Climate change prompts debate among experts about spread of tropical diseases

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.

In May, an international group of experts published a letter on climate change and malaria in the journal Nature stating that "widespread claims that rising mean temperatures have already led to increases in worldwide malaria morbidity and mortality are largely at odds with observed decreasing global trends." Further warming of the climate appears unlikely to broaden malaria's reach very much, they concluded.

Markers along the C&O Canal towpath memorialize the canal diggers who died of cholera there early in the 19th century. And cholera is still with us, its bacteria colonizing brackish waters of the Chesapeake Bay. If we didn't have water filtration plants in our region, we'd probably still have outbreaks of the disease.

Many biologists say there is little reason to think that climate change will trump human efforts against such diseases in the future. "It's intuitive to think that with global warming there's going to be increasing dengue and malaria or yellow fever, but it's not necessarily so," says Duane J. Gubler, a longtime dengue expert at the Centers for Disease Control and Prevention who now directs the Asia-Pacific Institute of Tropical Medicine and Infectious Diseases in Singapore. "We got rid of these diseases after World War II with good water management, improved standards of living and DDT.

Gubler says that dengue's resurgence in parts of the tropics has resulted from urbanization and the failure of mosquito eradication campaigns, rather than climate change. He sees little chance that dengue, which causes a painful, sometimes fatal infection, will make serious inroads in the United States and points to a 2008 study that compared dengue rates in Brownsville, Tex., and Matamoros, its sister city across the Rio Grande in Mexico. While 32 percent of Matamoros residents had been recently infected, the survey found, only 4 percent of Brownsville residents showed evidence of infection.

Among the likely explanations is that almost all residents of Brownsville have running water, so they don't need to keep it in pots around their houses where mosquitoes breed. They have air conditioning and are less likely to be outside during mosquito-biting periods, and they live in less densely populated houses and neighborhoods, lowering the chances of an epidemic.

Still, some studies have shown an increase in dengue in Puerto Rico, Venezuela and Thailand following particularly warm, wet periods, suggesting that warming could also bring the disease to the United States. Fauci and David Morens, co-author of the JAMA article, don't think it likely that dengue will spread massively around the United States, Morens said in an interview. But smaller outbreaks like one seen in Key West this year are more likely than in the past, he said.

Haiti's epidemic

A clear example of how tricky it is to figure out whether climate change or something else is affecting disease rates is apparent in Haiti, where a cholera epidemic is raging: More than 3,400 people had died and nearly 150,000 had been sickened as of a week ago, according to the most recent Haitian government statistics.

Rita Colwell, a University of Maryland environmental microbiologist and former National Science Foundation director who has spent 40 years studying cholera, believes climate change has played a role in the Haitian outbreak.

Colwell and her colleagues have shown that the cholera bacterium lives, often in a dormant state, in brackish estuaries and on zooplankton. As these tiny organisms multiply rapidly during warm periods, cholera can spring back to life. Colwell's research has convinced most of her colleagues that the zooplankton blooms - and cholera outbreaks - respond to warming periods that will increase with climate change, mainly by lengthening yearly epidemics.

Yet, one leading theory for what caused Haiti's epidemic focuses on humans. U.N. peacekeepers from Nepal, where cholera is endemic, may have caused the outbreak by allowing their septic tank to contaminate a river from which Haitians drink. If proved, this would underline the powerful impact of random events in a world of global travel and overpopulation. Many scientists, in fact, believe that such influences will dwarf the impact of climate change on infectious disease - or, perhaps, exacerbate it.

"Public health systems around the world are fighting a constant battle against diseases," said Joshua Rosenthal, environment and health program director at the National Institute of Health's Fogarty International Center. "Climate change is definitely affecting disease dynamics, but the big question - will diseases be a bigger problem in a warmer, wetter, sometimes dryer world? - can't be separated from the factor of diligent public health systems that are in place."

As a result, many scientists say it makes more sense to spend money addressing public health crises - climate-related or not - rather than greenhouse gases, though others argue it's unsafe to make it an either/or question. Campaigns to fund malaria vaccine research and put millions of bed nets in malarial African villages are expected to have an impact on the spread of the disease larger than any caused by climate change, many scientists agree.

Whether or not human engineering got us into this mess, perhaps it can get us out. That's the idea behind Jacobs-Lorena and his Baltimore mosquitoes.

The Hopkins center is one of many research groups seeking to change the nature of the mosquito and the diseases they carry. Jacobs-Lorena's group intends to infect his insects with malaria-killing bacteria and send them out to live and propagate around African villages.

A British company recently bred millions of sterile male mosquitoes and mated them with wild females in an experiment in the Cayman Islands that reduced the population of potentially dengue-carrying mosquitoes in one area by 80 percent. Still other researchers - at the Walter Reed Army Institute of Research and elsewhere - are getting closer to creating vaccines against malaria, dengue fever and other tropical diseases.

"I think it makes more sense to use our money on research and public health than carbon trading," Gubler said. "We could do an awful lot, around the world, with not very much money."

Allen, a Washington-based writer, is the author of "Vaccine: The Controversial Story of Medicine's Greatest Lifesaver" (W.W. Norton, 2007).

View all comments that have been posted about this article.

© 2011 The Washington Post Company