A Hot Zone For Disease

Fishermen were the first to see the signs. A typical haul of oysters always includes a few empty shells, or "boxes," and usually each one is encrusted with barnacles. But fishermen in Delaware Bay in the summer of 1990 were pulling up boxes that were bone white, as if they'd just been discarded from an oyster dinner; apparently they had died only recently. Many oysters trailed tiny bits of rancid meat. Susan Ford, a marine biologist at Rutgers University, identified the blight as the work of Perkinsus marinus, a deadly waterborne parasite. It had been wiping out stocks up and down the southern U.S. coast for decades but had never come as far north as Delaware Bay. As the parasite settled into its new home, over the next three years oyster hauls plummeted from 500,000 bushels annually to fewer than 20,000, decimating the fishing villages on the Jersey shore. Meanwhile, Perkinsus marinus continued its northward journey another 500 miles to Cape Cod, then another 200 to Maine. "The way the fishing industry operates has had to change markedly," says Ford. "Some managers just closed fisheries. It has been devastating."

Researchers now think such stories will become more and more common. According to a study published late last month in the journal Science, the real culprit behind much of the rise in infectious diseases around the world may be rising temperatures. Drew Harvell, a biologist at Cornell University, and Andy Dobson, an ecologist at Princeton, surveyed new disease outbreaks and correlated them to data on warming trends. Their report catalogs more than 50 outbreaks that coincided with a rise in temperature. It adds up to compelling, albeit circumstantial, evidence that climate change has already begun to trigger the spread of disease. "A warmer world will be a sicker world," says Dobson. "We will see more outbreaks of disease in animals and plants. Unfortunately, those diseases will spill over to humans."

The existence of a link between climate change and disease has been controversial for years, mainly because separating the effects of climate from other causes, such as public-health practices, is nearly impossible. Harvell and Dobson tried to get around this problem by choosing plant and animal pathogens that are unlikely to have been affected by other factors--such as the oysters in Delaware Bay or cases of Dutch elm disease in England. It makes for grim reading. Fungus, unchecked by abnormally warm winters, is rotting the roots of Mediterranean oaks in Australia. A debilitating parasitic nemotode, or worm, that preys on cattle and deer is moving faster through its larval stages.

As far as humans are concerned, mosquito-borne diseases are the most worrisome, say the researchers. In warmer climates mosquitoes mature more quickly, which means there are more of them, and their metabolisms speed up, which means they bite more frequently. Fewer are killed off during the winter. The malaria parasite matures more quickly in the mosquito's gut. The outbreak of West Nile virus in the United States (which has killed more than 20 people in the past three years) may have been spurred by climate change, say the authors.

This study won't resolve the scientific controversy over disease and warming. Although it assembles a compelling case, it doesn't prove cause and effect. And despite the scientists' precautions, some of the data is less than conclusive. For instance, some studies of the highlands of eastern Africa show that mosquito-borne diseases are on the rise, while others show no increases; researchers attribute the discrepancies to the heavy use of insecticides in the region. "We're a long way from understanding what will happen to human health if there is significant climate change," says Rita Colwell, director of the National Science Foundation and a professor of microbiology at the University of Maryland. "We just don't know if the evidence [supporting a link] is strong enough." Harvell admits: "Our monitoring data in wildlife diseases is horrible." More work clearly needs to be done. But if her thesis pans out, we may have to learn to like the smell of bug repellent.

© 2002