Did Deforestation Cause Ebola?
While Ebola continues to ravage Liberia and Sierra Leone, an old debate has returned over how best to discourage future transmissions in areas like the one from which the virus emerged.
A report by the National Institutes of Health last week confirmed that the outbreak began with a single case of transmission from an animal to a human. That likely raised the hackles of activists who've been insisting that human incursion into forests might increase the incidence of disease transmission. Mining and logging means roads and camps and workers who need food, often leading to increased consumption of, and new trade in, wild animals, known as bushmeat.
That's the thinking anyway. But it’s not clear that human activity in forests where diseases like Ebola are present increases the odds of outbreaks. The two positions—Ebola creates itself in the forest, or Ebola moves with animal populations that industry decimates—are opposite sides of a long-running, arguably life-and-death debate among scientists studying the disease.
“[Ebola] hasn’t been associated with natural resource exploitation,” said Peter Walsh, who studies primate ecology at Cambridge University. “The original outbreak on the Ebola River wasn’t, and the ones in southern [Democratic Republic of Congo] in 1995 and 1997 were also not. It’s just not happening.”
Though West Africa is heavily populated, the transmission that led to the current outbreak didn’t occur in an area with extensive road systems or a large population. It appears to have begun in one of the region’s remaining forested areas, Walsh said.
“There are plenty of logging and mining concessions out there in Central Africa. If there was a strong tendency for [human activity] to be associated, it would have happened.”
Walsh’s argument is part of a long-standing debate over how Ebola emerges in animals in the first place. Virologist Eric Leroy argued in a 2004 article in the journal Nature that the virus exists as multiple strains and could emerge anywhere. If so, road building would, in theory, increase the odds of people coming into contact with infected animals. (The following year, Leroy's team identified fruit bats—suspected as the source of the transmission that led to the current outbreak—as a potential “reservoir” for the virus.)
Leroy’s team had found evidence of independent strains in samples taken from primates in Gabon. “The five outbreaks were caused by five distinct Ebola viral strains, rather than by one strain that mutated into different forms,” they claimed.
At the heart of the question is why places where road building exists have not had large outbreaks. Walsh argues that areas developed for industry, or developed generally the way much of West Africa is, quickly lose habitat to support the animals carrying diseases like Ebola.
Ebola, he said, is “density dependent. When it gets in the gorillas, then more gorillas die, [and] you’ve got a lot of dead gorillas on the ground. If you’ve got a logging operation [nearby], they quickly get hunted out.” Instead, outbreaks tend to occur in pristine places, where small-scale hunters and bushmeat traders can reliably find animals.
If Leroy is right, then an epidemic like the one in West Africa could happen virtually anywhere.
If Walsh is right, then mapping animal populations should allow us to predict outbreaks and prepare.