A comparative assessment of culture and serology in the diagnosis of brucellosis in dairy cattle

To investigate the usefulness of culture for the confirmation of brucellosis in cattle, a comparison of culture and serology was undertaken on 248 animals in four dairy herds where the disease was active. Paired supramammary (SM), retropharyngeal (RP), and internal iliac (IL) lymph nodes were cultured, and five serological tests were deployed: the microserum agglutination test (MSAT), complement fixation test (CFT), the indirect (iELISA) and competitive ELISA, and the fluorescence polarisation assay (FPA). Brucella abortus was isolated from 86.8% of animals on combined culture of all three lymph nodes. Individually, the highest isolation rate was from the RP (90.5% of culture positives). Of culture positive animals, 13.7% and 6.2% were positive from the RP and SM alone, respectively.Approximately half of the positive cultures yielded <10 colonies/culture plate. Although 80.9% of animals were positive in at least one serological test, only 45.2% were positive in all five. For culture-positive animals, the MSAT was the most sensitive test (71.8%). Of the culture-negative animals 67.7% were positive in at least one test, while 12.9% were positive in all five. Titres were higher in animals culture-positive from the SM, and there was a direct correlation between higher titres and higher colony counts in SM cultures. Only 8.9% of animals were both culture-negative and seropositive (in at least one test), while 16.5% were culture-positive and seronegative in all five tests. The results highlight and validate the sensitivity of bacteriological culture in confirming a diagnosis of bovine brucellosis. While the MSAT and FPA were the most sensitive serological tests, a significant percentage of infected animals were undetectable using these standard serological assays.