Host factors affecting seroprevalence of bluetongue virus infections of cattle

Michael P. Ward From the Departments of Medicine and Epidemiology (Ward, Carpenter) and Pathology, Microbiology and Immunology (Osbum), School of Veterinary Medicine, University of California, Davis, CA 95616.

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 BVSc, MPVM
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Tim E. Carpenter From the Departments of Medicine and Epidemiology (Ward, Carpenter) and Pathology, Microbiology and Immunology (Osbum), School of Veterinary Medicine, University of California, Davis, CA 95616.

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 PhD
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Bennie I. Osburn From the Departments of Medicine and Epidemiology (Ward, Carpenter) and Pathology, Microbiology and Immunology (Osbum), School of Veterinary Medicine, University of California, Davis, CA 95616.

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 DVM, PhD

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Summary

Results of testing of 19,731 samples from a serologic survey of cattle with bluetongue virus (btv) infections in Australia were analyzed for association between age, species, or sex and test result. Bivariate analysis indicated that all 3 host factors were associated with test result. After adjusting for confounding caused by the location of each animal in the study (high, moderate, and low btv prevalence regions), cattle ≥ 4 years old had an odds ratio of 4.33 (95% confidence interval, 3.99, 4.71) for a positive test result, compared with that for cattle < 2 years old. Cattle 2 to 4 years had an odds ratio of 2.28 (2.14, 2.54), compared with cattle < 2 years old. Bos taurus cattle had an odds ratio of 1.76 (1.63, 2.05) of a positive test result, compared with crossbred cattle, and B indicus cattle had an odds ratio of 1.20 (1.09, 1.33), compared with crossbred cattle. Sexually intact (+) male cattle were found to have an odds ratio of 3.13 (2.66, 3.49) for a positive test result, compared with castrated male (−) cattle, and female cattle were found to have an odds ratio of 1.38 (1.29, 1.48), compared with male (−) cattle.

Multivariate analysis of btv testing results was performed, using stepwise logistic regression. The most parsimonious model selected included age, species, and sex factors, and first-order interaction terms between these factors. This model was only able to be fit to data from cattle restricted to the high (> 25%) btv prevalence region. Odds ratios were found to increase with age for male (−) cattle of all species. Odds ratios were found to be greatest at 2 to 4 years of age for female cattle of all species and for B taurus and crossbred male (+) cattle.

Summary

Results of testing of 19,731 samples from a serologic survey of cattle with bluetongue virus (btv) infections in Australia were analyzed for association between age, species, or sex and test result. Bivariate analysis indicated that all 3 host factors were associated with test result. After adjusting for confounding caused by the location of each animal in the study (high, moderate, and low btv prevalence regions), cattle ≥ 4 years old had an odds ratio of 4.33 (95% confidence interval, 3.99, 4.71) for a positive test result, compared with that for cattle < 2 years old. Cattle 2 to 4 years had an odds ratio of 2.28 (2.14, 2.54), compared with cattle < 2 years old. Bos taurus cattle had an odds ratio of 1.76 (1.63, 2.05) of a positive test result, compared with crossbred cattle, and B indicus cattle had an odds ratio of 1.20 (1.09, 1.33), compared with crossbred cattle. Sexually intact (+) male cattle were found to have an odds ratio of 3.13 (2.66, 3.49) for a positive test result, compared with castrated male (−) cattle, and female cattle were found to have an odds ratio of 1.38 (1.29, 1.48), compared with male (−) cattle.

Multivariate analysis of btv testing results was performed, using stepwise logistic regression. The most parsimonious model selected included age, species, and sex factors, and first-order interaction terms between these factors. This model was only able to be fit to data from cattle restricted to the high (> 25%) btv prevalence region. Odds ratios were found to increase with age for male (−) cattle of all species. Odds ratios were found to be greatest at 2 to 4 years of age for female cattle of all species and for B taurus and crossbred male (+) cattle.

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