Volume 12, Number 29: 22 July 2009
In a review paper dealing with the possible impacts of climate change on the spread of infectious diseases, Randolph (2009) notes that it is generally tacitly assumed -- and even explicitly stated -- that climate change will result only in a worsening of the situation, with the expansion of vector-borne diseases into higher latitudes and an increased disease incidence. In fact, she states that implicit in almost all of the literature on this subject -- both popular and scientific -- "is an assumption that environmental change is more likely to strengthen the transmission potential and expand the range, rather than to disrupt the delicate balance between pathogen, vector and host upon which these systems depend."
With this background, the zoologist from the UK's University of Oxford explores the evidence for these two opposing world views via an analysis of what the bulk of the accurately-informed scientific literature on the subject seems to suggest. In doing so, she finds that "the mercurial epidemiology of each vector-borne disease is the system-specific product of complex, commonly nonlinear, interactions between many disparate environmental factors," which include "not only climate but also other abiotic conditions (e.g., land cover) and the physical structure of the environment (e.g., water sources), and further biotic factors such as host abundance and diversity." She additionally indicates that a number of socioeconomic factors drive human living conditions and behaviors that determine the degree of exposure to the risk posed to them, and that nutritional status and concomitant immunity also determine the degree of resistance to infection.
In some interesting examples from the past, Randolph notes that the upsurge of tick-borne diseases within preexisting endemic regions in central and Eastern Europe "appears to be an unforeseen consequence of the fall of the iron curtain and the end of the cold war," which she describes as "a sort of political global warming." Also noted is the fact that "the introduction of the mosquito Aedes aegypti to the Americas within water containers on board slave ships from Africa was repeated four centuries later by the dispersal of the Asian tiger mosquito, A. albopictus, from Japan to the United States within water trapped in used car tires (Hawley et al., 1987; Reiter and Sprenger, 1987).
This phenomenon, according to Randolph, still continues today, augmented by trade in other water-carrying goods such as Asian Luck Bamboo plants, which activities have allowed this mosquito species "to establish itself in almost all New World countries, a dozen European countries, parts of West Africa, and the Middle East." All of these disease expansions, in her words, have "nothing to do with climate change," which also holds true for such chance events as "the introduction of West Nile virus into New York in 1999, most probably by air from Israel (Lanciotti et al., 1999)," and the introduction "of the BTV-8 strain of bluetongue virus into the Netherlands in 2006 from South Africa (Saegerman et al., 2008)."
In concluding her brief treatise, Randolph states that the real-world complexity within each disease system emphasizes that "any expectation of a simple consistent response to climate change, i.e., a universal worsening of the situation, is ill founded," noting further that "there is no single infectious disease whose increased incidence over recent decades can be reliably attributed to climate change." Consequently, she says that "if the purpose of predictions about the future is to guide policy and therefore government spending, exaggerated simplistic rhetoric about the universality and uniformity of the impact of climate change on infectious disease risk is morally indefensible [our italics]," especially "if it distracts public health agencies from more effective ameliorative action targeted at the real causes."
Sherwood, Keith and Craig Idso
References
Hawley, W.A., Reiter, P., Copeland, R.S., Pumpuni, C.B. and Craig, G.B. 1987. Aedes albopictus in North America: probable introduction in used tires from northern Asia. Science 236: 1114-1115.
Lanciotti, R.S., Roehrig, J.T., Deubel, V., Smith, J., Parker, M., Steele, K., Crise, B., Volpe, K.E., Crabtree, M.B., Scherret, J.H., Hall, R.A., MacKenzie, J.S., Cropp, C.B., Panigrahy, B., Ostlund, E., Schmitt, B., Malkinson, M., Banet, C., Weissman, J., Komar, N., Savage, H.M.,Stone, W., McNamara, T. and Gubler, D.J. 1999. Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States. Science 286: 2333-2337.
Randolph, S.E. 2009. Perspectives on climate change impacts on infectious diseases. Ecology 90: 927-931.
Reiter, P. and Sprenger, D. 1987. The used tire trade: a mechanism for the worldwide dispersal of container breeding mosquitoes. Journal of the American Mosquito Control Association 3: 494-501.
Saegerman, C., Berkvens, D. and Mellor, P.S. 2008. Bluetongue epidemiology in the European Union. Emerging Infectious Diseases 14: 539-544.