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Warmer Temperatures May Help Spread Disease

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St. Louis Post-Dispatch


ST. LOUIS, Mo. - As Earth's temperature rises, scientists warn that disease could also spread at a fever pace.

The most obvious influence of climate on human health was seen last summer as heat waves swept Europe killing an estimated 15,000 people in France.

"That's a pretty direct climate effect," said Jonathan Patz, director of the Program on Health Effects of Global Environmental Change at Johns Hopkins University in Baltimore.

Flooding, hurricanes, forest fires, mud slides, storms, droughts and other extreme weather events are predicted to increase with global warming. Already natural disasters kill about 123,000 people each year, many of them in Asia and Africa. Conditions in the developing world are likely to worsen as malnutrition due to crop failures increases and people pack into urban areas with poor sanitation and overburdened health care systems, Patz said.

But climate change could have more subtle effects on human health as well. Public health scientists fear that warming temperatures are already increasing the spread of some infectious diseases.

Diseases that pass from person to person are unlikely to be affected by changes in climate, Patz said. But sicknesses spread by insects, water or other environmental elements may change as drastically as the weather.

Epidemics of malaria, cholera, hantavirus infection and Rift Valley fever have all been tied to the temperature and weather fluctuations associated with El Nino.

As the weather warms, glaciers melt and plants begin creeping up mountainsides, said Dr. Paul R. Epstein, associate director for the Center for Global Health and the Environment at Harvard University in Boston.

"Mosquitoes are moving in lock-step with plants and temperature increases," Epstein said. As the insects invade new territory, they bring malaria, dengue fever and other diseases with them, he said. The ranges of snails, sand flies, tsetse flies and ticks - all disease carriers - may also expand as the environment changes.

But Epstein and others say it's oversimplifying to attribute epidemics exclusively to warmer conditions.

"There was a while where people were thinking `it's very simple. It's warmer so we'll have more mosquitoes and more disease,'" said Uriel D. Kitron, a veterinary biologist at the University of Illinois Urbana-Champaign who tracks disease-carrying insects such as mosquitoes and ticks. Moisture levels are also important, he said.

Several years of dust bowl conditions made St. Louis ripe in 1933 for the appearance of a mosquito-borne virus called St. Louis encephalitis virus. The rapid spread of West Nile virus, a cousin to St. Louis encephalitis, across the United States has also been attributed to drought conditions followed by rain.

Combinations of moisture and temperature help determine whether mosquito populations boom or bust, how fast parasites develop inside the insects and whether the pest can pass on the disease.

Other human activities, such as stripping forests, can also worsen the spread of disease, Epstein said. Flooding gets worse as forests fall. Fragmented forests also promote growth of rodent populations, and bring the animals - along with diseases they may carry - into greater contact with people, he said.

People also move diseases around the world with global travel and trade, he said.

But predictions are not uniformly dire.

Hotter temperatures may increase the spread of some diseases, but could actually limit others. The bacteria that causes Lyme disease, and the ticks that carry it, fare poorly in warmer climates, Kitron said.

Insects called kissing bugs, which carry Chagas' disease, may also expire in the South American heat, some computer simulations suggest.

Still, many experts predict a poor health prognosis for people, plants and animals if global warming continues.


(c) 2004, St. Louis Post-Dispatch. Distributed by Knight Ridder/Tribune News Service.


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