By Alex Kirby
BBC News Online environment correspondent
Animals killed by the Ebola virus could help give warnings to people of a possible outbreak, researchers say.
Gorilla bones: They can reveal traces of the virus
A report in the journal Science says some recent human outbreaks in Gabon and the Republic of Congo came soon after big declines in animal numbers.
The animals had been killed by several different strains of Ebola, and some species could become locally extinct.
The animals affected were chimpanzees, gorillas and duikers, a small and shy antelope common across much of Africa.
Ebola kills in up to 90% of cases, causing massive internal bleeding - and there are few options for doctors, though an experimental drug appears to offer some hope.
The researchers, from Africa, Europe and the US, say the most lethal Ebola strains occur in Gabon, the Republic of Congo (RC) and the Democratic Republic of Congo (DRC).
There have been five human Ebola outbreaks in western central Africa in the past two years, with 313 people affected of whom 264 died. All began after people had handled dead animals.
The team say there were many animal carcasses in the forests just before and during the 2001 human Ebola outbreaks in Gabon.
Only the hand of this gorilla retains its flesh
Dead animals are seldom found there, as they decompose quickly if they are not eaten: a gorilla decomposes completely within a month.
The researchers say they think the numbers of carcasses found mean hundreds or even thousands of animals may have died from the virus.
In one sanctuary signs of gorillas and duikers fell by 50% from 2002 to 2003, and chimpanzee indicators by 88%. Eight gorilla groups, totalling 143 animals, disappeared and have not been seen since.
The team writes: "Our data also confirm that Ebola outbreaks occur abruptly, exterminating exposed animal populations very rapidly and very locally."
Duikers breed rapidly and may recover, but "the slow reproductive cycle of the great apes, together with hunting and poaching, may lead to their extinction in western central Africa".
Doctors Eric Leroy and Pierre Rouquet examine a dead gorilla
Ape populations in Gabon declined by more than half between 1983 and 2000.
Ebola was the cause of death in 10 gorillas, three chimpanzees and one duiker.
But carcasses left in the forest were not infectious after three to four days, so transmission between different groups of apes is unlikely.
Cause and effect
The researchers think outbreaks in animals probably do not happen as a single wave spreading through the Congo basin, but through multiple infections from "still unknown natural hosts".
Their finding that human outbreaks tended to follow soon after infections in wildlife offers some hope for people in the area.
They conclude: "Almost all human Ebola outbreaks in Gabon and RC have been linked to the handling of dead animals by villagers or hunters, and increased animal mortality always preceded the first human cases."
The lead author is Dr Eric Leroy of the International Medical Research Centre, Franceville, Gabon.
He told BBC News Online: "If we can detect dead animals and diagnose Ebola infection in the carcasses, we can prevent the appearance of human outbreaks with health teams going into the villages."
Images courtesy of Dr Pierre Rouquet.