Scientists warn that wild ape populations, already devastated by poachers and habitat loss, now face a new threat from Ebola and other viruses

Vaccine campaigns to protect wild chimpanzees, gorillas and orang-utans could be the only hope of preventing the planet’s remaining great apes from suffering catastrophic population crashes, scientists have warned.

They say that recent surveys of several populations of great apes have revealed devastating drops in numbers and that protective steps must now be taken as a matter of urgency.

One report, published last week, revealed that numbers of Grauer’s gorilla, the largest of all the great apes, had plunged from 17,000 to fewer than 4,000 over the past 20 years. Other reports have said that devastating forest fires in Indonesia, in addition to the spread of palm oil plantations, have wiped out thousands of square miles of rainforest, the home of the orang-utan. Again, species numbers have plummeted.

“Several factors are involved in the terrible losses that are occurring in our fellow great apes,” said Peter Walsh, a primate ecologist at Cambridge. “However, diseases – some spread by humans – are the ones that we can do most about in the short term.”

Three main human-induced impacts are blamed for the devastation of great ape numbers in recent years:

■ Habitat loss. Swaths of jungle in Asia have been turned to palm oil plantations and have brought the orang-utan to the edge of extinction in many areas. Now oil companies are eyeing up central Africa for similar treatment;

■ Bushmeat. Eating chimpanzee and gorilla meat has become a widespread source of protein in several countries in central Africa and has depleted numbers further;

■ Disease. Epidemics such as Ebola fever have affected humans, but also chimps and gorillas. The latter suffered a death toll of tens of thousands during the last outbreak.

In addition, the sale of baby chimps and gorillas to China – where they are being bought as pets – has emerged as a new threat in the past few years.

In each case, serious depletion of great ape numbers has occurred, undermining decades of conservation work by primatologists such as Jane Goodall, who has worked extensively with chimpanzees.

The disease, probably spread by bats, killed gorillas and chimps in remote strongholds where we thought they were safe Peter Walsh, ecologist at Cambridge University

However, it is the danger of disease that most alarms scientists, including Walsh. “I did a survey of the impact of Ebola over the past 20 years and found that about a third of the world’s gorillas were wiped out by the disease. It also badly affected chimpanzees. The key point is that the disease – which was probably spread by bats – killed gorillas and chimps in remote strongholds where we thought they were safe.”

The problems facing the great apes are alarming, but scientists stress that there are still reasons for optimism. Chris Ransom, the conservation programmes manager for West Africa at the London Zoological Society, was one of a group of scientists who discovered several years ago that the cross-river gorilla – then thought to be extinct – had survived in small numbers on the Nigeria-Cameroon border. “We now think there are about 300 left in the wild. Once we thought there were none. So there is some ground for cheer,” said Ransom.

In addition, two small populations of mountain gorillas in Uganda and Rwanda are also thriving. These groups are the focus of a major wildlife tourism industry that has been hailed as playing a key role in protecting great apes. “That tourism has brought money and jobs to the countries,” said Walsh. “Gorillas are seen as an investment to protect.”

There is a downside to this story, however. Tourists also bring in germs. Guides have got around this by providing visitors with face masks and they impose guidelines to prevent people with colds, or possibly other viruses from getting near apes. “Nevertheless, there is always going to be a risk of passing on germs from humans to apes,” added Ransom.

This point was stressed by Walsh. “Our research has made it clear that viruses such as Ebola can affect gorillas and chimps, as can human respiratory viruses. We need to protect them against these viruses – using oral vaccines. And that is what we plan to do this summer. We are going to do a trial on an Ebola vaccine on great apes. We will leave out bait in the form of food that contains the vaccine. Within five years, I would want all gorillas and chimpanzees that come anywhere near humans to be vaccinated against Ebola or respiratory diseases. That is the only way we can go.”

Dr Carlos Drew, the director of the global species programme at WWF International, said he hoped it would be possible to develop a vaccine in the form of an attenuated Ebola virus that would then spread between individual animals, and so allow protection against the disease to pass through groups without the need for further vaccination.

“In about a year, we hope we will be able to announce concrete plans to carry out such a programme,” said Drew. “Certainly we have to act soon.”

This argument was backed by Walsh. “Unless we do something now, great apes will no longer be part of the functioning ecosystems of Africa or of Asia,” he said. “Their populations will be so small and isolated, and they will have to be managed so carefully, that they will only be able to exist on land that is run like a zoo or a park. Ecologically, they will be extinct – which makes what we do now so important and so urgent.”