The Ebola virus has killed more than 5,000 western lowland gorillas in the past four years according to scientists who warn that the world's largest ape is suffering a dramatic population decline that could soon lead to its total extinction.
The virus is one of the deadliest infectious agents known to man. It also affects other primate species and its rapid spread among chimps and gorillas in parts of central Africa has alarmed conservationists. A study published in the journal Science is one of the first to estimate accurately the number of western lowland gorillas affected by the epidemic, which appears to spread from ape to ape.
Before the latest study, scientists were not sure whether Ebola was spreading into apes from other animals in the forest which acted as natural "reservoirs" of the virus. The latest findings suggest there is direct transmission from one ape to another.
The type of Ebola virus killing the gorillas is known as the Zaire strain which has repeatedly infected humans in Gabon and Congo, said Magdalene Bermejo of the Ecoystemes Forestiers d'Afrique Centrale, based in Libreville, Gabon.
"During each human outbreak, carcasses of western gorillas and chimpanzees have been found in neighbouring forests," said Dr Bermejo, a primatologist whose study of the gorillas' deaths with colleagues from Germany and Spain is published in Science.
Dr Bermejo was part of a project that was studying 10 social groups of gorillas, totalling some 143 individuals, living in the vicinity of the Lossi Sanctuary in Congo.
In late 2001, human outbreaks of Ebola flared up along the border between Gabon and Congo. In June 2002 the first dead gorilla was found 15km from the Lossi sanctuary.
By October, gorillas were dying within the sanctuary and, over the next four months, the scientists counted 32 carcasses. A dozen were tested for Ebola and nine tested positive for the Zaire strain of Ebola.
Between October 2002 and January 2003, 130 of the 143 gorillas that were being studied as part of the gorilla project had died - a mortality rate of more than 90 per cent. In the following months further carcasses were reported in parts of the forest further south of the sanctuary.
The virus appeared to be spreading from one gorilla group to another in a sequential manner consistent with ape-to-ape transmission, the scientists said.
A survey of nesting sites used by gorillas living in a 2,700 sq km area surrounding the Lossi sanctuary found that the number of occupied nests had fallen by 96 per cent.
The scientists estimated that would suggest about 5,000 gorillas living in the region had been killed by the Ebola virus since the epidemic began in 2002.
The deaths among the western lowland gorillas are mirrored among eastern gorillas. The number of eastern lowland gorillas has fallen by 70 per cent during the past 10 years. In 1994, there were some 17,000, but now fewer than 5,000 exist.
"We hope that the study dispels any lingering doubts that [Ebola virus] has caused massive gorilla die-off," the scientists say in their Science study.
"The Lossi outbreaks killed about as many gorillas as survive in the entire eastern gorilla species. Yet Lossi represents only a small fraction of the western gorilla killed by [Ebola] in the past decade or indeed of the number at high risk in the next five years.
"Add commercial hunting to the mix, and we have a recipe for rapid ecological extinction. Ape species that were abundant and widely distributed a decade ago are rapidly being reduced to tiny remnant populations," they say.
Scientists believe there may be a reservoir species for Ebola, such as a bird or a bat, that continues to harbour the virus without being killed off itself.
Although such a species has not been identified, there was the possibility that it was close contact between gorillas and this second animal that was causing the virus to spread, rather than ape-to-ape transmission.
"An answer has proved elusive. Scientists had no idea which of hundreds or even thousands of forest species might serve as a reservoir, and it is extremely difficult to observe whether apes in the wild are passing a virus to each other," says an editorial in Science.
"But during the past year, a consensus has begun to emerge. Although both mechanisms of spread probably play a role, evidence has been mounting that apes are indeed passing the virus to each other," it says.
What is Ebola?
The Ebola virus causes haemorrhagic fever and is named after a river in the Democratic Republic of Congo, formerly Zaire, where the first outbreaks in humans were reported. It has a high mortality rate, killing at least 50 per cent of those who become infected.
Symptoms range from vomiting and diarrhoea to general body pain, fever and internal bleeding, which soon lead to death. The first outbreak of the Zaire strain of Ebola occurred in 1976. It has one of the highest rates of mortality.
Transmission in humans is through body fluids and the incubation period can be anything from two days to 21 days. It has only been in the past few years that gorillas and chimpanzees have been found to be infected with Ebola, which appears to produce similar rates of mortality in the primates.Reuse content