Brucella abortus (Strain 19) vaccine: (a) Determination of the minimum protective dose in cattle; (b) The effect of vaccinating calves previously inoculated with anti- Brucella abortus serum

Abstract

Four groups of ten heifer calves (aged 3–6 months) were vaccinated (s.c.) with the following doses of Brucella abortus S19 vaccine: one full dose (two groups), 0·04 dose, and 0·001 dose respectively. One of the groups of calves inoculated with a full dose of vaccine had previously received (i.v.) 300 ml (5720 i.u. (SAT) per ml) bovine anti- Brucella abortus serum. The heifers were challenged at 13–16 months of age (10 months after vaccination). The results showed that the group inoculated with one full dose of vaccine and with 0·04 dose were well protected, whereas the group inoculated with 0·001 dose showed little protection. The minimum protective dose of Br. abortus S19 vaccine given subcutaneously to 3–6-month-old calves thus appears to be between 4·5 × 19 9 viable units and 9 × 10 7 viable units. The calves vaccinated with one full dose of S19 vaccine subsequent to receiving hyperimmune serum showed a considerably diminished serological response and little or no protection to challenge. It thus seems that passive circulating antibody tends to suppress both the serological and immune protective responses to vaccination.