Ebola evolves deadly new tricks

The Ebola virus that is wiping out gorillas and chimpanzees could become even harder to stop. It now appears there are at least two strains of the so-called Ebola Zaire virus, not one as previously thought, and they can swap genes between them - a rare ability that will make it harder to develop vaccines to fight them.

The Ebola virus causes fever and haemorrhage and kills up to 90 per cent of people who catch it. Since its discovery in 1976, it has spread across Africa, probably carried by bats. An outbreak which started in August in the Democratic Republic of the Congo has so far affected 76 people.

Over the past few years, Ebola has killed thousands of apes, including so many lowland gorillas that the species is now classed as endangered. Despite this, no one had sequenced a virus sample taken from an ape until now.

Eric LeRoy and colleagues at the International Centre for Medical Research in Franceville, Gabon, retrieved all or part of the Ebola virus from the remains of six gorillas and a chimpanzee. The virus samples had similar genes, and they were akin to samples of the Ebola virus that has claimed human victims in the region.

However, when the researchers compared the Ebola gene sequences taken from the apes with all the other known sequences of the Ebola Zaire virus, they were in for a surprise: the viruses belong to two separate strains. Samples collected before 1996, all taken from humans, belong to one cluster of viruses with gene sequences more similar to each other than they are to virus samples collected since then, from both humans and apes. Models that gauge the speed of mutation of the virus suggest the two strains actually diverged from one another around 1976.

But the real shock, says LeRoy, is that some recent Ebola samples collected from people carry one gene specific to each strain. That means the virus has recombined, something rare in RNA viruses such as Ebola, and something never seen before in filoviruses, the family to which Ebola belongs.

Leading Ebola expert Bob Swanepoel of the National Institute for Infectious Diseases in South Africa is not surprised by this variation: "There could be totally new filoviruses out there living a quiet life under our noses. We just haven't looked," he says.

If Ebola can recombine, it will be able to produce a greater variety of strains. The upshot, says Peter Walsh of the Max Planck Institute for Anthropology in Leipzig, Germany, is that vaccines which target a specific surface protein on the Ebola virus might suddenly stop working if a strain equipped with a different protein should emerge. Such viral evolution could stymie attempts to protect both people and the endangered lowland gorillas.

Published on 2 November 2007