Climate Change and Vectorborne Diseases
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Abstract
T he effects of climate change are widespread and rapidly intensifying and are largely driven by greenhouse-gas emissions from burning fossil fuels. 1 Global mean temperatures have already increased by 1.1°C since 1900, 1 with most of the change having occurred in the past 50 years.The extent of change is most extreme in highland and polar regions (Fig. 1), and temperatures in tropical regions are creeping closer to the thermal limits of many organisms.Given the current policies and actions, a warming of 2.5°C to 2.9°C or more by the end of this century is expected. 2arming and other manifestations of climate change -including changes in precipitation, with increased flooding in some areas and drought in others -have important implications for vectorborne diseases through their effects on pathogens, vectors, and hosts, as well as on our ability to prevent and treat these diseases (Fig. 2).Yet attributing changes in the distribution and frequency of vectors and diseases to climate change is challenging because other factors, including land-use changes, 3 the abundance of reservoir hosts, 4 and control measures, 5 also contribute to these changes.Furthermore, it may be difficult to distinguish between natural climate variability and human-influenced change, 6 although scientific techniques to do so are emerging.Despite these complexities, it is clear that the components of vectorborne disease systems, including pathogens, vectors, and reservoir hosts, are highly responsive to the varied environments they inhabit and that observed changes in the rates of vectorborne diseases at given locations are often associated with concomitant changes in the local climate.For example, warming temperatures affect the behavior, physiologic characteristics, and life history of both vectors and pathogens as well as the abundance and behavior of reservoir hosts and definitive hosts.The interactions among temperature, vector, and pathogen can change the risk of human-to-human disease spread and of spillover to humans from reservoir hosts.Thermal performance curves illustrate the ways in which temperature affects the physiological traits of pathogens, vectors, and reservoir hosts, which determine the rate of disease spread in a susceptible population.These curves are commonly used to predict the potential effects of rising temperatures resulting from climate change on vectorborne disease systems. 7Curves for individual components of a disease system must overlap in order for transmission to occur.Thermal adaptation, acclimation to a warming climate, or both can potentially shift thermal performance curves and thermal tolerance limits, with important implications for expansion of the geographic range of certain diseases.Depending on their ability to adapt, vectors may no longer carry certain pathogens or may carry new ones as climate-mediated ecosystem changes bring different pathogens, vectors, and reservoir and human hosts together.