Sick of It?, continued

Indeed, though changes in the distribution of infectious disease can’t be definitively attributed to climate change, they definitely seem to be happening. Ticks carrying viruses associated with encephalitis are creeping northward in Europe. A 2004 outbreak of gastroenteritis from the consumption of raw Alaskan oysters extended the known range of Vibrio parahaemolyticus northward by 600 miles.

Climate change is predicted to expand the range of the black-legged tick, vector for Lyme disease. Nicholas Ogden, a researcher with the Centre for Foodborne, Environmental and Zoonotic Infectious Diseases in Quebec, predicts the tick could spread to much of southern Ontario and Quebec by 2020.

Until recently, the mosquito-borne West Nile virus was confined to tropical and subtropical Africa. Since it reached the United States in 1999, it has spread across the continent as far north as Alaska. Research has pinned increasing infections on warmer temperatures, elevated humidity and heavy precipitation.

Of all mosquito-borne diseases, malaria is the deadliest, killing close to 1 million each year. Many scientists predict malaria’s range will spread as the climate becomes more suitable for dozens of host species of mosquito. Taking into account both spreading malaria and increasing human population, the IPCC predicts an additional 220 million to 400 million people will be exposed to malaria during the next century.

Cautionary View

Despite these warnings, writes Sarah Randolph, professor of parasite ecology at Oxford University, “There is no single infectious disease whose increased incidence over recent decades can be reliably attributed to climate change.” Randolph calls the number of deaths due to climate change “inestimable.”

Scientists, she argues, have been beguiled by doom-filled scenarios while discounting situations where diseases are likely to decrease. They have assumed that climate will strengthen virulence rather than disrupt “the delicate balance between pathogen, vector and host.”

“She forced me to take a more critical evaluation instead of just nodding my head at what seemed to make sense,” says Kevin Lafferty, research scientist at the Western Ecological Research Center, UC Santa Barbara. Lafferty wrote a cautionary review on climate change and infectious diseases that appeared in Ecology in 2009.

“In going through the literature, I became more and more convinced that the story was much more complex,” he says. “At this point, if you look at the weight of the evidence for malaria and a lot of other infectious diseases, and at sort of the general theory about how organisms respond to the climate, the expectation is that we should see shifts, not net increases.”

Recent models, Lafferty says, show some diseases spreading in some areas, contracting in others. As the Climate Change Working Group summarized in 2007, “models project that, particularly in Africa, climate change will be associated with geographical expansions of the areas suitable for stable Plasmodium falciparum malaria in some regions and with contractions in other regions. … Some central Asian areas are projected to be at increased risk of malaria, and areas in Central America and around the Amazon are projected to experience reductions in transmission due to decreases in rainfall.”

A recent survey of local public health directors showed most believed climate change poses a formidable challenge to public health. Yet only a small minority had made adaptation a top priority because they lacked knowledge, expertise or resources.

 “One of the most important things to do is to gather information,” says David Neitzel, a vector-borne disease specialist for the Minnesota Department of Health. “It’s important to have strong disease monitoring in place. We want to be able to see when things are changing.”

Scientists are trying to develop more sophisticated models to predict changing threats (see sidebar). As the World Health Organization has pointed out, the biggest problem, especially in poor societies, is not predicting a threat, but responding once it arrives. For that reason, says Neitzel, modern infrastructure, good health care and competent emergency response are the best preparation, “so that if some sort of disaster happens we’re able to adapt, so that people aren’t living underneath tarps.”

GREG BREINING writes about science, travel and nature for The New York Times, Audubon and other publications. His books include Super Volcano: The Ticking Time Bomb Beneath Yellowstone National Park and Paddle North: Canoeing the Boundary Waters–Quetico Wilderness.

Further REading

World Health Organization: Protecting Health From Climate Change

Protecting Health From Climate Change: Connecting Science, Policy and People, a 2009 publication of the World Health Organization, looks at the implications of climate change for health and what we can do to minimize harm. It also describes changes we can make that simultaneously reduce greenhouse gas emissions and provide health benefits.