Global warming may intensify disease

By Kirsten Schnackenberg
Contributing Reporter
Wednesday, April 11, 2012

There may be more to fear from global warming than environmental changes.

According to several leading climate scientists and public health researchers, global warming will lead to higher incidence and more intense versions of disease. The direct or indirect effects of global warming might intensify the prevalence of tuberculosis, HIV/AIDS, dengue and Lyme disease, they said, but the threat of increased health risks is likely to futher motivate the public to combat global warming.

“The environmental changes wrought by global warming will undoubtedly result in major ecologic changes that will alter patterns and intensity of some infectious diseases,” said Gerald Friedland, professor of medicine and epidemiology and public health at the Yale School of Medicine.

Global warming will likely cause major population upheavals, creating crowded slums of refugees, Friedland said. Not only do areas of high population density facilitate disease transmission, but their residents are more likely to be vulnerable to disease because of malnutrition and poverty, he said. This pattern of vulnerability holds for both tuberculosis and HIV/AIDS, increasing the incidence of both the acquisition and spread of the diseases, he explained.

He said these potential effects are not surprising, since tuberculosis epidemics historically have followed major population and environmental upheavals.

By contrast, global warming may increase the infection rates of mosquito-borne diseases by creating a more mosquito-friendly habitat. Warming, and the floods associated with it, are like to increase rates of both malaria and dengue, a debilitating viral disease found in tropical areas and transmitted by mosquito bites, said Maria Diuk-Wasser, assistant professor of epidemiology at the Yale School of Public Health.

“The direct effects of temperature increase are an increase in immature mosquito development, virus development and mosquito biting rates, which increase contact rates (biting) with humans. Indirect effects are linked to how humans manage water given increased uncertainty in the water supply caused by climate change,” Diuk-Wasser said.

Global warming may affect other diseases in even more complicated ways, Diuk-Wasser said. The effect of global warming on the incidence of Lyme disease, a tick-borne chronic disease, is more difficult to examine and measure, though she said it will probably increase.

“One possible way in which temperature may limit tick populations is by increasing the length of their life cycle from two to three years in the north, where it is colder,” she said. “Climate change could be reverting that and therefore increasing production of ticks. The transmission of the Lyme bacterium is so complex, though, that it is difficult to ‘tease out’ a role of climate change.”

Diuk-Wasser added, however, that scientists do find an effect of climate change on the distribution of Lyme disease in their data, but are not yet sure of the reasons behind such results.

While the study of global warming itself is relatively new, research on the impact of global warming on disease is an even more recent endeavor that draws on the skills and expertise of a wide variety of scientists and researchers.

“The field is multi-sourced, and recently interest has been evolving among climatologists, vector biologists, disease epidemiologists, ecologists, and policymakers alike,” said Uriel Kitron, professor and chair of the environmental studies department at Emory University.

Kitron said that in order to mitigate the effects of global warming on disease, the public must turn its attention to water management and an increased understanding of the connecting between “global processes and local impact.”

Diuk-Wasser said that raising awareness about the public health effects of global warming might aid climate control efforts, because it made the potential impact of global warming more personal.

“There’s been a great interest in climate advocacy groups to look for negative effects of climate change on health, since studies have found that this motivates people to adopt measures to curb climate change,” Diuk-Wasser said.

The Yale Climate and Engery Institute recently won a grant to study the direct and indirect effects of climate change on dengue transmission in Colombia.

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