Monday, August 24, 2009

PNAS Article on Climate and Malaria Risk

It is often claimed that global warming will lead to a greater risk of malaria. If one looks at the malaria "hot spots" in the world (see here) it is not hard to see that climate influences malaria risk. There is no malaria in Alaska (a cold region of the world), and most of the 1 million malaria deaths in 2006 were among children in Africa (a warm region of the world). And thus it just seems intuitive that that warmer regions of the world will be yet even more prone to malaria risk if temperatures increase this century. But is this actually true?

Like most things in life, the story of malaria risk is much more complex than our intuitions would lead us to believe. Consider, for example, the impact the wealth of a country has on risk of malaria. When the United States was much poorer, Americans had a much higher risk of malaria (even though the climate was roughly the same as it is today). These useful maps show how malaria risk slowly declined in the United States from 1882 to 1935, and was eventually eradicated in 1951.

This paper in the latest issue of Proceedings of the National Academy of Sciences shows that the impact of climate on malaria risk is far from settled. Temperature fluctuation can substantially alter the incubation period of the parasite, and hence risk of transmission. And the influence fluctuation has on transmission rate is much more complex, and has been studied much less extensively, than most people think.

Here is the PNAS abstract:

The incubation period for malaria parasites within the mosquito is exquisitely temperature-sensitive, so that temperature is a major determinant of malaria risk. Epidemiological models are increasingly used to guide allocation of disease control resources and to assess the likely impact of climate change on global malaria burdens. Temperature-based malaria transmission is generally incorporated into these models using mean monthly temperatures, yet temperatures fluctuate throughout the diurnal cycle. Here we use a thermodynamic malaria development model to demonstrate that temperature fluctuation can substantially alter the incubation period of the parasite, and hence malaria transmission rates. We
find that, in general, temperature fluctuation reduces the impact of increases in mean temperature. Diurnal temperature fluctuation around means >21°C slows parasite development compared with constant temperatures, whereas fluctuation around <21°C speeds development. Consequently, models which ignore diurnal variation overestimate malaria risk in warmer environments and underestimate risk in cooler environments. To illustrate the implications further, we explore the influence of diurnal temperature fluctuation on malaria transmission at a site in the Kenyan Highlands. Based on local meteorological data, we find that the annual epidemics of malaria at this site cannot be explained without invoking the influence of diurnal temperature fluctuation. Moreover, while temperature fluctuation reduces the relative influence
of a subtle warming trend apparent over the last 20 years, it nonetheless makes the effects biologically more significant. Such effects of short-term temperature fluctuations have not previously been considered but are central to understanding current malaria transmission and the consequences of climate change.

And a sample from the article:

Our analysis reveals that diurnal temperature fluctuation will alter the length of parasite incubation compared with estimates based on the equivalent means, with both DTR and day length shaping the relationship. Under warmer conditions, for example,
diurnal fluctuation increases the EIP due to the nonlinear effects of short-term exposure to sub- and superoptimum temperatures. Consequently, in areas with mean temperatures in the range of 22–28°C (representative of large parts of sub-Saharan Africa),estimates of R0, or other metrics of malaria risk, based solely on measures of mean temperature could be too high so that by extension, malaria may be potentially more controllable than currently assumed. The effect is likely to be greatest for mean temperatures >26°C, which tend to be representative of areas with high transmission intensities. A more pronounced effect, however, occurs at lower temperatures, where malaria transmission is more likely to be epidemic rather than endemic. In these transition environments, EIP becomes markedly shorter as day length and DTR increase. Indeed, temperature fluctuation could enable parasites to complete development within the lifespan of their vector at lower mean temperatures than previously predicted. Hence, in areas with mean temperatures below 20°C, current estimates of risk could be too low.