The spread of Ebola

Changes in lifestyle, such as increased human connectivity and movement, mean that new outbreaks of Ebola virus are likely to be very different to previous outbreaks. Chris Smith spoke to Dr David Pigott, from the University of Oxford, about why the 2014 outbreak was so much larger and deadlier than before...

David - Given the recent outbreak in Guinea, we've been looking at how Ebola is actually spread around the African area. So, when you consider Ebola, it's important to recognise that there are two distinct processes going on. Firstly, there's what we call the zoonotic phase where there's cycling of Ebola viruses between different animals and we currently think that bat species are the main reservoir of this, but also great apes such as gorillas and chimpanzees can also be infected. Now occasionally, humans interact with this animal population, normally through bush meat hunting or butchering of animal carcasses and then it transitions from animals to humans, and then there is subsequent human to human transmission going on. So, in the outbreak in Guinea at the minute, genetic evidence has shown that there's only actually been one transmission event from animals to humans and that all subsequent cases are actually human to human interactions, so, burial practices, or caring after sick relatives for instance, or within the hospital setting itself.

Chris - What can you tell us about Ebola's history, because this is not a new infection, it's been knocking around and periodically surfacing for a number of years, isn't it?

David - Yeah, so, it's got about a 45-year history. In 1976, there was a series of unexplained symptoms that had never really been recognised before that occurred simultaneously in what was then called Zaire, so, Northern Democratic Republic of Congo and also in the southern borders of Sudan. And since then, we've seen small numbers of cases, no more than 400 to 500 individuals involved in an outbreak and spreading no further than one or two imported cases elsewhere in the world. So, with the Guinea outbreak, what we've actually seen is something that seems completely different to the previous history of Ebola. We've had more individuals infected than all other outbreaks of Ebola combined, and we've actually seen it spread to more countries than we ever have before. So, it's moved from Guinea to Liberia, Sierra Leone, and then through air travel to Nigeria and Senegal recently.

Chris - So, would you say then that the sort of dynamics, the way in which the disease is behaving, in this outbreak are clearly different than patterns you've seen in previous years?

David - So, in the work that we've been doing, we've been looking at how the connectivity of these populations has changed dramatically over the last 45 years. The population size themselves has nearly tripled and actually we only have international air connectivity data for the last five years or so, but within that period of time, some countries have actually increased their outbound flights by three or four times, so it's a pretty rapid increase.

Chris - And why do you think that those things changing, could increase the likelihood that Ebola will surface and alter the way in which the disease behaves in communities, like it has?

David - All these  human changes are actually going to be very important in how we see human to human transmission of the disease. So, increasing population size, increasing population density, is going to mean that if individuals are infected, they're more likely to come across others to infect them.

Chris - So, how did you go about doing this study? How did you get the data that's enabled you to begin to look at how the disease might change or how there are various factors that could influence the dynamics of this infection?

David - So, we looked at the last 45 years or so of published articles on Ebola virus, both within humans and animals, and basically, it was a bit of detective work identifying human outbreaks and then backtracking it all the way to see who was the first person that got infected and, if possible how they became infected.

Chris - And what implications are there? On the basis of what you have found in your analysis, for both preventing future episodes and outbreaks of Ebola but also perhaps managing better the current crisis we've got.

David - Yeah. So, there are two different aspects to that. I think one of the most important things that our paper shows is that the range of different countries that are at risk of Ebola is actually a lot greater than we first thought. So, we predict that there's 22 countries in Central and Western Africa that there's the environmental possibility that animal to human transmission can occur. Seven of those countries, we've actually seen that transmission occur but there are 15 countries that haven't actually seen Ebola in those areas. So, these would be great places to start surveying bat populations or other mammals that could be reservoirs of Ebola, i.e. animals that are infected with Ebola but don't show signs of infection, so they don't die necessarily, to get a better understanding of how the disease is cycling within animal populations. And that's a very important process to understand because then it allows us to assess the risk of, does hunting in this area actually mean that the transmission from animal to human could occur?

Chris - Now given that you've identified the potential for Ebola outbreaks is much bigger than the present pattern of activity, what sorts of strategies would you urge apart from obviously monitoring to try to prevent this happening in future or to get a handle on where it might happen and mobilise some sort of resource to stop it?

David - So, I think it'd be a very tall order to try and say, "Oh, the next area where Ebola is going to transition from animals to humans is going to be...," but one thing that we could do is look to see how these populations are connected. So, there are some people that use, for instance, mobile phone data to see how people move within a country. So, if we can identify areas that are very well connected, we could perhaps prioritise those transport nodes, as it were, to prevent further spread of any infection, should it actually occur.