The effect of climate change on the spread of predicted bluetongue in Australian livestock

IntroductionThe frequency of vector-borne disease in human and animal populations has increased in recent years leading to concerns that even greater increases will occur as a result of climate change, driven by changes in the geographic distribution of insect vector habitat areas. In this study we investigate the effect of climate change on the expected spread of bluetongue (a viral disease of ruminants spread by Culicoides midges) using the Australian Animal Disease Spread model (AADIS). MethodsEstimates of average daily temperature across Australia for 2015 were obtained from the Australian Bureau of Meteorology. Predicted average daily temperatures using the CanESM2 model (emission scenario RCP 8.5) for 2025 and 2035 at a resolution of 5 km x 5 km were obtained from the Australian Climate Futures decision-support tool, managed by CSIRO. Two study areas in Australia were selected: the first in North Queensland and the second in Northern New South Wales. A total of 24 outbreak scenarios were run: mid-summer and mid-winter incursions for each study area for 2015, 2025 and 2035 with direct movement of animals in the AADIS model disabled and enabled for each. Model results were expressed as the number bluetongue-positive herds, (herds in which at least one animal was BTV positive) at the end of each 365 day simulation period. ResultsFor North Queensland, there was little change in the median predicted number of bluetongue positive herds for mid-summer and mid-winter 2025 and 2035 incursions (compared with 2015) and a moderate increase in the variability of predicted outbreak sizes when direct animal movements were disabled. For Northern New South Wales there were moderate increases in both the predicted number of bluetongue positive herds and the variability of predicted outbreak sizes for 2025 and 2035, compared with 2015. Compared with the direct animal movement disabled scenarios, there were marked increases in the predicted number of bluetongue positive herds as a function of simulation year for North Queensland. For Northern New south Wales this trend was not as distinct, but as for the direct movement disabled scenarios, the variability of predicted outbreak sizes for the 2035 incursions were greater than the variability of predicted outbreak sizes for the 2015 incursions. ConclusionClimate change will result in a greater portion of the land area of Australia with conditions suitable for Culicoides midges. Our findings show that under conditions of climate change and an outbreak of virulent bluetongue in Australia, the rapid imposition of effective restrictions of animal movement will be the single most important control measure to limit further spread of disease.