Agglutination of Brucella abortus cells by sera from cattle experimentally infected with Escherichia coli

Infection of normal adult cattle and sporadic serological Brucella abortus reactors, which no longer had diagnostic titers to B. abortus, with a culture of Escherichia coli isolated from a cow (or its environment) resulted in the production of a primary IgM serum antibody response to both B. abortus and E. coli. Cattle injected with heat killed E. coli and guinea pigs or young calves lacking natural agglutinins to B. abortus injected with live E. coli, produced serum antibody only to E. coli. The subsequent reinjections of live E. coli into the former two groups of cattle resulted in all but one animal in each group producing IgM ‘secondary’ responses to B. abortus of decreasing magnitude, while the anti-E. coli responses increased and eventually switched to synthesis of IgG class antibody. The remaining two animals produced a series of ‘secondary’ responses of IgM antibody to B. abortus similar in amplitude and duration to the primary immune response. The anti-E. coli agglutinins of these two animals increased in titer and IgG class antibody to E. coli was evident after several injections of E. coli. These results indicate that infection of cattle by E. coli can cause a problem in the serological fiagnosis of B. abortus infection. Speculations on the cause of this cross-reaction and ways of minimizing misdiagnosis are discussed.