Global Warming and Malaria: It’s Kind of Complicated

Al Gore’s “An Inconvenient Truth” may be the first place a lot of people heard about it, but scientists have been worried for a long time about how climate change will affect infectious diseases. Malaria, dengue fever, West Nile virus and a host of other scary conditions all depend on animal vectors — mosquitoes and ticks, for example — to spread from one human to the next. And as the planet warms, these nasty little beasts can expand their range into new territory that used to be too cold. 

That’s the simple version, anyway, and it’s the version Gore laid out. But the truth is, as always, a bit more complicated. A new study in the journal Biology Letters is the latest evidence of that: using malaria-stricken mice, scientists at Penn State showed that the rodent-malaria parasite, Plasmodium yoelii multiplies faster in warmer temperatures, but becomes less infectious. On balance, say the researchers, the risk of infection actually goes down — and what’s true for mouse malaria, they suspect, is likely to be true for the human variety as well (for a fuller but still readable explanation, try the news article that just appeared in Nature).   

Although warming temperatures will increase the range of the animals hosting infectious diseases, it is not fully understood how it will affect the disease itself. Credit: eyeweed/flickr 

So we’re off the hook, right? Well . . . not necessarily. As I said, the truth is complicated. The new study addressed changes in temperature, but not in the range of the mosquitoes. If the mosquito-borne malaria parasite got less virulent but was spread over a much wider range, what would the upshot be? More infections? Fewer? Hard to say.

Beyond that, disease-carrying mosquitoes already have a much wider range than you probably think. Well into the 20th century, malaria was widespread in much of the American Southeast. In fact, the CDC (the Centers for Disease Control and Prevention) was founded in 1946 with the express mission of fighting malaria in the U.S. 

So the climate doesn’t have to change to make America hospitable to malaria, or to dengue fever. The diseases were already perfectly happy here — although warming temperatures could certainly expand their range. One reason we don’t see many cases is that the U.S. has a very good public-health infrastructure. We’ve drained lots of the swamps where mosquitoes thrive. We can afford pesticides and window screens and mosquito repellent. Other places — Russia, for example — have pretty dismal public health systems. If disease-carrying mosquitoes move into large areas of that country that hadn’t seen them before, things could get ugly.

The new study is important as far as it goes, of course. But when pondering a new piece of scientific research it’s always important to keep in mind how far it actually does go. It’s also important to remember that the newest scientific paper isn’t always the most definitive. It’s often just one more small piece of a very large puzzle — and it can take an awful lot of pieces to make the overall picture clear.