Global warming may cause spread of deadly hot-weather diseases Illnesses, such as malaria, to increase, says article

January 17, 1996|By Jonathan Bor | Jonathan Bor,SUN STAFF

Global warming could spread infectious diseases, such as malaria and dengue fever, to new regions of the world and speed transmission in areas that are already affected, a new study warns.

Warmer temperatures along with more extreme weather -- such as drought and heavy rainfall -- could could have their greatest impact on diseases spread by insects, rodents and other animals.

Dr. Jonathan A. Patz, a researcher with the Johns Hopkins School of Hygiene and Public Health, said climate change could cause mosquito-borne illnesses such as malaria and dengue to flourish at higher elevations and in regions that are now too cool.

The warnings, appearing in today's Journal of the American Medical Association, reflect a growing concern that the health consequences of global warming could be more immediate than rising sea levels or other effects that have captured greater attention.

"This is a new public health concern that we have just begun to recognize," said Dr. Patz, who prepared the article with colleagues at Harvard and George Washington University. "If climate changes, so too will disease transmission and distribution." Although some scientists still dispute predictions of warmer earth, the notion gained credibility last year when an international commission of climatologists estimated that the earth warmed a half-degree Fahrenheit over the last century and could heat up another 3.6 degrees by the year 2100.

The group, known as the Intergovernmental Panel on Climate Change, said in a draft summary that global warming was probably due, in part, to activities such as the burning of coal, oil and wood.

Such activities release excessive carbon dioxide and other "greenhouse gases" that trap heat inside the earth's atmosphere. People make the problem worse by cutting down forests that act as natural sponges for carbon dioxide, many experts believe.

The United States, Dr. Patz said, will probably remain outside the reach of many diseases that today thrive only in tropical areas. Nonetheless, health authorities should remain alert, he said, citing the emergence of dengue last year in southern Texas.

The parasite that causes malaria doesn't live outside the world's warmer regions, and the mosquito that carries dengue doesn't survive in freezing temperatures. But subtle increases in temperatures would expand the range of both diseases, he said.

"Where you have diseases that are already endemic, it's the areas at the margins that will be first affected," said Dr. Patz, who specializes in occupational and environmental health.

Warmer temperatures also spur the development of smaller dengue-carrying mosquitoes that bite more often. Heat also shortens the time it takes a mosquito to become infectious after it has bitten a human who carries dengue, increasing the chance that a mosquito will spread the disease.

Dengue, a viral illness, has for many years been a leading killer of young children in Southeast Asia, and recently it caused major epidemics in Latin America. The first attack usually causes a severe flu-like illness, but subsequent exposures can cause a potentially fatal illness marked by internal bleeding, very high fever and shock.

A warmer climate could create a need for more irrigation, thus increasing rates of schistosomiasis, an extremely common tropical disease acquired by wading in pools inhabited by infected snails, Dr. Patz said.

Climatologists have predicted that the greenhouse effect will also produce wilder swings between extremes of weather -- severe drought, for instance, followed by periods of heavy rainfall.

But Duane Gubler, chief of the vector-borne disease branch of the U.S. Centers for Disease Control in Fort Collins, Colo., said other factors are to blame for the continuing spread of infectious diseases. These include poverty, poor sanitation, increased world travel and the exploding population of developing nations.

"Dengue is an example that all the people who talk about this use, and it's expanded its geographic distribution," he said. "But none of this has been associated with global warming -- it's primarily due to demographic changes, societal changes and declining public health infrastructure."

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