In parts of the Republic of Congo in equatorial Africa, nearly all the gorillas are gone. Since 2001 gorilla and chimpanzee remains have showed up near and in the Lossi Sanctuary, close to the Gabon border. Just what was killing these great apes was unclear. Now researchers finger the Zaire strain of the Ebola virus as the culprit. "No doubt that's what killed them," says Peter Walsh, a primatologist at the Max Planck Institute for Evolutionary Anthropology. He and his team estimate that the virus has killed 5,500 gorillas in the northwestern part of the country.

Of four subtypes of the Ebola virus, Ebola Zaire is the nastiest, Walsh says. This virus has about an 80 percent mortality rate and infects primates, including humans. The disease begins with a headache and leads, in about a week, to hemorrhagic fever and organ failure.

Magdalena Bermejo, the lead researcher on this study from the University of Barcelona, had worked in the Lossi Sanctuary to habituate the gorillas to people. Then, in 2002, the researchers noticed that the gorillas were dying. Between October 2002 and January 2003, 91 percent of the 143 gorillas Bermejo worked with disappeared. Remains found in the area tested positive for Ebola Zaire. The next year, 95.8 percent of another group of gorillas Bermejo had worked with also died.

To estimate how many gorillas in the region perished, the researchers compared the difference in the number of gorilla nests found in an affected area with one that was unaffected. East of the Lossi Sanctuary, few gorillas had become ill. In the western part of the region, which included most of the sanctuary, researchers found just 4 percent as many gorilla nests than in the unaffected east. In their calculations, the researchers assumed, based on populations in the sanctuary, that the 2,700-square-kilometer western zone had a pre-Ebola gorilla density of 2.2 animals per square kilometer, or nearly 6,000 individuals, leading them to conclude in this week's Science that about 5,500 gorillas died of Ebola Zaire.

"Probably a lot more than 5,000 died," Walsh says, adding that they made a conservative estimate. Based on the number of nests, about 83 percent of chimpanzees died of Ebola, too, the researchers say.

It is possible that some deaths resulted from something other than Ebola Zaire, says Gary Nabel, a virologist at the National Institutes of Health who was not involved in the study. "It is always hard to be airtight, he says, noting that other viruses, including smallpox and Marburg, also lead to hemorrhagic fever and could be striking the great apes as well. In any case, the disease striking the gorillas fits the pattern of an epidemic, Nabel says. It is a warning shot over the bow in terms of threat to this species, and to us, too, he adds.

Gorillas and chimpanzees have some hope. Ebola is not popping up randomly, and scientists are developing vaccines. Walsh estimates, however, that it might take $5 million to $10 million to test and deliver the vaccine safely to the gorillas or chimpanzees--who are, he points out, our closest relatives.

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