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Tracing the Origin of the Ebola Outbreak Is Really Hard

Researchers suggest a tree full of bats could have transmitted the Ebola virus to the outbreak's patient zero, but we might never know for sure.
​Children in Meliandou, Guinea. Image: ​UNICEF Guinea/Flickr

​There have now been over 20,000 cases of Ebola in West Africa, according to the World Health Organisation's co​unt, and thousands of deaths. But where did it all begin?

Researche​rs believe that the current outbreak's "patient zero" was a two-year-old boy in Meliandou, a village in Guinea, who died in December 2013. But how did he get infected?

Fabian Leendertz, head of the Epidemiology of Highly Pathogenic Microorganisms research group at the Robert Koch Institute in Germany, was part of a team that attempted to find the source of the Ebola epidemic shortly after the outbreak started. In a paper published in EM​BO Molecular Medicine, he and his co-authors suggest one potential cause: a hollow tree full of bats where the boy may have played.

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The result is frustratingly inconclusive, as the tree was largely burned before the team got to it, and they were unable to definitively prove the bats had the disease. But Leendertz thinks it's as close to an answer as we'll get.

The burned-out tree. Image: ​Saéz AM et al.: Investigating the zoonotic origin of the West African Ebola epidemic. EMBO Molecular Medicine January 2015

MOTHERBOARD: Let's start at the beginning. How do you go about trying to trace the origin of this outbreak?
Fabian Leendertz: We followed the two classic hypotheses of how the virus can be transmitted from animals to humans. In most of the outbreaks of the Ebola virus, there was no transmission directly from the reservoir as far as we know (from the suspect reservoir, the bats, to humans). It was rather that great apes and other wildlife—especially gorillas but also chimpanzees and others—contracted the virus from the reservoir and then you had a large epidemic in wildlife.

There was always a population decline, especially of wild great apes, before epidemics in humans. So to look at that possibility, we went to two of the main forests in southern Guinea where we know that there are still chimpanzees and other wildlife, and we repeated the monitoring that the Wild Chimpanzee Found​ation had done several years ago to see if there were much less chimpanzees or duikers or any decline, or if we [could] even find carcasses. But during that, we found the wildlife population is largely stable and there is no major decline or anything observed in those last areas where you have a high mass population density of wildlife.​

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While we were there, the health authorities informed us about who the index case was and where this boy lived, so we split the team. We let the monitoring team continue doing their work in the forest, and I took three vets from my team and the anthropologist, and we went to that index village and spent more than a week there talking to people, trying to find out who hunts, how they hunt, and what is special about Meliandou, this village. What was obvious was that it's not a village in the remote rainforest like you think when you think about Ebola.

IT'S NOT A VILLAGE IN THE REMOTE RAINFOREST LIKE YOU THINK WHEN YOU THINK ABOUT EBOLA

That's not at all the case; it's a normal village in a deforested area, there's no forest nearby, it's just the old people in the village who still remember that there were chimpanzees and other larger wildlife in the area, but that's a long time ago. So it's really a modern African setting. That also contributed for us to exclude that last wildlife amplifier hypothesis. So we asked them, how could it have jumped—where did it come from?

And you suggest that it could have originated instead with a bat colony—what's the evidence for that?
People in the village hunt bats like they do in every village in West Africa; people will opportunistically kill fruit bats, because there's quite some meat on them and they're just easy prey. If there are a few fruit bats between some bananas or some coconuts, they will just kill them and take them home. People do that as well in Meliandou, but children don't—that's adults. So we wondered now if the index case is really a child, and our anthropologist also investigated that and she could confirm that it's really likely this boy is the index case. So how do children get into contact with bats?

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She interviewed lots of children, and we observed them, and we played with them, and we realised that children are also big bat hunters. They take their sticks and they poke around these roofs and try to disturb small bats—the insectivorous bats that live on these roofs. When they drop on the ground, they kill them and they put them on a stick and then grill them over the fire. So they also have exposure to bats.

But that's also something you find in every village. You know, all the kids, they don't have TV—they play, they go hunting. It's exciting for them, and extra protein and everything. So then we wondered what's special about Meliandou, is there some exceptional risk there? That led us to the discovery of this huge tree, which the villagers had burned a few days before we arrived. They didn't know we were coming, so they burned that tree. We asked more questions about that tree, and the kids used to play in that tree because it's near the path where the women go to wash themselves in the small river. So that used to be a common playground. And in that tree, they said there was a huge colony of bats. When they set the tree on fire—we don't know why, by chance probably, kids playing with fire in the tree or so—then it started "raining bats," as they said. There were lots and lots of them.

We were really excited when we heard that, because that's a similar situation to Marb​urg virus, which you know is sort of the sister of Ebola virus. There are places in Uganda where tourists​ got infected by just entering a cave to watch a huge colony of fruit bats, and they contracted the virus. So you are in contact with a huge number of bats, a much bigger number than normally by hunting individual bats.

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Our problem was then that the tree was obviously burned—you can see in the pho​tos in the paper—so we couldn't catch the bats any more to finally prove if the virus was really in those bats. I was really disappointed about that, obviously, because that would have been a really good proof. However, in order to confirm which bats lived in that tree we collected soil samples, and there's a method you can find the DNA still of the animals living on a certain ground by looking at the DNA of mammals in general. And so we could confirm it was fre​e-tailed bats living in the tree, and this species fit very well the description the villagers told us of what the bats looked like.

It's not a big find like we'd normally try to do; that's all we have, so we thought we'd have to publish it.

I THINK IT WON'T BE POSSIBLE TO PROVE THE REAL ORIGIN OF THE EPIDEMIC

Yeah, it sounds like you had quite a few challenges along the way. What are the main difficulties of trying to prove the origin?
We will never be able to. In that case, I think it won't be possible to prove the real origin of the epidemic. I mean, we went in April, just ten days after the WHO confirmed that Ebola caused this outbreak; we were there really fast but still we were there three months after the epidemic started. If you want to investigate now further you're even further away, and memory is bad, and the ecology changes and everything. I fear, for this outbreak, we've got as close as you can get.

Obviously we are now doing more research in that sense and we are looking into the ecology of these bats and we are sampling lots of these bats, entire colonies, to try to trace down the virus in bats. But that's a big challenge, and that's more a long-term project. If we find good evidence for the virus in these bats, obviously that strengthens the story we published.

And how did you get involved in studying zoonotic infections in the first place?
I've been working in West Africa for 15 years—mainly in the Ivory Coast, the neighbouring country, in Taï National​ Park. My speciality is great apes so we were working on Ebola for some years, because Ebola is a main problem for great apes. I have people on the ground and we have all the logistics next door, so it was easy for us to rush there.

What's next for the research?
​Next for Ebola virus research, we are closely working together with ecologists to try to understand the potential host ecology, especially for these free-tailed bats. We're now marking all communities, we're following them up over hopefully many years, we're taking samples of them to look for traces of the virus—antibodies against the virus—to try to get some depth in these reservoir studies.

What's been done mostly until now is a bit like what we did—when there was an outbreak we've rushed there and caught a lot of bats and tested them, and almost never found the virus. So we think we need more in-depth studies linked to the ecology, and also to understand how the virus circulates between regions.