EBOLA is wiping out Africa’s great apes. The virus has killed a third of the world’s gorillas in the past decade, and tens of thousands of chimps. Now the disease, which can have a mortality rate of more than 90 per cent, is working its way through the densest population of apes in the world – in Odzala National Park in the Republic of the Congo.
Because of the way researchers believed Ebola was spread there seemed little anyone could do. But it now appears that the calculations were wrong, and the apes could be saved with an existing vaccine.
Ebola was first reported at the town of Yambuku, in what was then Zaire in 1976. Since then the virus has cropped up sporadically across central Africa, leading epidemiologists to suspect that it was lurking across the region in some as yet unknown “reservoir” species that does not die of it. Humans got Ebola, the theory went, when they came into contact either with the reservoir species or with infected apes.
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But the pattern of outbreaks made Peter Walsh of the Max Planck Institute for Anthropology in Leipzig, Germany, and his colleagues suspect this model was wrong, and there is no reservoir species. Instead, Ebola has been advancing into uninfected territory in a wave, Walsh says (see Map).
The virus strain causing each successive outbreak is genetically descended from the strain in the previous outbreak – in a direct line from Yambuku. If the virus had existed for years throughout the region, variants should be much more mixed up, and should not all come from one ancestor (Public Library of Science Biology, DOI: 10.1371/journal.pbio.0030371).
The pattern was seen even when the wave changed direction, when it crossed the Ogooué river in Gabon in 1996. And the rate of movement has been a steady 50 kilometres a year, as expected of a virus moving in a wave through a widespread host population.
Epidemiologists thought Ebola had been a long-time resident of the region partly because of a calculation showing that it mutates slowly. But Walsh believes that there is no good evidence for slow mutation. A moving wave of virus would have been able to mutate as observed if it is merely as fast as most RNA viruses, he says.
“I was very impressed by the data,” says Tom Geisbert of the US Army Medical Research Institute of Infectious Diseases in Fort Detrick, Maryland, who helped develop a potential vaccine for Ebola (91av 11 June, p 5). “It’s incredible how they predicted where the virus would go next.”
A human outbreak in May and ape deaths in August have followed the predicted wave heading east through Odzala. Ominously, the Ebola wave is now heading for the region’s last large populations of apes north and east of Odzala, in Nouabale-Ndoki National Park. Walsh gives it three to four years to reach those areas.
“The virus, which can have a 90 per cent mortality rate, is working its way through the densest population of apes in the world”
If, as he suspects, the virus does not linger after the wave moves on, it should be possible to vaccinate the apes there to save them while the plague passes. Walsh organised a meeting of Ebola specialists in Washington DC in April to discuss this. But so far no one wants to fund the research needed for a vaccination programme, such as determining dosages and delivery of the vaccine. “In three or four years it will be too late,” Walsh says. “The apes will be gone.”
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