The deadliest 'Zaire' strain of the Ebola virus is spreading across parts of equatorial Africa at a rate of 50 kilometres a year, say researchers writing in the journal PLoS Biology this week (24 October).
Efforts to protect villagers, and some of the region's last remaining wild apes, from the disease it causes should focus on tackling the front of the virus's "advancing wave", says the team.
The finding goes against the theory that the virus lies dormant most of the time, erupting in random outbreaks because of unknown environmental factors, and helped by increased contact between human and ape populations.
Peter Walsh of the Max Planck Institute for Evolutionary Primatology in Germany, and colleagues, looked at the genetic sequences of some of the Ebola viruses that caused outbreaks in equatorial Africa over the past 30 years.
They found that they were all closely related to the virus that hit Yambuku in the Democratic Republic of Congo (then Zaire) in 1976.
What's more, the further the later outbreaks were from Yambuku, the more their genetic sequences differed. This suggested that the viruses all originated from the 1976 Yambuku virus, and changed gradually as they spread out outwards.
Maps of Ebola Zaire outbreaks
Photo Credit (Walsh et al.)
Walsh began developing the theory that the Ebola virus was spreading in the 1990s when conservationists and villagers noticed that, west of Gabon's Ogooue river, the gorilla population was falling. To the east, however, the gorillas seemed to be fine.
Then, in 2001, the researchers noticed that the gorillas were dying-off in large numbers on the east side too. They reasoned that the river had been protecting the apes by acting as a barrier to the virus — but that the barrier had now been breached.
The genetic data suggests that the virus crossed the river during a 1996 human outbreak at the riverside village Booue.
Some researchers have contested Walsh and colleagues' findings, saying that a study of antibodies — proteins that the immune system makes to fight a virus — indicated that the Ebola virus has already been present throughout the region.
But Walsh believes these antibody findings may be skewed, and could indicate the presence of another strain of Ebola, rather than the Zaire strain that his team studied.
In May 2005, while the paper was in publication, an outbreak of Ebola occurred at Etoumbi village in Gabon.
The location of the outbreak lends support to the researchers' theory that Ebola outbreaks in the region are caused by the virus spreading, rather than by a dormant strain emerging at each site.Link to full paper in PLoS Biology