HOST FACTORS AFFECTING SEROPREVALENCE OF BLUETONGUE VIRUS-INFECTIONS OF CATTLE

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 a nalysis indicated that all 3 host factors were associated with test re sult. After adjusting for confounding caused by the location of each a nimal in the study (high, moderate, and low BTV prevalence regions), c attle greater than or equal to 4 years old had an odds ratio of 4.33 ( 95% confidence interval, 3.99, 4.71) for a positive test result, compa red with that for cattle < 2 years old. Cattle 2 to 4 years had an odd s 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 tes t result, compared with ratio of 1.20 (1.09, 1.33), compared with cros sbred cattle. Sexually intact (+) male cattle were found to have an od ds ratio of 3.13 (2.66, 3.49) for a positive test result; compared wit h castrated male (-) cattle, and female cattle were found to have an o dds ratio of 1.38 (1.29, 1.48), compared with male (-) cattle. Multiva riate analysis of BTV testing results was performed, using stepwise lo gistic regression. The most parsimonious model selected included age, species, and sex factors, and first-order interaction terms between th ese factors. This model was only able to be fit to data from cattle re stricted to the high (> 25%) BTV prevalence region. Odds ratios were f ound to increase with age for male (-) cattle of all species. Odds rat ios 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.