Climate change prompts debate among experts about spread of tropical diseases

A girl with cholera symptoms receives treatment in Haiti. Some experts link the outbreak there to climate change.
A girl with cholera symptoms receives treatment in Haiti. Some experts link the outbreak there to climate change. (Emilio Morenatti/associated Press)

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By Arthur Allen
Special to The Washington Post
Monday, January 10, 2011; 7:47 PM

The room where 10,000 Anopheles stephensi mosquitoes hatch each week is hot and humid and smells like the tropics - an appropriate surrogate for a warming world. The Johns Hopkins Malaria Research Institute in Baltimore, where the insects are raised, was created with a billionaire's anonymous donation a decade ago, after a map printed in Scientific American suggested that by 2020 malaria could be breaking out in Baltimore, and across the eastern United States and Europe.

The idea that climate change will bring malaria and other tropical killers to our door turns out to be an extremely controversial one among ecologists, climatologists and biologists such as Marcelo Jacobs-Lorena, who runs the "insectary" at Johns Hopkins. "It's a very complicated story," says Jacobs-Lorena.

The malaria map accompanied a 2000 article, written by Harvard biologist Paul R. Epstein, that raised the alarm about the impact of global warming on the spread of infectious diseases. It helped influence a research agenda that last year resulted in more than 4,000 studies of climate change and disease.

Epstein believes that evidence of the disease risks of climate change have only grown in the past decade. "The earlier models did not take into account the dramatic increase in extreme weather that we're seeing," he said.

Extreme weather events such as heavy flooding and drought - thought to be linked to the warming of the oceans and to changes in the precipitation cycle - create conditions for waterborne illnesses that may be becoming more common in the United States, said Jonathan Patz, a professor of environmental public health at the University of Wisconsin at Madison. A cryptosporidiosis outbreak that killed 50 people in Milwaukee in 1993, preceded by the heaviest rainfall month in 50 years, could be a sign of things to come, he said, given that record rainfalls have become more common in recent years.

In 2008, the director of the National Institute of Allergy and Infectious Diseases, Anthony Fauci, warned that physicians should be on the lookout for dengue fever, a tropical disease that has exploded in South and Central America and across much of Asia in recent years. The mosquito Aedes albopictus, which can carry dengue, has extended its range across the United States since arriving in the 1950s, probably in a shipment of tires from Japan.

Cases of dengue have been reported in Texas since 1981, and there have been small outbreaks in Hawaii and, most recently, in the Florida Keys. The disease "threatens temperate zones of the continental United States where mosquito vectors continue to expand," Fauci wrote in the Journal of the American Medical Association.

Biological first principles suggest that warmer weather, by causing organisms to grow faster, will expand the range of disease-carrying insects and microbial pathogens. And some models published in the medical and scientific literature suggest that tropical illnesses such as Chagas, which spreads in Latin America through the feces of a beetle, and leishmaniasis, carried by sandflies, could soon find niches in the United States.

Offsetting effects?

Last year, an article in the journal Ecology raised questions about these theories and suggested that, rather than broadening the range of tropical infectious diseases, climate change would just shift the burden. New outbreaks in some areas would likely be offset by reductions in disease elsewhere, wrote the author, Kevin Lafferty.

"It seems plausible that the geographic distribution of some infectious diseases may actually experience a net decline with climate change," with, say, malaria declining in areas too hot for the malarial mosquitoes to live even as the disease spreads into previously cooler highland areas, wrote Lafferty, an ecologist at the University of California at Santa Barbara. "While this is the reverse of the conventional wisdom, it is consistent with the increasingly accepted view that climate change will reduce biodiversity."

Climate change has not been the main cause of shifts in infectious-disease patterns over the previous couple of centuries, infectious-disease specialists note. Humans have played an important role.

Mosquito-spread diseases such as malaria, dengue fever and yellow fever appeared in the United States as late as the early 20th century, in periods that were cooler than today. There were massive malaria epidemics in places as far north as Poland and Siberia in the mid-20th century. These diseases went away as a result of public health campaigns and improved sanitation and living standards.


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