Does chopping down forests spread diseases?

Publié le
30 janvier 2014

Some people go to Panama for the seafood and sunshine. Helen Esser, doctoral candidate at Wageningen University (at the chair groups of Herbert Prins and Frans Bongers) went for bloodsuckers. She collected 20,000 ticks across forest floors 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 was first identified in heavily logged forests in Congo. To determine if ticks might be a key to human disease outbreaks, Helen Esser dragged down cotton cloths on the forest floors to collect ticks. She did that for the Smithsonian Tropical Research Institute in Panama City, where she is a fellow.

Ticks can transmit infectious-disease agents to mammals, including people, while feeding on their blood. Some ticks feed exclusively on one or two animal species, while generalist species feed on many. Helen Esser and 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. And they are more likely to carry infectious agents. “To really understand how diseases spread, you can’t just look at one species,” Helen says. “You have to consider the different interactions between species throughout an entire community.”

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, Helen 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.

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. And of course, she collected ticks. “It involved a lot of climbing steep hills and grabbing onto roots and trees,” she says. “Tick bites are part of the job,” she says, “but it’s worth it, if it helps to preserve forests. Habitat destruction, fragmentation… these things are backfiring on us, because in many cases they’re paving the way for human infection.”

Source: Smithsonian Magazine