Life Science

Assessing the Malaria Threat

Could increasing global temperatures cause this tropical disease to spread?

May 5, 2009
Scientists debate how global warming will effect the spread of malaria to patients like this one, in <br> a Taiwanese clinic.  [Credit: Ashley Jonathan Clements, flickr.com.]
Scientists debate how global warming will effect the spread of malaria to patients like this one, in
a Taiwanese clinic. [Credit: Ashley Jonathan Clements, flickr.com.]

Impact of Global Warming

With so many human influences on malaria, will global warming have any effect on the disease? According to the 2007 Assessment Report put out by the Intergovernmental Panel on Climate Change, malaria is likely to see mixed effects from our changing climate—expanding in some regions while contracting in others. Severe weather, such as rainfall and drought, may actually have a larger impact on the disease spread than temperature.

“I think when people think about climate change they only think about the old term of the climate heating up, and really it’s the rainfall patterns which I think are more important for malaria transmission,” says Burkot. “More rain is generally better for malaria because you get more breeding sites being created.”

Alternatively, periods of drought could be a double-edged sword for the disease. A drought would dry up the essential mosquito breeding pools and reduce the number of mosquito vectors in the affected area, which would in turn decrease the incidence of malaria. However, less malaria in any given region could also mean an increased population of people in that area who are susceptible to the disease—populations that have not been exposed to the disease and have very little immunity against it. When the rainfall returns and malaria resurges, there may be a larger number of people available to serve as disease hosts.

Increasing our ability to predict weather patterns, such as increases in rainfall, will help public health workers to better control the disease, says Burkot. “It gives us a chance to be proactive about doing something where there might be an outbreak of malaria,” he says.

There has been some debate about whether or not rising temperatures are to blame for an increase in malaria in the East African highlands. Many researchers agree that the number of malaria cases at these high altitudes has increased since the mid 1960’s, but they are still unsure about the role of climate change. For example, one study found that hospital admissions for malaria in a high altitude Kenyan town had increased by 26 percent over the period between 1965 and 1997, but the scientists did not find evidence for significant climate changes in this area. And another study in 2002, which analyzed climate data from four locations in the East African highlands, did not find a warming trend. However, a few years later, another group reexamined the data from the 2002 paper and did find evidence of warming—about a half a degree Celsius since the late 1970s. But even this does not establish that temperature changes caused the increase in malaria. It is important to note that malaria outbreaks have occurred at high altitudes in the past, and that a number of factors could have contributed to the rise in cases, such as resistance to anti-malaria drugs and a lapse in the implementation of disease control methods.

Despite warming temperatures, some parts of the world are experiencing dramatic reductions in the disease. For example, in Sri Lanka, a country with 15 million people, the number of malaria cases dropped from around 260,000 in 2002 to 197 cases in 2007, according to Burkot. This significant decease is due in part to a rise in malaria funding that allows for more resources to control the disease.

The Biggest Factor

Overall, the largest contributors to malaria spread are most likely not environmental. “I think the biggest factors that affect the disease spread are cultural things,” says Burkot. “How we live, how we raise animals, [and the] increase in urbanization” all help determine how far malaria can reach.

Reiter, the entomologist, agrees “Temperature has some kind of role, but the way we live is much more important than anything else.”

So what happens when the disease migrates away from endemic areas? The public health systems of developed countries, along with anti-malaria drugs, usually take care of any malaria cases that show up, says Reiter.

In fact, small malaria outbreaks periodically occur in the US, but they are usually stamped out before they affect many people. And while the Anopheles mosquito is native to the country, the disease-causing parasite is no longer there.

The best way to determine how malaria will move may be to look at the current public health prevention methods available in the area. “Where we’re going to see malaria is really going to depend on the resources available to detect, treat and prevent transmission,” says Burkot.

Unfortunately, if good living conditions can be said to contribute to malaria’s decline, then poor conditions certainly aid in its rise. “The problem is that malaria really is a disease of poverty. There’s a feedback there: poverty breeds malaria and malaria breeds poverty,” says Reiter.

“Despite the present circumstances, hopefully over the next 20, 30 [or] 40 years, people’s living standards will improve, and if their living standards improve, then malaria will dwindle away just as it did in Europe and North America,” he adds.

Currently, several methods are used to help prevent malaria in endemic areas. One line of defense is to use mosquito nets treated with insecticides. These nets help protect the inhabitants from mosquitoes feeding at night. Another method, called indoor residual spraying, involves applying long-lasting insecticides to the inside walls of houses. There are also relatively new drug therapies, known as artemisinin combination therapies, to treat those infected with the disease. These anti-malaria treatments help to deal with forms of the disease that have become resistant to other drugs.

Some feel that these prevention strategies may eventually lead to the global eradication of malaria. “A high proportion of the countries which applied for support from the Global Fund to control malaria are basically talking about heading towards elimination,” says Burkot. The Global Fund to Fight AIDS, Tuberculosis and Malaria is an international partnership that finances efforts to combat these diseases.

“But we can’t be too complacent,” adds Burkot, who notes that several effective anti-malaria treatments and insecticides, such as chloroquine and DDT, worked very well until the parasite became resistant to them.

Others argue that present public health initiatives are not enough to combat the disease. “The methods that we use today are very simple, and unfortunately, I don’t think they will do more than put a dent in transmission,” says Reiter. “There are problems of [insecticide] resistance, there are problems of people not using [insecticide nets].”

Perhaps with more funding, says Reiter, countries would have better tools to control and prevent malaria. “We need methods, good methods to control the disease, and I wish they would be spending more on controlling the disease than the billions that are being spent on climate change.”

While funding for malaria has been low in the past, there has recently been an upswing in support.

“The last few years have seen significant investment from the global fund,” says Stephen Connor, director of the environmental monitoring program at the International Research Institute for Climate and Society in NY. “[There has been] more money available to countries to implement effective control programs than they’ve seen in previous decades.”

The key to effective prevention, says Connor, is not just funding itself, but steady supplies of resources.

“Certainly more investment is needed,” says Connor, “and it’s needed consistently over a period of time, a fairly short period of time, so you can make a dramatic impact on malaria, because you’re always running against the danger of new drug resistance developing.”

Recent declines in malaria, like those in Sri Lanka, have some optimistic about the extent of the disease in years to come. “If funding remains at the level that it has been the last two years,” says Burkot, “then I would expect that we’re going to see a lot less malaria in the future than we have right now.”

Related on Scienceline:

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