Brucellosis serology: reduced dose S19 vaccination of yearling heifers versus the use of the standard dose at 5-7 months of age in a clean herd.

In the 427 heifers vaccinated at 12 to 14 months of age with the reduced dose (3 X 10(8) to 1 X 10(9) viable organisms per dose) 3 had antibody titres in the complement fixation test of 196, 688 and 748 IU ml-1, respectively, at 5 months post-inoculation. At 23 to 25 months of age 8/128 (6.3%) of these heifers had rose bengal test (RBT) reactions, at least 6/128 (4.7%) a serum agglutination test (SAT) antibody titre in excess of 30 IU ml-1 and 1/128 had a suspicious reaction in the complement fixation test (CFT) of 30 to 49 IU ml-1. In the 116 heifers inoculated with the standard dose (4 to 12 X 10(10) viable organisms) at 5 to 7 months of age and tested at 22 to 24 months of age, a reactor rate of 37/116 (31.9%) in the RBT was seen. At least 34/116 (29.3%) had antibody titres in excess of 30 IU ml-1 in the SAT. Only 1 animal had a suspicious reaction in the CFT of 18 to 24 IU ml-1. No abortions could be attributed to the vaccine strain and no other isolates of the vaccine strain were made. The possible role of vaccine residues in syringes as a cause of persistent reactions is discussed. The use of disposable syringes is recommended. On serological grounds there appears to be little advantage in using the reduced dose vaccine in yearling heifers as opposed to the standard dose vaccination of 5 to 7 month old heifers.