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Ebola Outbreaks May Have Had Independent Sources

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Science  16 Jan 2004:
Vol. 303, Issue 5656, pp. 298a-299a
DOI: 10.1126/science.303.5656.298a

Since 1995, when the world was horrified by a deadly Ebola outbreak in Kikwit, Zaire, central Africa has suffered nearly a dozen outbreaks of the hemorrhagic fever, which can kill more than 80% of its victims. The most recent bout killed at least 29 people in the Republic of Congo between October and December 2003. The human toll is tragic enough, but the disease is also threatening one of the world's largest populations of chimpanzees and gorillas: Researchers estimate that it has killed thousands of great apes in the past 5 years (Science, 13 June 2003, p. 1645).

Public health authorities and conservationists are urgently trying to pinpoint the source of the continuing—and apparently increasing —outbreaks. They suspect that the virus lurks in a species that is somehow impervious to the disease but can infect vulnerable animals. One of the main puzzles is whether the recent outbreaks are all part of one larger epidemic that is spreading steadily through the forest from diseased animals to new victims, or whether each cluster stems from an independent introduction of the virus from its elusive host. Now, a team of virologists, epidemiologists, and veterinarians reports on page 387 that the evidence points to multiple, independent introductions.

Eric Leroy of the Institute of Research for Development in Franceville, Gabon, and his colleagues sequenced virus samples from human and animal victims of five outbreaks in Gabon and the Republic of Congo between 2001 and 2003. In the five outbreaks, they found eight distinct strains of the virus. Previous studies have suggested that the Ebola virus is relatively stable; isolates from nine human patients infected during an outbreak in 1996 and 1997 were identical, and sequences from a 1976 outbreak in Zaire and a 1996 flare-up 3000 kilometers away in Gabon differed by less than 2%. Therefore, the authors of the new study conclude, the eight distinct strains they found in Gabon probably diverged over decades or even centuries, suggesting that they came from different sources.

Tracking a killer.

Ebola passes from an unknown reservoir to humans. In 1995 it killed 255 people in Kikwit, Zaire.

CREDIT: STR/REUTERS

Not everyone is convinced. Peter Walsh, an ecologist at Princeton University in New Jersey, who has argued that the human and ape outbreaks are part of a single epidemic, says the apparently different strains do not rule out a spreading wave front of Ebola. If the virus passes through multiple animal hosts as it moves through the forest, it could quickly collect enough mutations to produce the observed genetic differences, he says. “Under their theory, [Ebola] should be popping up all over the place, but it is always in areas adjacent to previous outbreaks,” he says. The pattern “provides strong evidence that there is not a magic bat cave that is spitting out infections in multiple directions.”

Both scenarios are theoretically possible, says Les Real, a disease ecologist at Emory University in Atlanta, Georgia. The new set of data “leaves us where science almost always leaves us: unsure. It is really pointing to the necessity of finding the reservoir,” he says.

The answer may not be so far off. Teams searching for evidence of the reservoir “are making good progress,” says William Karesh of the Wildlife Conservation Society in New York City, a study co-author. “I'm confident they'll have it nailed down this year.” Although researchers have been looking for evidence of the virus in hundreds of species, a leading suspect has long been bats, Karesh says. Lab experiments have demonstrated that bats can be infected with the virus and even shed it without becoming deathly ill, he says. But it has been difficult to pin evidence on any of the dozens of bat species native to the region. And co-author Pierre Formenty of the World Health Organization in Geneva raises a disturbing possibility: The different strains suggest that there may be more than one host species, perhaps insects as well as bats, rats, or shrews, he says.

One observation is clear: The human outbreaks can all be traced to hunters or other villagers coming in contact with dead animals in the forest. Both sides agree that more careful monitoring of animal outbreaks will help prevent further human cases, and continued efforts to discourage hunting of great apes and other wild animals would benefit both apes and humans. The new study “does give us more leverage to work with local people about why they shouldn't be hunting these animals,” Karesh says, because it suggests that the virus is endemic in the forest and that the danger does not decrease after an epidemic has passed through a region. If that message gets across, he says, it's “going to reduce a tremendous amount of pressure on the great ape populations.”

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