Evaluation of cattle for experimental infection with and transmission of Brucella suis biovar 4

Objective-To determine whether cattle can become persistently infected with Brucella suis biovar 4, whether the organism can be transmitted vertically or horizontally, and whether tests for bovine brucellosis are diagnostic. Design-Observational study. Animals-24 pregnant cows and their calves and 6 bulls. Procedure-Cows and bulls were housed separately in groups of 6 with each group consisting of 3 cattle experimentally infected with 8 suis biovar 4 and 3 naive animals. Cattle were observed for clinical signs daily; blood samples were collected weekly. Clotted blood from each sample was submitted for bacterial culture. Serum was tested with an indirect ELISA and the standard tube agglutination test (STAT), buffered plate agglutination test, brucellosis card test (BCT), and complement fixation test (CFT). Tissues collected at necropsy were submitted for bacterial culture and histologic examination. Results-All 15 inoculated cattle seroconverted on 2 or more serologic tests, and bacteria were isolated from 4 inoculated cows at necropsy. There was no bacteriologic evidence of vertical or horizontal transmission, and none of the cattle developed clinical abnormalities or gross or histologic lesions. Results of the indirect ELISA were positive for all inoculated cattle. The other tests gave variable results; the CFT, STAT, and BCT yielded negative results for at least 1 of the 4 cattle from which the organism was isolated. Conclusions and Clinical Relevance-Results suggest that cattle-to-cattle transmission of 8 suis biovar 4 is unlikely. Serologic tests for bovine brucellosis should be used cautiously when attempting to identify cattle with rangiferine brucellosis, as they do not discriminate between the 2 diseases and vary in their ability to detect exposed cattle.