Monitoring of dairy herds for Brucella abortus infection when prevalence is low.

A total of 2,698 dairy herds were surveyed in 1981-1982 in New South Wales and north eastern Victoria in a review of the methods used to monitor them for the presence of Brucella abortus. The methods used to monitor dairy herds were testing of all breeding cows over 1 year of age using the rose bengal test (RBT) and complement fixation test (CFT), the bulk milk ring test (BMRT), and testing of blood samples collected at abattoirs using the RBT and CFT. The surveyed herds had at least one whole herd test, and BMRT was done at regular intervals in the period of the survey. Of the 99 (3.7%) herds that reacted to the BMRT, 91 (3.4%) herds had false positive reactions and 8 (0.3%) herds were declared infected on follow-up herd testing. False-positive reactions were obtained in 22 herds on more than one occasion. Common causes of false positive reactions to the BMRT were thought to be previous vaccination with Strain 19 and sampling in very early or late lactation. Of the 98 (3.63%) herds that reacted to the whole herd serological tests, 80 (2.96%) herds had false-positive reactions and 18 (0.67%) herds were declared infected. Strain 19 vaccination was thought to be an important cause of false-positive reactions. Fifty-three (2.0%) herds showed suspicious reactions on abattoir monitoring but none was declared infected on follow-up testing. Of the 18 herds with infected or equivocal status, the BMRT identified?(ABSTRACT TRUNCATED AT 250 WORDS)