Effect of dose of Brucella abortus strain 19 in yearling heifers on the relative risk of developing brucellosis from challenge exposure with strain 2308

Richard P. Crawford From the Departments of Veterinary Public Health (Crawford, Richardson) and Veterinary Pathology (Adams), College of Veterinary Medicine, Texas A&M University, College Station, TX 77840.

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L. Garry Adams From the Departments of Veterinary Public Health (Crawford, Richardson) and Veterinary Pathology (Adams), College of Veterinary Medicine, Texas A&M University, College Station, TX 77840.

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Barbara E. Richardson From the Departments of Veterinary Public Health (Crawford, Richardson) and Veterinary Pathology (Adams), College of Veterinary Medicine, Texas A&M University, College Station, TX 77840.

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SUMMARY

Yearling heifers were given SC injections of 108 (n = 40), 109 (n = 44), or 1010 (n = 44) colony-forming units of Brucella abortus strain 19 (S19). The proportion of heifers with positive serologic test results at 1 month following vaccination increased as the dose of S19 increased. These proportions decreased with time, and all heifers had negative card, rivanol, and complement fixation test results within 4 months. Positive elisa results persisted beyond 4 months in all three S19 dose groups; however, all heifers were elisa-negative within 9 months after vaccination. Comparable lymphocyte transformation activity was stimulated by S19 doss of 109 or 1010 and approximately half of the heifers in both groups had a positive stimulation index at 9 months.

Immunity of the pregnant heifers was challenged 9 months after vaccination with 107 B abortus strain 2308 as follows: diluent controls (n = 69); 108 B abortus S19 (n = 40); 109 B abortus S19 (n = 39); and 1010 B abortus S19 (n = 39). Tissue specimens from heifers were obtained at parturition and necropsy for culturing of B abortus. The proportion of heifers that developed brucellosis, ie, had positive culture results, increased as gestation days at challenge exposure increased. The effect of gestational age was controlled in the analysis using logisic regression. The relative risk of brucellosis was reduced to 0.38, 0.15, and 0.06 for B abortus S19 doses of 108, 109, and 1010, respectively, compared with diluent controls at 1. Risk of brucellosis in heifers vaccinated with 108, 109, or 1010 S19 was, respectively, one-third, one-seventh, or one-seventeenth that of diluent controls or nonvaccinated heifers.

SUMMARY

Yearling heifers were given SC injections of 108 (n = 40), 109 (n = 44), or 1010 (n = 44) colony-forming units of Brucella abortus strain 19 (S19). The proportion of heifers with positive serologic test results at 1 month following vaccination increased as the dose of S19 increased. These proportions decreased with time, and all heifers had negative card, rivanol, and complement fixation test results within 4 months. Positive elisa results persisted beyond 4 months in all three S19 dose groups; however, all heifers were elisa-negative within 9 months after vaccination. Comparable lymphocyte transformation activity was stimulated by S19 doss of 109 or 1010 and approximately half of the heifers in both groups had a positive stimulation index at 9 months.

Immunity of the pregnant heifers was challenged 9 months after vaccination with 107 B abortus strain 2308 as follows: diluent controls (n = 69); 108 B abortus S19 (n = 40); 109 B abortus S19 (n = 39); and 1010 B abortus S19 (n = 39). Tissue specimens from heifers were obtained at parturition and necropsy for culturing of B abortus. The proportion of heifers that developed brucellosis, ie, had positive culture results, increased as gestation days at challenge exposure increased. The effect of gestational age was controlled in the analysis using logisic regression. The relative risk of brucellosis was reduced to 0.38, 0.15, and 0.06 for B abortus S19 doses of 108, 109, and 1010, respectively, compared with diluent controls at 1. Risk of brucellosis in heifers vaccinated with 108, 109, or 1010 S19 was, respectively, one-third, one-seventh, or one-seventeenth that of diluent controls or nonvaccinated heifers.

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