Observations on single brucellosis reactors in large herds

Over a 3 year period specimens were collected from single reacting cattle to the complement fixation test for brucellosis from large herds in north Queensland. Twenty-four such reactors were destroyed for culture, and Brucella abortus was isolated on 3 occasions. The existence of such low levels of Br. abortus in large herds should not be overlooked in the eradication campaign. Explanations were found for 17 of the single reactors, and included introduction of cattle to the herd, exposure to adjoining infected herds or vaccination with Strain 19.