SIMULATION MODELING OF THE EFFECT OF CLIMATIC FACTORS ON BLUETONGUE VIRUS-INFECTION IN AUSTRALIAN CATTLE HERDS .2. MODEL EXPERIMENTATION

Using a state transition model based on matrix formulation, the effect of climatic factors on bluetongue virus infection was investigated in a simulated cattle herd in Queensland, Australia, Changes in spring r ainfall had the most influence on predicted prevalence of infection, a nd changes in seasonal minimum temperature had more effect on predicte d prevalence than changes in seasonal maximum temperature. Four medium -term (i.e. 25-30 years) climatic scenarios were simulated: warm, wet spring and autumn seasons, and dry, cool spring and autumn seasons. Th e predicted prevalence of infection was suppressed approximately four- fold when a series of dry, cool autumn seasons were simulated, compare d with the other scenarios. Occurrence of dry, cool autumn seasons at least once every 4 years or less had the greatest effect on predicted prevalence of bluetongue virus infection. Results suggest that drought conditions in Australia may affect the endemic stability of bluetongu e virus infection. Instability in the system could lead to cyclical ep idemics of infection, may explain previously observed variability in b luetongue virus infection, and might lead to outbreaks of bluetongue d isease in commercial sheep flocks in Australia.