June 8, 2010

Trying to Hit a Mosquito with a Sledgehammer

Filed under: Health Effects

One of the standard tenets of the global warming bible is that malaria will get worse as temperatures rise. We’ve addressed this many times before, primarily by noting that the link between high temperatures and high malaria infection rates is anything but straightforward. Infectious disease expert Paul Reiter is quick to point out that malaria has been observed inside the Arctic Circle…and this is obviously not typical of a so-called “tropical” disease.

Nevertheless, the case for a malaria-temperature relationship stands on reasonably solid ground. Mosquitoes are more active at higher temperatures so they can expand their range. Biting frequency also depends on temperature, to some extent, so this should increase the infection rate, assuming the little buggers can find enough people to bite. Fairly sophisticated models have been developed that estimate the impact of weather variables on malaria infection rates. On the face of it, this seems like a reasonably solid argument.

But in a recent paper in Nature, Oxford University’s Peter Gething and colleagues from Oxford and the University of Florida took a careful look at global malaria data to see if the predicted trend was correct. They uncovered data from around the year 1900 showing where malaria was observed. These data not only show where malaria occurred, but also different categories of endemicity (in locations where the disease is continually present, the categories depict the approximate percentage of mosquitoes that carry the malaria parasite). 1900 is a key time because of the lack of prior malaria intervention efforts. The authors then used a current model of the parasite’s transmission to create a map at the same scale for the year 2007. The 1900 and 2007 maps are shown in Figure 1a and 1b, respectively. It’s then a simple matter to subtract the two maps to show how malaria endemicity has changed over the last 100 plus years (in this case, this is a subtraction of categories). This is shown in the bottom Figure (1c), where red shows increasing malaria and blue decreasing malaria.

There is virtually no red on the map.


Figure 1. Malaria endemicity in 1900 (a, top) and 2007 (b, middle) by increasing severity category. The difference in endemicity (c, bottom) from 1900 to 2007 indicates worsening malaria in red areas and improvements in blue (Gething et al., 2010).

If you give this issue a moment of thought, this result should be obvious. Of course malaria is not as bad now as it was 100 years ago. Global health interventions should have reduced the problem significantly.

But it has also been warming since 1900, including nearly all of the regions were malaria was endemic. Look at the problem this way: if you had available to you a) the current malaria/climate models, b) the 1900 malaria map, and c) a fairly accurate prediction of future temperatures, there is no possible way you would have predicted anything close to the map shown in Figure 1b for 2007. That’s because the climate models do not consider factors other than climate (this is also why heat-related mortality/climate model projections don’t work either).

It’s fair to say that everyone who works on this issue is pleased that malaria is less of a problem now. This speaks to the importance of intervention and awareness programs in fighting transmission. And the trend really shouldn’t be that surprising. But one might argue that regardless of the Gething et al. result, this does not mean that climate is not important.

The key part of the Nature paper, however, is the author’s attempt to quantify the effect of climate compared to other factors. To estimate these, they calculated something called the “basic reproductive number” of the malaria parasite (this is a measure of how efficiently the disease spreads within a population that has no inherent resistance to it). Even though the exact reproductive number is hard to predict, you can estimate the magnitude of the changes (also called the “effect size”) that might arise from different factors, such as climate or intervention programs.

Climate projections vary, of course, depending on the models and assumptions used, but the maximum effect sizes for the year 2050 arising from climate changes are around 2 or 3 (a doubling or trebling of the reproductive number). By comparison, the observed changes in effect size (between 1900 and 2007) were much greater than the projected climate change impact. More specifically, Gething et al.

…found that, of the 66 million km2 of the Earth’s surface thought to have sustained stable/endemic malaria in 1900, 12%, 18% and 57% had exhibited proportional decreases in the reproductive number of up to one, between one and two, and greater than two orders of magnitude, respectively; 11% had shown no evidence of change; and 2% had shown evidence of an increase in the reproductive number by 2007. Although imperfect, this simple comparison illustrates that despite warming global temperatures, the combined natural and anthropogenic forces acting on the disease throughout the twentieth century have resulted in the great majority of locations undergoing a net reduction in transmission between one and three orders of magnitude [emphasis added, Eds.] larger than the maximum future increases proposed under temperature-based climate change scenarios…When compared to the substantially smaller proposed magnitude of climate-induced effects, an important and simple inference is that [climate change impacts] can be offset by moderate increases in coverage levels of currently available interventions.

In other words, if we are really interested in stopping the spread of malaria, there are more effective ways of dealing with it than undertaking draconian global legislative efforts to reduce greenhouse gas levels—the equivalent of pummeling a mosquito with a sledgehammer.

Reference:

Gething, P.W., Smith, D.L., Patil, A.P., Tatem, A.J., Snow, R.W. and S.I. Hay, 2010. Climate Change and the Global Malaria Recession. Nature, 465, 342-346.




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