The instability of Brucella abortus strain 45/20 and a note on significance of using an unstable rough strain in the diagnosis of bovine brucellosis.

The serum agglutinin titres of six heifers experimentally inoculated with live Br. abortus strain 45/20 were similar to those produced in six bullocks inoculated with virulent Br. abortus strain 544. However, the agglutinin response appeared slowly and disappeared earlier in the heifers than in the bullocks. It is concluded that Br. abortus strain 45/20 is an unstable strain, could revert to the smooth form and become highly agglutinogenic in non-pregnant heifers. The significance of using an unstable rough strain of Br. abortus vaccine in the diagnosis of bovine brucellosis is briefly discussed.