The influence of maternal antibody and age of calves on effective vaccination against Leptospira interrogans serovar hardjo

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Abstract
Twelve seronegative cows were vaccinated with an experimental bivalent Leptospira interrogans serovars hardjo and pomona vaccine late in their first pregnancy. Calves born of these dams were divided into 4 equal groups that received this vaccine at 4, 6, 10 and 18 weeks of age, respectively. Before vaccination the group geometric mean titres of maternally-derived circulating antibodies ranged from 2 to 25 for the microscopic agglutination (MA) test and 3 to 35 for enzyme-linked immunosorbent assay (ELISA) using a serovar hardjo outer envelope antigen. Post-vaccination peak titres were 645 to 1612 for MA and 562 to 1037 for ELISA, respectively. Calves vaccinated at the youngest age, had the highest pre-vaccination circulating maternal antibody titres, but showed the smallest rise in post-vaccination antibody titres. Circulating maternal antibody was detected in calves up to 13 weeks of age. All immunised calves were protected against a virulent challenge with serovar hardjo type Hardjobovis, regardless of their age or maternally-derived antibody titres. These findings indicate that calves as young as 4 weeks old, vaccinated in the presence of maternally-derived antibody, can be fully protected against homologous virulent challenge.