The Ebola virus is extremely virulent, killing 50-90% of victims (Image: NIH)

The Ebola virus may have killed more than 5000 gorillas in West Africa, enough to send them into extinction if people continue to hunt them too, researchers say.The virus is spreading from one group of the already endangered animals to another, the international team of experts report today in the journal Science And it appears to be spreading faster than it is among humans."The Zaire strain of Ebola virus killed about 5000 gorillas in our study area alone," write primatologist Dr Magdalena Bermejo of the University of Barcelona in Spain and colleagues.Ebola haemorrhagic fever is one of the most virulent viruses ever seen, killing 50-90% of victims.The World Health Organization says that it killed 1200 people infected between its discovery in 1976 and 2004.The virus is transmitted by direct contact with blood, organs or other bodily fluids. There is no cure or good treatment, although several groups are working on vaccines.Several experts have noted that chimpanzees and gorillas are also killed by the virus, and suspect that people may have caught it from infected apes, perhaps when hunting them.But it is not clear whether the gorillas are infecting one another, or being repeatedly infected and reinfected by another species of animal, perhaps a bat.Bermejo's team has been studying a group of western gorillas in the Lossi Sanctuary in northwest Republic of Congo."By 2002 we had identified 10 social groups with 143 individuals," they write.In 2001 and 2002, several outbreaks of Ebola had begun killing people along the Gabon-Congo border.By October 2002, the researchers had found 32 dead gorillas, and of the 12 they tested for Ebola, nine were positive."[Bermejo] knew these animals individually, and in the course of three months they all died," says Dr Peter Walsh, an ecologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who worked on the study.Eventually the researchers counted 221 dead gorillas. Based on what they and other experts knew, Walsh extrapolated what the total impact must be to come up with the estimate of 5500 gorillas killed by Ebola in that area.He says no one knows precisely how many gorillas are in the world and how many have died."But I know what's the typical mortality rate in those areas that are affected. It's an educated guess. A quarter of the gorillas in the world have died from Ebola in the last 12 years. It's huge," Walsh says."Add commercial hunting to the mix, and we have a recipe for rapid ecological extinction," the researchers write.Their report supports a study published in July that showed gorillas were spreading the virus within their social groups."Our work is complementary to that; we have shown it is spreading between groups," Walsh says.Walsh says gorilla groups share territories, often eating fruit from the same tree, although at different times. Faeces from a sick gorilla could easily infect other gorillas.Gorillas and chimpanzees also touch and handle the bodies of other apes when they find them, something known to transmit Ebola between humans."The issue here is that there is a certain amount of work that needs to be done to take these vaccines that already exist and put them into gorillas," Walsh says.