent organism became established. Moraxella bovis with bacterial cultural characteristics similar to those of the virulent strain placed in the eyes of groups 1 and 3 was cultured from > 83% of the eyes of calves in all groups. The incidence of clinical IBK was > 50% in each group. There was no significant difference in the ability of vaccinated calves to resist experimental or natural challenge infection. Compared with that in nonvaccinated controls, no decreased incidence or severity of clinical IBK was noticed in vaccinated calves. Though the challenge strain used was not homologous to those used to prepare the vaccine, it was no different than those that may be expected to cause disease under field conditions.