Why Ebola is Killing Gorillas

By MICHAEL D. LEMONICK

There's nothing like an outbreak of Ebola virus to guarantee screaming headlines. That's largely due to the mid-1990s bestseller The Hot Zone, which described the disease's horrifying course in gruesome detail, leaving many readers to believe that Ebola posed a looming threat to human existence. The truth is, however, that since the first recorded human cases in the 1970s, only a few hundred people have died from it. Of all the diseases you need to be afraid of, Ebola is near the bottom of the list.

Unless, that is, you're a gorilla. Over the past decade or so, tens of thousands of the great apes have died of Ebola in central Africa, along with similar numbers of chimpanzees. That the disease was responsible was established in a paper published in December in Science. Now a report in the American Naturalist explains just why Ebola is spreading among the animals so furiously--and shows how it could be stopped, according to lead author Peter Walsh of the Max Planck Institute for Evolutionary Biology in Leipzig, Germany. The epidemiological tactics used to treat outbreaks of human scourges like E. coli hold the answer.

Ebola is transmitted by contact with body fluids, and it's rapidly fatal. When people get it, they become so sick so fast--their organs literally liquefy--that others try to stay away from them. What's more, the mere fact of their quick immobility means they can't carry the virus very far. Ebola usually burns through an isolated village or community and then has nowhere else to go.

"People always assumed it was the same for gorillas," says Walsh. This belief made particular sense since gorillas live in relatively compact packs that don't interact much with other packs. Ebola, however, is oddly aggressive in great apes, ignoring pack boundaries and advancing across great-ape habitats at a rate of about 29 miles a year. Heading into the field to study the outbreaks, as well as animal behavior that could be contributing to them, Walsh and his team soon cracked the mystery

It turns out that animal epidemiologists had based all their Ebola assumptions on mountain gorillas--the kind studied by Dian Fossey--and not on Western gorillas, which were actually dying. The mountain variety subsists mostly on leaves, which are available all over the forest. Western gorillas, by contrast, live mostly on fruit, a scarcer resource that draws different groups of gorillas and chimpanzees to the same trees at different times of day. "They defecate and urinate in and around the trees," says Walsh, leaving infected body fluids to sicken the next group. Gorillas also examine the bodies of dead apes they come upon, perhaps because they're smart enough to want to know if whatever claimed that life is a threat to them. This provides another means of direct transmission.

Now that the mechanics of the epidemic are known, putting the brakes on it could be comparatively easy. Ebola vaccines exist, but public-service announcements won't exactly bring gorillas to a vaccination center where the entire population can be inoculated. Instead, epidemiologists can use selective-vaccination techniques, which work with human communities when universal vaccination isn't practical. Just inoculate a few gorilla groups along the infection chain, and when the virus reaches them, it is stopped cold.

"We're not talking about massive vaccination anymore," says Walsh. "We're talking about getting a vaccine into key gorilla populations." And the cost? Perhaps as little as $2 million--chump change, Walsh calls it, to save our closest evolutionary kin from extinction.