Diseases on the Move

In the mid-1990s, Brazilian mangrove crabs the size of dinner plates began dying en masse. They climbed out of their muddy burrows, flopped over, and expired. Crabbers who sold to local restaurants were used to harvesting a dozen bags in one trip; all of a sudden they were emerging from the forest with just one.

At first, scientists thought a parasite from the local shrimp industry was to blame. When they dug deeper, they found that the culprits were actually black yeast-like fungi previously seen only in frogs, orange juice, and German drinking water. Why were they suddenly decimating the crab population? Turns out, climate change may have had made conditions better for the fungi to survive and attack new hosts.

Those aren’t the only offenders moving to new locales. From the poles to the tropics, climate change is helping pathogens and parasites find new hosts to victimize. A worm living in white-tailed deer in Finland now infects moose and wild forest reindeer, and stronger storms in the American Southwest lead to more pinyon pine seeds, feeding more hantavirus-carrying rodents that come in contact with humans. Some of these organisms are annoyances. Others could wipe out an entire species or devastate a local economy.

To combat the problem, we need to change how we think about pathogens, say Eric Hoberg, a zoologist at the U.S. National Parasite Collection, who sees them as a fundamental part of the ecosystem. “They really deserve far greater attention than we’ve been giving them." In a recent paper in Philosophical Transactions of the Royal Society B, Hoberg and Dan Brooks, a zoologist at the University of Nebraska-Lincoln, point out how climate change is affecting the spread of infectious diseases.

Scientists thought for a long time that parasites evolved to feed and develop on specific hosts, but they seemed to adapt to new ones, leading to the so-called “parasite paradox.” Biologists and parasitologists studied that puzzle for decades and eventually came up with a new theory called the “Stockholm paradigm,” explaining that under the right conditions, opportunistic organisms can jump to a number of species. “To the parasite, this is just more of the same kind of real estate,” says Brooks.

From looking at historical records and models, researchers knew this occurred in the past. But as long as the climate is stable, pathogens usually stay put. Climate change forces hosts—and the little buggers that live on them—to new habitats, where they encounter other species, which might make perfect vectors for infectious disease.

To prepare for future outbreaks, Hoberg works on an inventory of mammals and their associated parasites; the Wildlife Disease Association tracks illnesses; and parasitologists like Scott Gardner from the University of Nebraska collect information on biodiversity. Gardner is putting together a database of Mongolian species and their moochers to create what he calls a “snapshot in time.” That baseline will let us see how species and their miscreants change.

It’s a start. But what we really need, say Brooks and Hoberg, is a worldwide database of outbreaks and parasites, which would help us predict how they could spread if introduced to a new environment. That, and to give these organisms the respect they deserve.