Weather-Driven Variation in Dengue Activity in Australia Examined Using a Process-Based Modeling Approach

Melanie Bannister-Tyrrell 0 1 Craig Williams 0 1 Scott A. Ritchie 0 1 Gina Rau 0 1 Janette Lindesay 0 1 Geoff Mercer 0 1 David Harley 0 1 0 Epidemiology and Population Health, Australian National University , Building 62, Canberra 0200, Australian Capital Territory, Australia 1 National Centre for Epidemiology and Population Health, and Fenner School of the Environment and Society, Australian National University , Canberra, Australian Capital Territory, Australia ; Sansom Institute for Health Research, University of South Australia , Adelaide, Aouth Australia, Australia ; School of Public Health, Tropical Medicine and Rehabilitation Sciences, James Cook University , Cairns, Queensland , Australia The impact of weather variation on dengue transmission in Cairns, Australia, was determined by applying a process-based dengue simulation model (DENSiM) that incorporated local meteorologic, entomologic, and demographic data. Analysis showed that inter-annual weather variation is one of the significant determinants of dengue outbreak receptivity. Cross-correlation analyses showed that DENSiM simulated epidemics of similar relative magnitude and timing to those historically recorded in reported dengue cases in Cairns during 1991-2009, (r = 0.372, P < 0.01). The DENSiM model can now be used to study the potential impacts of future climate change on dengue transmission. Understanding the impact of climate variation on the geographic range, seasonality, and magnitude of dengue transmission will enhance development of adaptation strategies to minimize future disease burden in Australia. - Dengue is a cosmotropical mosquito-borne virus, affecting 50100 million persons annually, and approximately 40% of the worlds population live in dengue-endemic regions.1 Clinical manifestations range from a nonspecific viral syndrome to severe and life-threatening dengue hemorrhagic fever and dengue shock syndrome.1 The four dengue viruses are transmitted to humans by the bite of infected female Aedes (Stegomyia) mosquitoes, of which Aedes aegypti is the predominant vector.2 Climate influences dengue transmission through impacts on the vector (e.g., growth and development, availability of habitat, survivorship)3 and impacts on the virus (e.g., length of extrinsic incubation period, [EIP]).4,5 Correlations have been found between weather and climate variability and dengue incidence, including distinct seasonal variability,6 El Nin o Southern Oscillation index variability,711 and monthly1214 and weekly15 weather variability. The potential impact of climate change on dengue has been extensively explored and estimated via scenario-based modeling, with prediction of expansion in the geographic distribution of dengue under climate change scenarios.5,1618 Dengue is not endemic to Australia; in far northern Queensland, the only area where dengue occurs, transmission is greatly reduced or absent during the dry season.19 Viremic travelers from dengue-endemic regions, especially Southeast Asia, are the source of frequent introductions.1921 Because Ae. aegypti is established across northeastern Queensland, Australia, well inside theoretical and modeled temperature limits for dengue transmission, there is potential for dengue outbreaks in this region.22 However, dengue outbreaks are largely restricted to major population centers such as Cairns and Townsville, where they have occurred almost annually in the past 15 years.19 Dengue outbreaks did not occur in Australia during 19551981,23 but were previously widespread in eastern Australia, from northern New South Wales to northern Queensland.24 Since 1990, regular outbreaks have occurred, including outbreaks with up to 1,000 reported cases.19,25,26 There is concern that the geographic range and magnitude of outbreaks may increase in Australia as the climate changes.27 The World Health Organization recommends estimating the future burden of disease attributable to climate change by quantifying current climate-disease relationships and making statistical projections under future climate scenarios.28 All local studies of dengue and climate in the Asia-Pacific region have used an empirical approach29 relying on statistical models, in which the input variables and their coefficients are assumed to remain unchanged.17 The mechanistic processes of disease transmission are not explicitly represented, but rather are implicit in statistical models, meaning that the impact of variation in these processes on disease transmission cannot be assessed.30 Process-based models, in contrast to statistical models, explicitly describe each aspect of pathogen transmission and its underlying biological processes, which can be individually influenced by climate change. To examine the effects of weather and other determinants on dengue transmission, a process-based dengue simulation model (DENSiM) (Supplementary Table 1), has been developed that uses daily temperature, precipitation, and humidity to first produce simulations of the vector population (in the entomologic model Container Inhabiting Mosquito Simulation [CIMSiM]).3,31 The DENSiM then combines this information with meteorologic, virologic, demographic, and epidemiologic data to model virus transmission.4 A modified version of DENSiM has been used to investigate global dengue-climate associations.5 Global empirical models of dengue distribution under climate change scenarios have limited validity for Australia because their predictions do not accord with the current or historical distribution of dengue.24 Data in these models may be too coarse to predict the occurrence of dengue in small regions of the continent. There is ongoing debate about the relative contributions of climate compared with social conditions, economic and resource factors, and topographic conditions, to future vector-borne disease risks.22,27,32 Some studies seeking to model dengue at local and regional scales, and using monthly weather variables, have returned weak correlations, and authors have identified the need to incorporate epidemiologic, demographic, and socioeconomic data to improve the predictive capacity of these models.33,34 To improve understanding of dengue-climate relationships at a regional scale, there is a need for dengue models based on local biological, meteorologic, and epidemiologic characteristics. In 20082009, the largest dengue outbreak in Australia since vector control programs were initiated in the 1950s occurred in Cairns, far northern Queensland, with more than 1,000 reported cases.35 This outbreak overwhelmed the capacity of the local Dengue Action Response Team to contain transmission, necessitating provision of additional Queensland State Government resources to achieve control.26 Anomalous weather conditions, the arrival of a dengue-infected traveler to Cairns who did not immediately seek medical attention, and a dengue virus with a reduced EIP may have contributed to the magnitude and force of transmission o (...truncated)