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Climate & malaria

March 22, 2010 - 13:51 -- Sander Koenraadt

These days, not only scientists are debating about the existence and potential impact of climate change. Since the mishaps in the fourth assessment report of the IPCC were revealed and received enormous media exposure, even my parents in law (non-scientists) have been asking me about it. Now I wonder when my 3-year old daughter will start casting doubts…

Nonetheless, there is sufficient reason to be extra careful when investigating the relationships between the existence of warming trends and changes in malaria risk. In our review in the Quarterly Review of Biology (Climate change and highland malaria: fresh air for a hot debate), Luis Chaves and I argue that, although we do find robust evidence for warming trends in highland areas of eastern Africa, it can be risky to focus solely on climate as the driving force of malaria (‘the panclimatic paradigm’). As previous reviews have shown (e.g. Lafferty, 2009, Ecology), many other factors are at stake. Given the limited opportunities for the manipulation of observations, teasing out the relative impact of climate change in studies on malaria risk remains a tough challenge. But what can we do then? Recently, some ecologists, including myself, have focused on how temperature affects the development and survival of individual mosquitoes, so at a very fine (local) level. This has revealed important insights. For example, experiments have shown that, at higher altitudes, adult malaria mosquitoes can survive inside the microclimate of local village houses, despite the fact they do not naturally occur there (for your correct understanding: I’ve put them in cages and brought them higher up myself…). By contrast, larvae of these mosquitoes, were not capable to complete their life cycle in water at the same altitude. Malaria levels were very low in this area (<10% in school children), but temperature increases of one or more degrees could lead to completion of the life cycle and thus an abundance of mosquitoes. In the absence of natural immunity in the human population and in the presence of a small parasite reservoir, this could lead to outbreaks as observed elsewhere in eastern Africa. Another recent study (from colleagues Paaijmans and Thomas) has shown that taking into account daily temperature fluctuations has dramatic implications for parasite development inside the mosquito and consequently the risk of malaria. In fact, this study concluded that, irrespective of the warming trend in the east African highlands, taking into account daily temperature fluctuations in risk models could explain the pattern of malaria outbreaks much better. Such realizations seem simple and straightforward, but apparently had not been investigated before. In contrast with such detailed studies, there are studies that evaluate the impact of the various IPCC scenario’s for climate change on malaria risk at the continental scale. Often remote sensing techniques, whereby modelers observe the earth through the use of satellite imagery, are used in these exercises. I think it should be clear that, for accurate assessment and evaluation of scenario’s these two ends of the spectrum should meet. This is in line with another recent study on 10 years of malaria hospitalization rates in different locations throughout Kenya (Okiro and colleagues). They conclude that spatially constrained observations may be misleading in addressing country-wide trends of malaria hospitalization. In other words, there is a need to fill in the gap between detailed observations at small spatial scale and the crude, but also critical assessments of future malaria risk at the large scale. I would argue that future studies should focus on the interaction between the role of climate change and other drivers, instead of arguing it’s either this or that. Migration and population increase, for example, are often cited as alternative explanations for changes in malaria patterns. But why do these occur? What drives people to move from one place to the other? It may well be that devastating rains or extreme droughts, that themselves may be the result of climatic change, lead to crop losses that eventually force populations to move to urban centers, affecting disease risk in the end. So this calls for much more interaction among climate scientists, ecologists, epidemiologists, sociologists and economists. We hope that by having provided ‘fresh air’ we’ll keep the heated debate going. Only through informed discussions, we’ll obtain the required insights for developing effective mitigation strategies in this malaria elimination era.