Fears of a global rise in infectious conditions may be unfounded.
Climate change takes the blame for many dim future prospects: rising sea levels, more frequent droughts and disappearing glaciers, to name just a few. But perhaps the warming trend should be absolved of responsibility for a predicted bump in the global burden of infectious disease.
That's the bottom line of a paper in the April issue of the journal Ecology, which argues that the geographical ranges of infectious diseases are more likely to shift than to expand (K. D. Lafferty Ecology 90, 888–900; 2009). "You often see a list of the 12 terrible things that are going to happen with climate change, and increases in infectious diseases is often on that list," says Kevin Lafferty, an ecologist with the US Geological Survey in Santa Barbara, California. But data from diseases such as yellow fever and malaria, he says, provide "a different reality".
Five other papers in the same issue of the journal respond to Lafferty's work, with widely varying views. Sarah Randolph, a parasite ecologist at the University of Oxford, UK, agrees with Lafferty and says that focusing on climate change can distract scientists and public-health agencies from the actual reasons that infectious diseases spread.
But Mercedes Pascual, a disease ecologist at the University of Michigan in Ann Arbor, says that other causes are themselves often affected by climate change. To polarize the causes as climate versus anything else is, she says, "not helpful".
Work in the mid- to late 1990s suggested that warming temperatures would prompt infectious diseases to sweep across the globe. For instance, a 1999 report by Britain's Royal Society called Climate Change and Human Health predicted that by 2020, the incidence of malaria would have risen by 60%. But ecologists now say that those studies were too simplistic, having failed to take into account the fact that even a large increase in temperature can result in a very small chance of disease in areas where disease transmission is very low.
Still, says Lafferty, the idea has been hard to shake. As the planet warms, insects and other disease carriers can move into higher latitudes and altitudes that were previously too cold. According to Lafferty, some of the warmer parts of the disease's current range may become too hot. Besides, the vectors may not find appropriate host animals in expanded ranges, because biodiversity tends to decline away from the tropics.
Critics charge that Lafferty's arguments are flawed. Most climate predictions suggest a greater rise in minimum temperatures, such as night-time or winter temperatures, than in maximum temperatures, points out Richard Ostfeld, an animal ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York. Because of that, disease carriers may remain comfortable in their current ranges. Andrew Dobson, an infectious-disease ecologist at Princeton University in New Jersey, adds that the lower diversity of hosts in new ranges could actually pose a problem for humans because the vectors may choose to bite or sting humans and their livestock, rather than wild animals.
Besides, says Pascual, even if the range of a disease shifts rather than expands, the effect on human health is still likely to worsen dramatically. For instance, the high-altitude areas of Africa that are currently free of malaria have much larger populations than the low-lying malarious regions. People in the highlands are also less like to have acquired immunity, and so may be affected more by the disease. "It does matter if we get three million more cases in the highlands versus a much smaller contraction [of cases] somewhere else," she says.
Paul Epstein, a doctor and public-health expert at Harvard Medical School's Center for Health and the Global Environment, says that people are likely to move in large numbers because of climate change, often increasing the population density in already-crowded areas such as Africa's highlands. The implications of shifting disease ranges, he says, are not that "everything balances out. There are much more profound impacts."
Scientists are also arguing over whether recent examples of rises in infectious disease can be pinned on climate change. Lafferty and Randolph say that agriculture, travel and shipping, drug and pesticide resistance, and other economic and social factors may all have a larger role than climate. "There is not a single case of upsurge or emergence of an infectious disease that can be shown to be due to climate change," Randolph says. She and Pascual have each studied increases in malaria in the East Africa highlands in the past few decades and come to opposing conclusions about whether the climate data mirror the disease rise.
The key to settling these questions, many scientists say, is finding long-term data sets that include information on both disease and climate. Because of ecosystem dynamics and the complexity of disease, generalizing over large regions will always be fraught. "What happens in one place may not specifically predict what's going to happen in another place," says David Relman, a microbiologist and infectious-disease specialist at Stanford University in Palo Alto, California, who chaired a 2008 Institute of Medicine report Global Climate Change and Extreme Weather Events on similar topics. "These are really big unanswered questions. You'll find a lot of opinion, but we don't have a lot of data."