Haemophilus somnus infections. II. A Canadian field trial of a commercial bacterin: clinical and serological results.

To evaluate the efficacy of a Haemophilus somnus bacterin a total of 1114 beef calves or yearling bulls were used in the province of Saskatchewan. The six herds were included in a vaccination trial in which the vaccine was administered subcutaneously in one or two doses. The prevalence of H. somnus in nasal swabs of these animals at the time of the initial vaccination was 0.35% Postvaccination information on morbidity and mortality for a four month period was requested from the five ranches where there were nonvaccinated control calves. Postvaccination outbreaks of infectious thromboembolic meningoencephalitis occurred in two of these herds and, although the numbers were limited, there was a trend to reduced morbidity and mortality in the vaccinated animals compared to controls. Seroconversion rates, as determined by the complement fixation test, in the six herds were 28.3% for control calves, 57% for animals vaccinated once and 80.3% for animals vaccinated twice. On the basis of these results the bacterin was considered to be sufficiently efficacious to warrant its further evaluation under field conditions.