Climatic factors associated with risk of seroconversion of cattle to bluetongue viruses in Queensland

The risk of seroconversion to bluetongue viruses of 464 cattle in 44 sentinel herds in Queensland, Australia, was examined using survival analysis. The association between risk of seroconversion and three climatic variables, mean daily maximum temperature (° C), mean daily minimum temperature (° C), and mean daily rainfall (mm) was assessed. Using stepwise regression techniques, the statistical model explaining the most variation in risk of seroconversion included: maximum temperature 1 and 2 months previously, minimum temperature 2 and 3 months previously, and rainfall 1 and 3 months previously. The model also included an interaction term between maximum and minimum temperature 2 months previously. All variable coefficients in the model except for maximum temperature 2 months previously were significantly (P < 0.001) different from zero. The model predicted that nearly 100% of cases of seroconversion to bluetongue viruses at a typical location in south-east Queensland could be attributed to climatic events occurring in autumn and early winter. Results from this study suggest a causal association between infection of cattle with bluetongue viruses and temperature and rainfall. The lag period identified is most likely related to the effect of temperature and rainfall on vector population dynamics.