Malaria Cases Could Rise with Temperatures, Study Says

In at least two countries, higher elevations had more cases in warm years Andrei Pungovschi/AFP/Getty

After Dr. William Moss, a pediatric infectious disease expert and epidemiologist, returned from a trip to Zambia in 1998, he came down with a fever. He had gastrointestinal problems, including nausea, and he felt tired. He went to the hospital, where blood work revealed he had a low white blood cell and platelet count. It turns out that Moss, who has specifically studied the epidemiology of malaria in southern and northern Zambia as well as in Zimbabwe, had malaria. He felt, he says, "really wiped out; I had no energy... I was having fevers and chills, which are very characteristic of malaria." Moss spent several days in the hospital and recovered, but many aren't that lucky. Malaria is a disease that claimed 627,000 lives globally in 2012, and got 200 million people sick. Young children and pregnant women are especially susceptible. Severe anemia is a common way it kills. A new study suggests that as global temperatures rise, the areas where malaria strikes could change, possibly affecting new populations.

Malaria, which is transmitted by mosquitoes carrying a parasite, is influenced by numerous, complex factors—rainfall amount, for example, is key. (The mosquitos need a place to breed.) But one controversial question that researchers have wondered about is the potential effect of global warming on the disease's spread. Higher elevations are generally cooler, and mosquitos don't like the cold, which also slows down the life cycle of the parasite inside it. But what happens if global temperatures rise? Will people living in higher elevations be more at risk of contracting the disease?

To answer that question, researchers led by Mercedes Pascual, an ecologist at the University of Michigan, focused on studying highland malaria rates in two separate countries: Ethiopia and Colombia. Her team looked at malaria records in western Colombia between 1990 and 2005, and central Ethiopia between 1993 and 2005. They concentrated on two factors: average temperature in a given year, and the median altitude of the reported malaria cases. What they found was straightforward. "We showed that highland malaria in both Ethiopia and Colombia basically moves up in altitude in warmer years," Pascual told Newsweek. "And the importance of this is that climate change will, without mitigation, result in an increase of malaria burden in these densely populated highlands of Africa and South America." (The journal Science published the study today.) In fact, the researchers had estimated before this study that a one degree increase in temperature could lead to hundreds of thousands of more infections each year in Ethiopia alone, and possibly millions more among children under 15. "The effects of climate change need to be mitigated in the future, and it is important to sustain the efforts to control the disease and eventually eliminate it in these regions," Pascual adds.

"This article is coming within a context of a larger debate," says Moss, the doctor who was kind enough to describe for Newsweek what it feels like to have malaria, "on what is the potential impact of global warming on the global disease burden of malaria." He's focused on malaria epidemiology since 2007, and is an epidemiology professor and member of the Johns Hopkins Malaria Research Institute. What this article has done, he says, is "make a strong argument" that in those two countries, the altitude change was associated with temperature change. In other words, when it got hotter, the malaria moved up in elevation. But it's not so simple to assume that global warming is going to cause more malaria, he said. For one thing, over the past century the map where malaria is prevalent globally has actually gotten smaller, not larger, noting that at one point (over a hundred years ago) you could get malaria in Baltimore, Maryland. There's been a lot more intervention in combating the disease globally, especially in the last 10 years. (Incidentally, the amount of malaria in the United States is the highest it's been in the past 40 years, says the Centers for Disease Control, due mostly to immigrants traveling back home and then bringing the disease back to the U.S. with them.")

"My main take-home message on this," Moss added, "is that what this article highlights is that the epidemiology or distribution of malaria is a dynamic thing. And temperature changes may be one thing that is driving the changing burden of malaria." But temperature change might not be the most important thing—a more worrisome trend is that the mosquitos in sub-Saharan Africa are becoming more resistant to insecticides, he says.

Then there's what's called the "theory of cheap linoleum," which is a metaphor that the chief of the Malaria Branch at the Centers for Disease Control, S. Patrick Kachur, references when asked about the study. Linoleum might have a bubble in it, and when you push down on the bubble, it just pops up somewhere else. Climate could affect malaria in that way—maybe the disease becomes more prevalent in the highlands, but other areas like the edges of deserts become drier, so malaria becomes less of a problem there. "We want to make sure that we're vigilant for the epidemiology to change," Kachur said.

Malaria isn't the only thing that can be affected by warmer temperatures—over the past 20 years, dengue fever, also carried by mosquitoes, has been found in the Gulf Coast of the Unites States. Before it was found mostly in South America, but crept up to the Caribbean and beyond, says Dr. Michael Parry, the chief of infectious disease at Stamford Hospital in Connecticut. "If global warming is real," he says, "we are clearly going to see a change in the spectrum of disease that we see in what traditionally have been temperate climates and are now warmer."