SEASONALITY OF INFECTION OF CATTLE WITH BLUETONGUE VIRUSES

A Cox proportional hazards model with time-dependent covariates descri bing the risk of seroconversion of cattle to bluetongue viruses, as a function of climatic variables (maximum temperature, minimum temperatu re and rainfall), was used to investigate the seasonality and geograph ical distribution of bluetongue virus infection in Queensland, Austral ia. Cases of seroconversion in Queensland (which mostly occur in autum n and winter) were associated with summer and autumn temperature and r ainfall at all but one of the 50 locations studied. In far north Queen sland, most cases of seroconversion were associated with temperature a nd rainfall in the summer months, whereas in the remainder of Queensla nd most cases of seroconversion were associated with autumn temperatur e and rainfall, Thus, simulation results suggest that two ecological c ycles of bluetongue virus infection of cattle exist, and may support a hypothesis of differential transmission by vector species in Queensla nd.