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.