Infection of cattle with bluetongue viruses in Queensland, Australia: results of a sentinel herd study, 1990-1992.

Between 1990 and 1992, 47 sentinel herds of 10-20 cattle each were established throughout Queensland, Australia to monitor bluetongue virus infection. Sixteen herds at 12 locations seroconverted to bluetongue viruses during the study. Herd incidence rates ranged from 0.0 to 3.45 seroconversions per cattle-year at risk. The mean incidence rate was 0.29 seroconversions per cattle-year at risk (95% confidence interval 0.23-0.36), and the median incidence rate was 0.32 seroconversions per cattle-year at risk (95% CI 0.0-0.54). Only seroconversions to bluetongue virus serotypes 1 and/or 21 were detected in the study; no evidence was found of infection with other serotyped previously isolated in Australia (3, 9, 15, 16, 20, 23). Seroconversion to serotype 1 occurred more frequently than seroconversion to serotype 21. Seroconversion generally occurred between April and July, being most common in May, but varied considerably within each of the 3 years of the study. This study confirmed the presence of only serotypes 1 and 21 in Queensland and demonstrated the sporadic and restricted nature of bluetongue virus infection in the Queensland cattle herd.