No abstract

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.

The indirect haemolysis test (IHLT) and the rough antigen complement-fixation test (RCFT) were compared with several conventional tests using serum samples from 9 cows known to be infected with Brucella abortus. In 7 cows all the tests except the RCFT (which was developed to detect antibodies resulting from 45/20 vaccination) became positive and remained so until the cows were autopsied 6 months after infection. In the other 2 cows the Rose Bengal test was occasionally negative and the titre of the serum agglutination test only reached 100 iu briefly. The routine complement-fixation test, the IHLT and the anti-bovine globulin test reached diagnostic levels at most of the weekly test intervals but occasional low titres occurred in them all. Only very low or negative titres were detected by the RCFT.

The complement fixation test (CFT) and indirect hemolysis test (IHLT) were applied to sera collected from 60 cattle challenged with Brucella abortus 544. Of the 60 cattle, 48 were vaccinated with either B. abortus 19 or B. abortus 45/20 as calves or as adults. The remaining 12 cattle were not vaccinated. Of the 27 sera from cattle found to be infected, 9 showed aberrant reactions to the CFT. The advantages of the IHLT for these cattle were as follows. After challenge, the titers to the IHLT became positive earlier than or at the same time as the titers to the CFT, they persisted longer than the titers to the CFT, and they failed to show prozone reactions, which are a problem with the CFT. An additional advantage was that before challenge and after vaccination with strain 19, the titers to the IHLT rose later and declined earlier than the titers to the CFT. We concluded that the CFT used in conjunction with the IHLT improves the detection of infected cattle.

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.