The Taming of the Mosquito?

by Bill Chameides | May 28th, 2010
posted by Erica Rowell (Editor)

Permalink | 3 comments

Malaria is spread by more than 100 species of the Anopheles mosquito. Because mosquitoes thrive in warm, moist environments, scientists worry that a warming world might lead to more outbreaks of malaria. But will it?

A new study finds that global warming will not bring malaria back to the United States.

First the Facts About Malaria

Malaria is an infectious disease caused by a microbe called Plasmodium. The little guy starts out by infecting the liver and then moves on to the red blood cells, causing such unpleasantries as fever, anemia, convulsions, and even death. Because the microbes hide themselves inside our red blood cells, our immune system does not recognize them as the enemy and so they are able to hang out, breeding wave after wave of malarial attacks over time. (Read more about malaria here and here.)

What makes tackling malaria difficult is that it is a so-called vector-borne disease carried by the mosquito — not just any mosquito but the one called Anopheles, and not just any Anopheles, the female Anopheles. That shouldn’t be all that surprising. Wasn’t it a woman who aptly warned to “best beware my sting”? And it has been sung that “the female of the species is more deadlier than the male.” (Listen/watch here.) On the other hand, that same sting-warning female concluded that: “Of all things living, a man’s the worst!” But I digress.

The Global Warming Connection

It’s the mosquito-vector part of the equation that has got people worried about malaria and global warming. After all, we all know that malaria is most common in the tropics and mosquitoes like to hang out in warm, humid places. It stands to reason that a warming world would lead to more malaria, maybe even large outbreaks here in America. Indeed, some model calculations (like those that use future climate simulations to extrapolate the spread of Anopheles from its current, largely tropical habitats to future habitats based on increasing temperatures) generally conclude that the world will see a significant increase in malaria in the coming decades. (Fore more on this complicated issue, see here, here, and here.)

Paul Reiter, an entomologist at the Institut Pasteur in France, has been questioning that conclusion for more than a decade. His argument is that human intervention against the Anopheles — such as using screening and netting, draining swamps, and spraying insecticides, as well as medical treatment and building human immunity — are far more important in determining the incidence of malaria than climate is. I first heard him make that argument in Sicily at a conference some 15 years ago, and he’s still at it today.

New support for Reiter’s thesis comes from a letter by Peter Gething of Oxford University and coauthors appearing last week in the journal Nature. The authors used historical records to reconstruct the incidence of malaria around the world in 1900 and compared that to the incidence in 2007. They found that “during a century in which global temperature increases have been unequivocal, we have documented a marked, global decrease in the range and intensity of malaria transmission.”

In the southeastern United States in 1900, for example, malaria was epidemic — meaning that there were outbreaks of the disease periodically but not continuously; clearly that is not the case today. Even in Africa, where incidence is more severe than anywhere else, there have been large regions of improvement. The exception remains in central and western parts of the African continent, where most of the population has malaria parasites and the transmission continues year-round.

And so, the authors reasonably argue that if climate change were not the overriding factor controlling malaria transmission over the past century, there is no reason to assume it will be so in this one. They conclude that: “the success or failure of our efforts against the [malaria] parasite in the coming century are likely to be determined by factors other than climate change.”

Interesting Conclusions but …

Not being a medical entomologist, I will be interested to see the tenor of any comments that come into Nature on this work. I also wonder to what extent things like the outbreak of West Nile in the United States contradict their conclusions.

Secondly, won’t global warming make controlling mosquito-borne diseases more difficult? I wonder if changing climate patterns (e.g., more flooding) will undermine the mosquito-control measures that have been put into place to prevent malaria transmission, especially in developing countries.

But bottom line: I find this paper reassuring. Happy Friday. Have a good, safe Memorial Day weekend. One approach: “Strive mightily, but eat and drink as friends.”

filed under: Africa, climate change, faculty, global warming, health
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  1. Ken Towe
    Jun 5, 2010

    “In the southeastern United States in 1900, for example, malaria was epidemic — meaning that there were outbreaks of the disease periodically but not continuously; clearly that is not the case today.” Southeastern US (as defined by NCDC-NOAA) is probably not a good model for temperature and malaria. In the last 20 years the southeastern US has experienced a virtually flat annual temperature trend (high of 65°F in 1990; low of 61.8°F in 1996; mean of 63.2°F). However, from ‘epidemic’ 1900 to 1921 the southeastern US experienced a sustained warming trend. The mean annual temperature climbed from a low in 1901 of 61°F to a high of 65°F in 1921, mean of 63.1°F.

  2. MattN
    Jun 1, 2010

    “After all, we all know that malaria is most common in the tropics and mosquitoes like to hang out in warm, humid places.” This is where your logic breaks down. Malaria is not a tropical disease. Mosquitoes do not need tropical temperatures: they need no more than 15 degrees Celsius to breed. The largest malaria outbreak of modern times was in Siberia in the 1920s and 1930s, when 13 million were infected, 600,000 died and 30,000 died as far north as Arkhangelsk, on the Arctic Circle. The re-emergence of malaria has absolutely NOTHING to do with temperature, and EVERYTHING to do with public health.

    • Bill Chameides
      Jun 7, 2010

      MattN: Perhaps you are confusing with some inferior blog out there because your point was actually the message of the post…. Did I say malaria IS a tropical disease, or just that malaria is most common in the tropics today? The rest of the post described why that does not necessarily mean increased temps will lead to more malaria.

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