What Effect Does Climate Change Have on the Spread of Disease?

Health and medicine explained.
Feb. 6 2008 10:52 AM

The Chikungunya Question

What effect does climate change have on the spread of disease?

The Aedes Albopictus mosquito
Scientists are searching for a link between weather and disease

Before the summer of 2007, Castiglione di Cervia, Italy, was known as a quiet village near Ravenna. In July, however, doctors noticed complaints of excruciating joint pain, fever, headaches, and rash. Their patients were experiencing a fever called "chikungunya"; the word originates in the Makonde language in Tanzania and Mozambique and means "to dry up or become contorted." This epidemic had two years previously raged unexpectedly through islands in the Indian Ocean. But it was new to Europe.

And so Castiglione found itself at the center of scientists' efforts to map the effect of climate change on the spread of infectious disease. In December, at a Washington, D.C., conference sponsored by the Institute of Medicine, scientists and doctors wrestled with these questions: Did global warming bring chikungunya to Italy? Will it lead to a return of scourges like malaria, pushed out of Europe and the United States in the mid-20th century? Will epidemics worsen in poorer countries?

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Already, there are suggestions of change. Rising water temperatures off the coast of Alaska allowed the bacteria Vibrio parahaemolyticus to move much farther north than previously, contributing to a 2004 outbreak of gastroenteritis in cruise ship passengers from contaminated oysters. Higher temperatures in Peru during the 1997-1998 El Niño phenomenon were associated with a doubling of diarrhea cases requiring hospital admission in children. Earlier in the 1990s, El Niño-associated coastal temperature changes off Peru cultivated an unusual abundance of plankton and are believed to have contributed to an unexpected cholera epidemic affecting nearly 1 million people and killing almost 8,000 in Latin America.

While they readily accept the associations between climate and infectious agents, scientists balk at stating exactly what a change in climate might cause. This reluctance lies both in the complexity of disease and in the nature of science, in the need to build a case incrementally, fact by fact. Asking a scientist to predict the spread of disease is like asking him or her, while standing in the midst of a tornado, to predict how the landscape will change by measuring the direction and amount of debris flying by.

Take the chikungunya debate: Initially hailed as a clear example of how warmer weather can lead to new epidemics, some experts pointed out that it was not global warming but a new mutation of the virus that made the fever increasingly infectious. Others argued that the reasons for the chikungunya epidemic lay in new routes for disease opened up by the global economy; in the 1990s, imported tires carried into Italy a mosquito known as "the Asian tiger," which can serve as a vector for chikungunya as well as dengue fever.

The association of malaria epidemics with warmer temperatures in the Ethiopian highlands proves equally difficult to pin down. Many factors other than temperature play a role in transmission. Paul Reiter, a medical entomologist at the Pasteur Institute in France, names several: forest clearance, wetlands drainage, agricultural fertilizers, herbicides, stagnant water, people's daily activity patterns, the location of homes in relation to mosquito breeding sites, the designs and materials of those homes, the presence or absence of screens and bed nets, chemotherapy, vaccination, and mosquito control. Can we extricate climate change from such a web?

There is no way to conduct the kind of experiment—one group of people exposed, another similar group not exposed—that would more definitively establish the effect of climate change on the spread of a given disease. To make things more complicated, climate and the other factors that affect diseases interact with each other. What if increasing temperatures cause people to stay in their homes and thus actually prevent disease transmission? What if drought causes people to wash their hands less and spread bacteria more? What if floods in Asia cause chaotic mass migrations and further disease spread?

Scientists also use mathematical formulas and, increasingly, computer models to try to predict the future. But because of the number of variables a computer model would have to account for, it would seem that unless God was your statistician, you would be hard pressed to "prove" that climate change X will cause disease Y. At the IOM conference, Michael Osterholm, director of the Center for Infectious Disease Research and Policy, voiced his frustration with these limitations in demonstrating cause and effect. "We are struggling to prove it with scientific data, and we can't," he said. "We don't need West Nile virus to know we are in deep doo-doo. … [I]f we are trying to solve it on individual studies, we will be in the court of science for a long, long time, and then it will be too late."

What is the alternative to endless discussion? Recent editorials in the New England Journal of Medicine and the Lancet call for accepting, even without 100 percent certainty, the accumulating body of evidence that climate change will affect infectious diseases. Doctors regularly make recommendations to patients based on what they think might happen. Gina Solomon, a physician and senior scientist for the Natural Resources Defense Council, points out that not knowing whether hypertension will hurt an individual patient does not stop a doctor from trying to bring his or her blood pressure down.

A different slant, put forth by Peter Schwartz and others at the consulting agency Global Business Network, sidesteps the need for scientific proof. Their "systems vulnerability approach" does not try to predict what will happen to the climate; rather, it identifies existing vulnerabilities in our world that could easily tip a region toward a new epidemic given the additional stress of climate change. A damaged Iraqi town without a clean water source is more vulnerable to a cholera outbreak if waters there warm, as they did in Peru. An unusually hot summer in a sprawling slum in Mexico City becomes the breeding ground for an epidemic of infectious diarrhea. We may not know precisely what causes what. But we don't have to sit back and wait to see what the weather will do.

Maria Said is an internal medicine resident at Johns Hopkins Bayview Hospital in Baltimore, Md., and will start a fellowship in infectious diseases at Johns Hopkins Hospital in July.

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