Yesterday, the New England Journal of Medicine (NEJM) published an article entitled “Emergence of Zaire Ebola Virus Disease in Guinea – Preliminary Report”. The authors found that the strain of Zaire ebolavirus (ZEBOV) that is currently spreading throughout West Africa is significantly different from the ZEBOV strains that we’ve seen before in Gabon and the DRC… Different enough that it ought to be considered a totally different strain of ZEBOV – despite being the same species.
This is where the lingo gets a little confusing.
In short, there are five different species of Ebolavirus. Four of them yield pathogenesis in humans: Zaire ebolavirus (ZEBOV), Taï Forest ebolavirus (TAFV), Sudan ebolavirus (SUDV), and Bundibugyo ebolavirus (BDBV). A few weeks ago, we learned that the virus responsible for the current outbreak in West Africa shared a lot of genetic similarities with the Zaire species. What was really interesting about this at the time was that ZEBOV had historically been constrained to Central Africa. A lot of us were curious about how it may have gotten to West Africa in the first place.
The NEJM article tells us that – while the virus causing the current outbreak is in fact ZEBOV – it’s not the same ZEBOV as the ones that have caused Ebola in Gabon and the DRC. This is where the word “strain” becomes very important; Guinea ZEBOV, Gabon ZEBOV, and DRC ZEBOV are all the same species, but NOT the same strain. According to the authors of the NEJM paper, Guinea ZEBOV evolved separately from the Gabon and DRC strains. With this in mind, they suggest that ZEBOV has been floating around Guinea for quite some time; it just didn’t rear its ugly head among humans until now.
Before the NEJM report was released, I proposed that industrial logging and habitat displacement in Central Africa might have forced ZEBOV-infected bats to fly out West. However, if the NEJM brief is right, it would mean that the Central African bats didn’t have to bring Ebola with them because a separate strain of ZEBOV was already alive and kicking in Guinea… Though it didn’t show up in humans until very recently.
So, what changed?
The authors don’t try to answer this question in the NEJM report, but I would suggest that increased prevalence of the virus among West African reservoir bat species is probably a piece of the puzzle. (Previous studies have shown that we need about a 5% prevalence among bat reservoirs to see a human outbreak of Ebola.) It’s not yet clear what pushed West African bat infections over this threshold, but environmental stress might have been a contributing factor.
While deforestation in the DRC may not have brought ZEBOV to Guinea, it’s possible that it still played a significant role in the emergence of Ebola among West African human populations. If enough bats migrated out West due to habitat displacement, increased population density would boost ZEBOV transmission… Resulting in above average Ebola prevalence among reservoir bats. This enabling condition – paired with the common Guinean practice of poaching bats – might have created viable circumstances for emergence among humans… But this is just one possible theory.
Understanding Ebola’s emergence in Guinea might help us prevent similar outbreaks in the future. While the NEJM article gives us valuable insight into the virus that’s responsible for #Ebola2014, it can’t be considered in isolation when addressing this challenge. The animals that carry the disease and the environment in which they live are equally important. We must also be willing to take a step back and look at the ecosystem system that connects them.