Scientists are warning of a real risk that the Ebola virus disease could become endemic in west Africa if efforts to end the epidemic slacken as the number of cases falls.

All previous outbreaks of Ebola were stamped out within months and the virus disappeared from the human population each time. Guinea, Sierra Leone and Liberia, however, have been in the grip of the virus for more than a year. While the numbers of cases dropped dramatically in December and early January, they have now plateaued and there are fears that the disease may not be totally eradicated.

“There is that risk,” said Prof Mike Turner, head of infections at the Wellcome Trust. “You can’t quantify how great that risk is but that risk is there. It is not going to be a smooth ride.”

In the week ending on 15 February, 128 new cases were confirmed in the three countries – 74 of them in Sierra Leone, 52 in Guinea and two in Liberia. In the past three weeks, the numbers falling sick in Sierra Leone have stabilised, but they rose from 39 to 65 in Guinea and then dropped again to 52.

Dr Margaret Harris in the director general’s office of the World Health Organisation (WHO) said they were at “a bump in the road”.

She said: “We were all very pleased to see the cases drop so rapidly; we would almost talk about an exponential drop. But we were expecting this plateau. It’s where we get down to harder cases. Communities in some places still don’t believe there is Ebola or have anxieties about notifying cases within their families because they fear quarantine.”

She quoted David Nabarro, the United Nations special envoy for Ebola, who said that going from 100 to 10 cases was easier than from 10 to zero.

Harris said the target of zero cases must be met. “There is no such thing as a little bit of Ebola, because it is such a lethal disease,” she said. “It is such a dangerous disease. It is not something we can consider as a possibility. We absolutely have to eliminate all Ebola.”

If Ebola were to become endemic, said Turner, the virus would have to mutate a little. At the moment, it does not linger in human hosts but causes severe illness very rapidly and goes on to kill at least half of them, which does not help its survival.

The difference between the current epidemic and those in the past is one of scale. Previous Ebola outbreaks affected a few hundred people, but more than 23,000 people have been infected in west Africa, of whom more than half will have died. “There has been more exposure of the virus to humans and a greater chance that it will adapt to a virus that will remain in humans,” said Turner.

“It would need to mutate. It could just be a subtle difference in transmission – the viral load in different [bodily] fluids, for example. If the viral load was low, you might get people who are less obviously sick, so they might still be mobile.”

That could mean that people would not be quick to recognise they had Ebola and were infectious, which could increase the spread. Turner said: “This is not the time to start backing off and thinking we have got this under control. We have got it beaten. This is a time for redoubling the effort.”

Harris said the WHO was seeing a degree of mutation, but not massive change. Some people were experiencing less severe symptoms now. “We’re realising there is a much wider spectrum of disease,” she said, adding that it could have been so from the start.

Dr Peter Walsh, researcher in emergent disease dynamics at the University of Cambridge, said there was no compelling evidence of changes that would make the virus more likely to become endemic.

“We are still a long way from it becoming endemic,” he said. “With an acute outbreak this large, we expect the attenuation to take quite some time. Although it is, in theory, possible that Ebola will become endemic, a far more likely outcome is that the epidemic will eventually burn out, probably by the end of this year.



“The greatest risk factor for it becoming endemic is the international community taking its eye off the ball. If we follow through with existing and planned efforts, the chances of endemic establishment are low.”

Although the virus was very lethal, he said, it did not spread as easily as measles or flu. “Some major evolutionary changes would be required to make it more transmissible; not just tweaking around the edges but big changes in the way it behaves.

“Second, even without major interventions such as treatment centres, people naturally become more educated about the virus and avoid high risk behaviours. This takes a while to kick in, but it already has in much of the outbreak zone and should continue to spread. This is why more should have been spent on public education. It is much more cost effective than expensive treatment facilities.”