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 est ablished throughout Queensland, Australia to monitor bluetongue virus infection. Sixteen herds at 12 locations seroconverted to bluetongue v iruses 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 blueton gue virus serotypes 1 and/or 21 were detected in the study; no evidenc e was found of infection with other serotypes previously isolated in A ustralia (3, 9, 15, 16, 20, 23). Seroconversion to serotype 1 occurred more frequently than seroconversion to serotype 21, Seroconversion ge nerally occurred between April and July, being most common in May, but varied considerably within each of the 3 years of the study. This stu dy confirmed the presence of only serotypes 1 and 21 in Queensland and demonstrated the sporadic and restricted nature of bluetongue virus i nfection in the Queensland cattle herd.