A study of cattle infected with Brucella abortus and which showed aberrant serological reactions.

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.