Chimpanzees and gorillas in West Africa are caught in a pincer movement between hunting and the Ebola virus, researchers have warned. The bushmeat trade is threatening their populations near towns, while Ebola is killing almost every animal in some remote areas.

In a paper on page 611 of this issue, ecologist Peter Walsh of Princeton University, New Jersey, reports that populations of both species have plunged by about half over the past 20 years. Walsh calls for the conservation status of each to be shifted from 'endangered' to 'critically endangered'.

The finding that apes that live near people are in decline because of hunting confirms the long-standing suspicions of conservation workers. But the extent of Ebola's reach in the ape populations has taken experts by surprise. In one remote area where there is little or no hunting, the virus has cut the population by more than 90% since 1991.

Everyone agrees that ape populations are under threat, but John Oates of the City University of New York, author of the IUCN's 1996 status survey and action plan for African apes, thinks it would be premature to reclassify them based on this evidence. It is not known whether these declines are repeated across Africa, he says, or how the picture might differ between chimpanzees and gorillas.

Government officials in Gabon, where the survey took place, say that chimpanzees and gorillas are protected there, and that every effort is made to stop illegal hunting. “The population of apes has been quite stable recently,” says Pierre Ngavoura, director of water and forests at the Ministry of Water, Forests, Fisheries and the Environment.

Walsh's team counted animals by surveying overnight sleeping nests in many areas, giving a combined population for chimpanzees and gorillas. Gorilla populations in neighbouring Congo — the other remaining population stronghold of these apes in West Africa — are thought to be experiencing similarly high mortality.

The Ebola epidemic may be a result of high ape population density, says Alexander Harcourt, a primatologist at the University of California, Davis. “The normal density of gorillas is about one every two square kilometres,” he says. “But in some of these regions there are ten in every square kilometre.”

Another possibility, says wildlife-disease expert Andrew Cunningham of the Institute of Zoology in London, is that environmental changes, such as human encroachment on the forest, have brought apes and the virus into closer contact. “The mortality suggests that there has been some trigger leading to the emergence of Ebola as an important cause of ape mortality,” he says. The Ebola virus is thought to reside in an unidentified reservoir species — possibly a fruit bat or other small mammal.

Ebola spreads from apes to humans when a hunter kills and eats an animal, or when someone comes into contact with an infected ape corpse. The government in Gabon is seeking to educate its people about the risks of ape hunting, Ngavoura says.

An Ebola outbreak in Gabon killed 50 people between December 2001 and March 2002, mostly in the remote areas where the ape disease is worst. A current outbreak in Congo has killed 120, according to the World Health Organization.

A further potential hazard is created by commercial poachers who hunt bushmeat for sale in urban Africa, says Richard Ruggiero, African programme officer for the US Fish and Wildlife Service. “It's a very dangerous situation in terms of global health,” he says. “I don't think there's a city in the world with a West African immigrant population that doesn't receive ape meat.”

Options to control the disease include cutting or digging barriers to quarantine infected populations, moving apes away from where the epidemic is raging, or culling the reservoir species, if it can be identified.

An experimental vaccine has shown good results in monkeys (see Nature 408, 605–609; 2000), and may be ready for human trials in a year or two, says Gary Nabel, director of the US National Institutes of Health's Vaccine Research Centre in Bethesda, Maryland. The vaccine might work for apes too, he says. “We'd very much like to use it to help.”