EVALUATION OF A HARDJO‐POMONA VACCINE TO PREVENT LEPTOSPIRURIA IN CATTLE EXPOSED TO A NATURAL CHALLENGE WITH LEPTOSPIRA INTERROGANS SEROVAR HARDJO

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Abstract
The efficacy of a vaccine against L. i. pomona and L. i. hardjo was evaluated in a group of dairy heifers that were serologically negative at the time of vaccination and later subjected to natural challenge with L. i. hardjo. Heifers (39) were vaccinated twice, at a 1 mo. interval, with a commercially prepared bivalent vaccine; 43 unvaccinated heifers were used as controls. After vaccination, microscopic agglutination titers (MA) of serums to L. i. hardjo ranged from 32-512; titers to L. i. pomona ranged from 32-2048. Titers resulting from vaccination were short-lived; after the 1st vaccination, the serums of 95% of vaccinated heifers did not react in the MA test by 24 wk. The 1st indication of infection in the heifers was noted at wk 6; by wk 16, elevated MA titers (.gtoreq. 128) to L. i. hardjo had occurred in 62% of unvaccinated heifers and had increased to 85% by wk 24. At wk 18, 18% of the vaccinated heifers and 56% of the unvaccinated heifers had leptospiruria (P < 0.01); after 22 wk, 13% of the vaccinated heifers and 58% of the unvaccinated heifers showed evidence of leptospiruria (P < 0.01).