New study on malaria and climate change

Last week, Nature published an important new study by Peter Gething and colleagues from Oxford University, examining changes in the global distribution of malaria over the past century1.   The findings suggest that during the past 100 years, the protective effects of socioeconomic development and disease control have been significantly greater than the transmission-enhancing effects of increasing temperatures.  The study therefore builds on and supports previous qualitative reviews, as well as analyses of the effect of climate and other factors on long term trends in malaria transmission over smaller areas2.

The study does not directly exonerate climate from a role in malaria transmission, either in the past or in the future.  Indeed, the results of other studies by the same group suggest that, compared to a situation where climate remained stable, human-induced climate change will tend to increase malaria risks for a large number of people in the least-protected regions3.  It does, however, provide important context, by indicating that future development and disease control programmes, if maintained and expanded from their present scope, could have considerably larger protective effects.

The paper relates the findings to previous modelling studies of the effects of climate change on the present burden and future risks of malaria, and their use in assessments by WHO and other agencies.  The WHO assessment does attribute a small proportion of the current malaria burden to the more favourable climate conditions associated with global warming.  In doing so, however, it takes into account that this existing burden has already been strongly affected by non-climatic factors – so the results are not necessarily in contradiction.

Initial reactions by other scientists also suggest that this will not be the final word on the analysis itself.  In the past, some of the study’s authors have criticized other researchers for relying on simple models of malaria which do not account for the complexities of local disease transmission. Yet here they use models of the global retreat of malaria (mainly reductions in Plasmodium vivax at high latitudes) to draw conclusions about current and future malaria burdens – which are overwhelmingly a function of the very different transmission dynamics of Plasmodium falciparum in Africa.  Scientists may also question why the study looked only at the effects of future temperature increases, since changes in rainfall may be more important4.

The most important question, however, is not whether the research is perfect, but what it means for policy and practice.  The authors main conclusion is that concern over climate change should not divert malaria control programmes from their current efforts to control the disease.  While there is no evidence that this distraction is occurring, it does strengthen the case made by WHO and others, that increased risks from climate change should be addressed through existing control programmes rather than as a stand-alone activity, and need to be supported by additional resources, rather than existing health budgets.

As is often the case in studies relating to climate change, subsequent coverage is likely to both amplify and distort the research findings.  While the authors aim to focus attention back on to the basics of malaria control, it is telling that this study has been heavily covered, while their more applied work in mapping current malaria distributions goes effectively unreported.  Some are also apparently using this work as part of a general opposition to climate science and policy, for example claiming that models based on climate change, and previous IPCC reports, have led to disease control efforts being diverted away from malaria in Africa. For its part, WHO argues that the wide range of health risks posed by climate change call for more, rather than less, investment in the kind of preventive disease control measures highlighted in this study.

References:

1. Gething PW, Smith DL, Patil AP, Tatem AJ, Snow RW, Hay SI. Climate change and the global malaria recession. Nature;465(7296):342-5.

2. Kuhn KG, Campbell-Lendrum DH, Armstrong B, Davies CR. Malaria in Britain: past, present, and future. Proc Natl Acad Sci U S A 2003;100(17):9997-10001.

3. Hay SI, Tatem AJ, Guerra CA, Snow RW. Foresight on population at malaria risk in Africa: 2005, 2015 and 2030. Scenario review paper prepared for the Detection and Identification of Infectious Diseases Project (DIID), Foresight Project, Office of Science and Innovation. London, UK, 2006: 40.

4. Small J, Goetz SJ, Hay SI. Climatic suitability for malaria transmission in Africa, 1911-1995. Proc Natl Acad Sci U S A 2003;100(26):15341-5.

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