Climate change and the distribution and intensity of infectious diseases

Many infectious diseases of humans, including ma-laria, dengue, cholera, and schistosomiasis, are restrictedto, or more prevalent in, tropical and subtropical zones.Within the tropics and subtropics, they are moreprevalent at lower than at higher altitudes. Warmertemperatures characteristic of lower latitudes andaltitudes generally increase rates of survival, develop-ment, and replication of parasites and of blood-feedingvectors such as mosquitoes. Warmer conditions alsoincrease activity (including biting) rates of vectors,resulting in higher rates of parasite transmission(reviewed by Harvell et al. 2002). A series of papers inthe 1990s (e.g., Shope 1991, Martens et al. 1995, Colwell1996, McMichael et al. 1996, Patz et al. 1996) contendedthat recent and future trends in climate warming werelikely to increase the incidence and geographic distribu-tion of infectious diseases, particularly those caused byvector-borne and water-borne parasites and pathogens.Owing to media attention and popular concern, thespread of infectious diseases was featured in the growinglist of negative outcomes known or anticipated to arisefrom anthropogenic climate change (e.g., Intergovern-mental Panel on Climate Change 2001; the 2006documentary lm, An Inconvenient Truth). However,unequivocal demonstrations of a causal link betweenclimate change and human infectious diseases are rare(albeit increasing). Some diseases are likely to decreasein incidence and range with climate warming (Harvell etal. 2002), and others are likely to respond to precipita-tion or humidity more than to temperature, leading topoor predictive power under warming scenarios. Manydiseases are strongly inuenced by other ecological,sociological, economic, and evolutionary factors besidesclimate change. These latter observations have stimu-lated the emergence of critics of a climate-change–infectious-disease linkage. Lafferty (2009) provides anoverview of recent criticisms, emphasizing three majorcategories: (1) In many cases, we should expect diseasesto shift geographically without net expansion underclimate change; (2) non-climatic factors are moreimportant than climate; and (3) models that predictincreasing disease transmission with climate warmingare awed if transmission rates fail to exceed a specicthreshold (R