Groups of female calves were vaccinated subcutaneously with the standard dose of Brucella abortus strain 19 (S19) or with B. abortus 45/20 (S45/20). These calves and non-vaccinated control calves were mated at 15 months of age and challenged by way of the conjunctival sac with B. abortus strain 544 (S544). The incidence of abortion, stillbirths, weanling calves and healthy calves was observed after challenge and specimens were collected for culture at parturition and slaughter. Fifteen healthy calves were born to 18 animals vaccinated with S19, 12 were born to 18 animals vaccinated with S45/20 and 2 were born to 8 animals that were not vaccinated. B. abortus was isolated from 5 of the animals vaccinated with S19, 13 of the animals vaccinated with S45/20 and 9 of the 12 animals that were not vaccinated. Only one of the 5 infected animals vaccinated with S19 was vaccinated as an adult.
Sixty cows, 48 of which had been vaccinated with live Brucella abortus strain 19 (S19) or with killed B. abortus strain 45/20 (S45/20) and 12 of which were unvaccinated animals, were challenged with B. abortus strain 544. Ten of the 27 cattle found to be infected after challenge showed aberrant serological reactions to the Rose Bengal Plate test, serum agglutination test and/or complement fixation test. These 10 cattle were all previously vaccinated with S19 or S45/20. It was concluded that infection in cattle vaccinated with S19 or S45/20 may be more difficult to detect than infection in animals that have no history of vaccination.
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