WHERE DOES EBOLA HIDE?

By Andrew Purvis/Nairobi

WHEN TWO CHILDREN FROM THE remote village of Mayibout, in Gabon, discovered a dead chimpanzee lying in the undergrowth near their home a few weeks ago, they were delighted. Bush meat is a delicacy throughout the rain forests of central Africa, and chimps are particularly prized for their size and scarcity. Villagers helped carry the primate back to Mayibout, where it was skinned, cooked and eaten. There the festivities ended. Within a week, nearly all those who had prepared the animal for the pot had fallen ill with a high fever. Some began bleeding from the eyes and mouth. By last week, 13 were dead.

Ebola had struck again. For the third time in 12 months, one of the deadliest diseases known has emerged from the forests of Africa. The outbreak underscores how frighteningly common Ebola is on the continent. But it also marks a new chapter in a medical detective story. In a matter of days, World Health Organization officials and local experts made a nine-hour river trip to the inland village, both to help contain the epidemic and to learn more about the virus that causes it. "If we can understand how the virus is transmitted from the wild, we might be able to interrupt it," explains Dr. David Heymann, head of the emerging-diseases division at WHO.

Scientists had suspected that the disease, which killed 245 in the Zairian town of Kikwit last year, was transmitted to humans from chimpanzees or other forest creatures. Thanks to the latest outbreak, that theory now looks pretty solid. But where is the virus coming from? Chimps seem to be as susceptible as people to the pathogen, so it's unlikely that they can harbor it for long periods of time.

After the Kikwit outbreak, health officials stepped up the search for the virus' natural reservoir--an animal that can house the microbe and not suffer ill effects, or at least not fatal ones. Last June, U.S. researchers collected thousands of tissue samples from insects and small animals living around Kikwit. But tests on the specimens at the U.S. Centers for Disease Control in Atlanta have come up empty.

So how are the chimps getting sick? To learn more, WHO researchers last month began studying chimpanzees in the remote Tai forest of the Ivory Coast, where the virus is known to have struck the animals twice since 1993. The scientists plan to scrutinize any changes in chimp behavior or eating patterns during the midyear rainy season, when the earlier outbreaks occurred. Already local workers are building platforms high in the forest canopy to trap creatures that might serve as a conduit between the infected animals and people.

The best guess is that Ebola resides in a small, forest-dwelling animal, possibly a rodent. Insects such as mosquitoes, abundant in the rainy season, could transfer the blood-borne pathogen to chimps--or to humans. But, cautions Heymann, that's no more than speculation. "We're still in the dark," he says. Meanwhile, officials in Gabon are playing it safe. Last week they warned villagers not to touch dead animals found in the forest or kill any that are "behaving strangely." Until researchers know what they're dealing with, that's probably prudent. As the villagers of Mayibout now know too well, Ebola seldom gives victims a second chance.

--By Andrew Purvis/Nairobi