Does Chopping Down Forests Spread Diseases?

A young scientist in Panama devises a novel way to study ticks and disease

Helen Esser
“You have to consider...interactions between species” in a community, says Helen Esser, right. Yorick Liefting

Some people go to Panama for the seafood and sunshine. Helen Esser, a fellow at the Smithsonian Tropical Research Institute in Panama City, went for the bloodsuckers. She spent three months on the Panama Canal, dragging cotton cloths across forest floors to collect 20,000 ticks. After dropping them into alcohol-filled jars, she carried them back to the lab and cataloged them, tick by tick, to get at a pressing global-health question: Does chopping down forests spread deadly diseases?

Scientists have long observed that infectious-disease outbreaks sometimes occur where and when forest habitats are degraded. The most notorious example is the Ebola virus, which causes an often fatal illness in humans; it was first identified in Congo, whose forests have been heavily logged. Another thing scientists have documented is that forests divided by roads, farms and settlements tend to hold fewer mammal species.

Esser, a doctoral candidate at Wageningen University in the Netherlands, has come up with an innovative way to determine if that might be a key to human disease outbreaks.

Ticks can transmit infectious-disease agents to mammals, including people, while feeding on their blood. (U.S. residents may be most familiar with Lyme disease and Rocky Mountain spotted fever, which are both caused by tick-borne bacteria.) Some ticks feed exclusively on one or two animal species, while others, called “generalists,” feed on many. Esser and her colleagues speculate that when forests are degraded and mammal diversity declines, generalist ticks outcompete species with narrower diets; that, in turn, would increase the risk to humans because generalists are more likely to bite us as well as more likely to carry infectious agents.

“To really understand how diseases spread, you can’t just look at one species,” she says. “You have to consider the different interactions between species throughout an entire community.” Esser’s innovation was to devise a field test that would allow her to trace those interactions in different communities.

The ideal field test would be to compare separate but similar parcels of forest, each with a self-contained animal community—an arrangement that’s not easy to come by in nature. But at the heart of the Panama Canal, Esser has found what she calls “the perfect fragmentation study”: the islands within the Barro Colorado Nature Monument. Those environments, isolated by water, range from miles-wide forests, with howler monkeys in the canopy and brocket deer, peccaries and even jaguars roaming the verdant floor, to woods a few hundred yards in diameter that barely support rodents. All of them have ticks: Panama is crawling with more than 40 species.

By comparing the numbers and kinds of ticks in these environments, Esser and her colleagues should be able to show whether, as they suspect, those higher-risk generalist ticks prevail in places with less mammal diversity. That dynamic has been modeled mathematically, says Allen Herre, a STRI staff scientist who assisted Esser, “but ain’t nobody nowhere shown this.”

In addition to spotting the opportunity hidden on Barro Colorado’s islands, Esser figured out a better way to count the mammals on them: She attached motion- and heat-sensing cameras to trees. (Human observers, she says, are prone to error, and some “walk around like an elephant” and scare the animals away.) And of course, she collected ticks. “It involved a lot of climbing steep hills and grabbing onto roots and trees,” she says. And tick bites—on some days, dozens. “Bites are part of the job,” she says, laughing.
And worth it, she says, if her work helps preserve forests. “Habitat destruction, fragmentation...these things are backfiring on us,” she says, “because in many cases they’re paving the way for human infection.”

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