An assessment of investigations conducted in the USA on Brucella abortus strain 45/20 bacterins.

The results of five investigations in cattle using 45/20 Bacterins are presented and evaluated. Seemingly contradictory results may be the result of one or more modifications in project design or in procedures. In these trials, there appeared to be no significant difference in the level of protection for cattle when using either intramuscular or subcutaneous routes of injection or when 10 or 12 week intervals between doses were used in cattle 6 months of age or older. Also, the relative resistance to subsequent exposure was not significantly different in which only 45/20 Bacterins were administered. There were apparent differences in protection between cattle receiving one versus two doses of 45/20 Bacterin or initially vaccinated with 45/20 Bacterin at 3 months of age and again 15 weeks later compared to older age groups vaccinated at 10 or 12 week intervals between doses. The disproportionate infection rate among 45/20 vaccinated animals in the last completed investigation suggests that cattle in infected herds subjected to multiple exposures to virulent organisms do not develop adequate resistance. Local and systemic reactions following the administration of 45/20 Bacterin were also noted in some groups.