An estimated 5,500 gorillas have died in Gabon and the Republic of Congo since 2001 after an outbreak of a strain of the Ebola virus, researchers said.

The deadly effects of Ebola, when combined with commercial hunting, may be enough to push the great apes to extinction, according to the study published Thursday in the journal Science.

A western lowland gorilla and her son at the Congo Gorilla Forest exhibit at New York's Bronx Zoo. A study published in December suggests the Ebola virus is responsible for the deaths of 5,500 gorillas from 2002 to 2003.
(Mary Altaffer/Associated Press)

"Ape species that were abundant and widely distributed a decade ago are rapidly being reduced to remnant populations," the study found.

"Add commercial hunting to the mix, and we have a recipe for rapid ecological extinction," the report says.

Reports of the gorilla deaths were widely reported in 2003, but the cause of the mortality was not known.

But the study conducted by Magdalena Bermejo, the lead researcher from the University of Barcelona, and colleagues at Max Planck Institute and Uppsala University outlines strong evidence Ebola was responsible for the deaths.

Bermejo witnessed first-hand the effect of the virus in the Lossi Sanctuary, a village-created area designed to acclimatize gorillas to humans, located between the towns of Kelle and Mbomo in the Congo.

Between 2002 and 2003, 93 per cent of the 238 western gorillas at the sanctuary died. Remains found later tested positive for Ebola Zaire, one of four strains of the virus.

Surveys of nesting areas showed gorillas had a 95 per cent mortality rate west of the sanctuary, and that chimpanzees were also heavily affected, with a mortality rate of 77 per cent.

The Ebola hemorrhagic fever causes massive internal and external bleeding and can kill up to 90 per cent of those infected.

It has killed over 1,200 people since its first recorded outbreak among humans in 1976.