Underestimating malaria risk under variable temperatures

Abstract

T he risksposed to human health by vector-borne diseases continue to provide grounds for the important exchange of scientific ideas (1, 2). The emerging consensus is that while early attempts to model climate driven changes in the distribution of vector-borne disease were based on very shaky assumptions, attempts to refute them were also based on less than perfect data and limited analyses that overemphasized stasis. The paper by Paaijmans et al. (3) in this issue of PNAS suggests that the relationship between climate and malaria is even more subtle than previously appreciated. If we are to assess the impact of both climate and weather on malaria transmission, we need a deeper understanding of the nonlinear ways in which the biology of the parasite and its mosquito vector integrates temperature fluctuations. The authors show that diurnal fluctuations in temperature can modify the parasite's external incubation period inside the mosquito (“sporogony”), relative to estimates based on the coarser temporal resolution of daily mean temperatures. In areas where temperatures are close to the threshold for completing sporogony (≈16 °C), increasing variance allows development to proceed despite the nocturnal interruptions, as warm temperature pulses can compensate during the warmer parts of the day. However, the authors also show that for mean temperatures at the other end of the spectrum, where sporogony should proceed apace, the opposite can potentially occur. This suggests that transmission models and “risk maps” that ignore diurnal variability will tend to underestimate malaria risk in cooler environments and to overestimate risk in warmer areas. Despite extensive laboratory knowledge acquired in the first half of the 20th century on both the vector and the parasite, important questions remain on the precise role and interactions of the various biological processes driven by temperature. For example, we still do not fully understand how … 1To whom correspondence should be addressed. E-mail: pascual{at}umich.edu