The sensitizing effect of a Brucella abortus antigen in cattle after repeated intradermal inoculations

An antigen prepared from a mucoid strain of B. abortus was repeatedly injected intradermally into cattle to determine whether it sensitizes cattle so that they test false positive with the skin delayed-type hypersensitivity (SDTH) test. Cattle (n = 14) that were experimentally inoculated with microorganisms that serologically cross-react with B. abortus, and uninfected cattle (n = 15) were tested with the antigen 2 to 5 times over a period of 16 to 17 weeks. An increase in skinfold thickness of greater than or equal to 2.0 mm on the injection site was considered a reaction elicited by the antigen. The sera from the cattle were tested with the serum agglutination test, complement fixation test, and an enzyme-linked immunosorbent assay for antibodies against B. abortus. Results showed that none of the animals had an increase in skinfold thickness of greater than or equal to 2.0 mm on the injection site of the antigen, which is considered a positive reaction. However, cattle experimentally inoculated with microorganisms other than B. abortus tested seropositive for brucellosis after one SDHT test only. We conclude that the B. abortus antigen inoculated intradermally does not sensitize cattle after repeated inoculations, and therefore does not interfere with the subsequent use of the SDTH test in the diagnosis of brucellosis.