ArticlesPotential effect of population and climate changes on global distribution of dengue fever: an empirical model
Introduction
There is increasing scientific interest in the potential effects on health of global climate change. One area that has received particular attention is the association between climate variation and vector-borne diseases.1, 2 Dengue fever is the most important viral vector-borne disease in the world.3 The disease affects hundreds of millions of people every year, and is transmitted predominantly by one species of mosquito, Aedes aegypti, which has adapted to living near areas of human habitation. It feeds during the day and prefers human beings to other animals. No effective vaccine or drug treatment for dengue fever is yet available, so management of the disease has relied on vector control measures, such as reduction of breeding sites and use of insecticides. Such measures have succeeded in eradicating mosquitoes in some regions, but have proved difficult to maintain in the long term.
Mosquito-borne disease transmission is climate sensitive for several reasons; mosquitoes require standing water to breed, and a warm ambient temperature is critical to adult feeding behaviour and mortality, the rate of larval development, and speed of virus replication.4, 5 If the climate is too cold, viral development is slow and mosquitoes are unlikely to survive long enough to become infectious. Although a suitable climate is necessary for disease transmission, other factors are needed for an epidemic to take place, including a source of infection, vector populations, and a susceptible human population.
Mathematical models project substantial increases in transmission of vector-borne diseases in various climate change situations.1, 6, 7 However, these models have been criticised on the grounds that they do not adequately account for rainfall,8 interactions between climate variables,9 or relevant socioeconomic factors.10 An empirical model of malaria transmission risks, which accounted for interactions between climate variables,9 predicted little change in the size of the at-risk population by the year 2050.
The decline of malaria in Europe and the southern USA in the 20th century was attributable to social and economic development and improved public health services, and not to change in climate.10 However, this finding does not mean that ambient temperature and rainfall are irrelevant from a health perspective. Climate is one of the fundamental forces behind epidemics, and its effects become evident if adaptive measures falter or cannot be extended to all populations at risk.
Our aim was to describe the current geographical limits of dengue fever transmission on the basis of climate. In an empirical model, we incorporated future projections of climate, to estimate changes in the geographical limits of dengue fever transmission and the size of populations at risk.
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Data
Dengue fever was defined as present if any outbreak of the disease had been reported between 1975 and 1996.10 Every country was a geographic unit, except where more detailed information was available, in which case subnational administrative regions were used.
Population projections were based on the spatial pattern in 1990,11 and region-specific projections for 2055 and 2085.12 Monthly averages of vapour pressure, rainfall, and temperature recorded between 1961 and 1990,13 and global
Results
We showed that annual average vapour pressure was the most important individual predictor of dengue fever distribution. Therefore, we chose a cautious final model that contained humidity as the only explanatory variable, which is in keeping with recommendations of the US Committee on Climate, Ecosystems, Infectious Diseases, and Human Health.2 Vapour pressure proved a significant predictor of dengue fever risk (OR 1·3; SE 0·003; p<0·0001). When areas at risk of dengue fever were defined by a
Discussion
Our findings confirm that the geographical limits of dengue fever transmission are strongly determined by climate. On the assumption that other factors affecting dengue fever transmission remain the same, we forecast that climate change will contribute to a substantial increase in the number of people and proportion of global population at risk of dengue fever. The finding that the baseline distribution of dengue is well predicted on the basis of reported vapour pressure is biologically
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