A report in the scientific journal, Nature, says at least six wild chimpanzees in Ivory Coast have died from anthrax. The findings of the study mean that infectious disease has now been added to the problems of poaching and habitat loss as the main threats to great apes in Africa.

The study shows an unusually high number of sudden deaths within three communities of chimpanzees was caused by anthrax. The chimpanzees and great apes are in Ivory Coast's Tai National Park, along the border with Liberia.

Anthrax has never been recorded before in a tropical rain forest environment. One of the scientists working on the study, Fabian Leendertz, says there are two theories on how the bacteria arrived.

"They are speculating in a very old French newspaper that it may have come from the north, with cattle transport and so on," he said. "Second possibility is that it has always been there, but nobody has seen it, because, in the forest, you don't see when there are dead animals."

Mr. Leendertz says the infected chimpanzees left their communities, and their bodies were later discovered.

"The thing is, wild animals, they hide their weakness, until it's too late, because they can't show weakness, because the others will take their position, or a predator will take them," said Fabian Leendertz.

The chimpanzee deaths occurred more than two years ago, but since anthrax poisoning among chimpanzees had never been recorded, it was not until recently that the toxin was identified.

Another member of the study team, Heinz Ellerbrok, says there is great concern that infectious disease could be passed between the chimpanzees and humans.

"These animals are already highly endangered," said Heinz Ellerbrok. "There is much more contact between ecological niches that have been well separated over ages. And now, people are going into the forest, and there is an exchange, or there might be an exchange of pathogens between chimpanzees, or in general, great apes and humans, and this can go both ways."

Observation groups have monitored the chimpanzee and great ape populations in Tai National Park since 1984.