Reactions to heartwater vaccination in crossbred Zebu cattle.

One thousand and ninety-four crossbred Zebu cattle were immunized against heartwater with a sheep-blood vaccine containing the Ball 3 strain of Cowdria ruminantium. Animals experiencing febrile reactions were treated at various stages of the reaction with tetracycline. Four hundred and sixty-two (42.2%) reacted, six (0.6%) of which died. Deaths were significantly less frequent in cattle treated on the first day of reaction (2/323, 0.6%) than those treated at a later stage (4/61, 6,6%). The mean incubation period was 15,6 d (range 7-23 d). Incubation period and frequency of reactions varied significantly between farms, between vaccine batches and between vaccine doses. Variations in frequency of reactions, incubation period and severity of reaction did not affect the efficacy of immunization, as assessed by seroconversion.