Global Health and Foreign Policy Blog

During the first week of December (coincident with the opening of the United Nations Climate Change Conference in Bali) the Institute of Medicine’s Forum on Microbial Threats convened an expert meeting to explore the links between climate change and infectious disease. The event brought together a diverse group of experts from fields such as medicine, public health, ecology, plant science, remote sensing, agriculture, entomology, demography, and public policy to grapple with an important, but somewhat controversial, question within the overall climate change debate: will a changing climate lead to more infectious disease?

In seeking an answer to the general question, presenters tackled multiple perplexing sub-questions, such as: What are the mechanisms by which climate change leads to shifts in disease? How do human social factors interact with the ecological processes leading to shifts in disease? Which countries will be most affected and which will be spared? How potent are the tools that we currently have at predicting, identifying and ameliorating future changes in disease incidence? After two full days of rigorous discussion, it seems fair to say that participants found that clear, convincing answers to these questions were often hard to come by.

There is solid scientific evidence that increasing temperatures can lead to increased transmission of disease, through direct action on infectious agents (e.g. malaria parasites develop in the mosquito more rapidly in higher temperatures), effects on vectors (e.g. greater geographic range and longer active season for mosquitoes, ticks, etc), or changes in host behavior (e.g. shifts in migratory bird patterns). Rainfall and humidity also have proven biological effects. Not all the effects are negative – e.g. influenza virus transmission is reduced in humid, warm environments. In addition, the bulk of the negative effects will be concentrated at the “disease margins” – areas on the border of endemic regions are much more vulnerable to climate change.

While these biological effects can be proven, what is increasingly clear is that such changes will be dwarfed by the interacting variables of future human behavior in reaction to climate changes (urbanization, migration, and agricultural practices), and the pre-existing and future capacity of public health systems. A poor country with a weak health system and a large infectious disease burden will likely have a hard time adapting (or even identifying) relevant changes in disease incidence in the future, leaving it vulnerable to the effects of climate change. In contrast, a rich country with an effective, existing public health system is likely to respond well. Whatever the temperature is in Atlanta in the future, we are unlikely to ever again see endemic malaria there.

Putting yet another layer of perspective on it, one can note that the additional cases of disease likely attributable to climate change as predicted by our best forecasting models represent only a small fraction of the current global infectious disease burden. Why are we worked up about possible climate change-induced disease when we have totally unacceptable levels of disease right now? This viewpoint was captured beautifully by Dr. Donald Burke, the keynote speaker and main author of the National Research Council’s comprehensive 2001 report on climate and infectious disease Under the Weather, when he answered an audience member’s question about what guidance he would suggest for ministers of health in countries at risk for climate-aggravated vector-borne disease problems. Burke responded that countries could reduce and even eliminate vector-borne diseases with existing tools and knowledge, all it would take is adequate financial resources and political will. Thailand could end its dengue problem now, he said, and never have to worry about whether or not climate change will increase dengue in the future.

The point is that with or without climate-change induced changes to disease, we still need to focus on the fundamentals – good health systems, adequate surveillance, predictive capacity, health prevention, laboratory capability, transparency, and effective vector control. Concerns about climate change could raise awareness about weaknesses in these areas and could lead to corrective actions (which would be a welcome development), but a changing climate is will not be a principal determinant of the future global distribution infectious diseases in general. Poverty, lack of adaptive capacity, and human social factors (including trade and travel practices) will play a much bigger role and should be placed at the center of all these discussions of the health effects associated with climate.