Ebola: Why here, why now, and why so deadly?

This is the first of two posts on Ebola this week. Tune in tomorrow for a look at the big questions facing Ebola researchers.

You’ve probably heard that the current outbreak of Ebola virus is the largest ever, and thus the deadliest. Today’s death toll is 729 out of 1,323 infected since the outbreak began late last year.

Unlike other diseases, Ebola is not always around. Measles infects twenty million people a year; Ebola, outside of outbreaks, infects (as far as we know) zero. Before the current outbreak, the previous infection was more than a year earlier. In between outbreaks, the disease doesn’t exist at a baseline level in the population; it is simply gone.

So why is a huge outbreak occurring now? Why in western Africa, thousands of miles from the place this strain of virus was last seen, in 2009? And why has it gotten so large? Those are the questions Daniel Bausch discusses in an article published today in PLOS Neglected Tropical Diseases. Bausch is an expert in infectious tropical diseases who has been part of the response to the current outbreak, treating patients and training medical staff.

Doctors and scientists know precious little about Ebola virus: how it travels, how it kills, why it emerges when it does. Ebola is suspected to circulate among fruit bats, but nobody has yet isolated the virus in a wild bat.

What’s a deadly little virus like you doing in a place like this?

There’s more than one kind of Ebola virus. Analysis of the genome of the virus in the current outbreak shows that it isn’t related to the Tai Forest virus from nearby Cote d’Ivoire, but rather to the Zaire ebolavirus whose stomping grounds are half a continent away.

How did the virus travel so far without any human cases detected in the meantime? Bausch thinks it wasn’t due to a human traveler. Getting to Gueckedou, where the outbreak seems to have started, requires 12 hours of driving just for the last leg of the trip from any of the nearby capital cities, hardly something you’d do if you were suffering from Ebola. I asked Bausch if an asymptomatic carrier could be the culprit in a Typhoid Mary-like situation, but he says that a person’s ability to shed the virus seems to correlate strongly with how severe their symptoms are. “The likelihood of this being introduced by a sick human is very low,” he says, and by an asymptomatic human even lower.

That leaves the bats. Bats do migrate, and if the virus is in regular circulation among bats it may be less severe, letting a sick bat make the trip more easily than a sick person.

When did the virus make its trip? A recent analysis of blood samples collected from the area over the last 18 years (when they were collected from patients suspected of having Lassa fever) shows that years before the current outbreak, the Ebola virus may have been popping up occasionally in humans in this part of Africa.

Why now?

The current outbreak started in December 2013, at the beginning of the dry season; based on previous outbreaks Bausch believes there may be a connection with the weather, but without knowing more about the ecology of the virus, it’s hard to say if that’s a factor or just a coincidence.

We don’t know how many places in Africa may have Ebola virus circulating in bat populations and occasional Ebola cases in humans that miss diagnosis. But the key factor that sustains an outbreak may not be biological at all.

Why is this outbreak so bad?

Bausch traveled to Guinea every year for a decade while investigating other diseases in the area, and writes that every time he traveled from the capital Conakry to the forest region, “the once-paved road was worse, the public services less, the prices higher, the forest thinner.”

That area where the outbreak started, around Guéckédou, is in a pocket of forest where the borders of three countries converge: Liberia, Sierra Leone, and Guinea. They rank 174, 177, and 178, respectively, on the UN’s Human Development Index; in other words, they are three of the poorest countries in the world. Sierra Leone and Liberia were embroiled in civil war until the early 2000s; Guinea has suffered from a devastatingly corrupt government. None of the countries are in a good position to respond quickly and efficiently to a disease outbreak, and health centers in the region are not always equipped with necessities like gloves and clean needles.

Another complication is that the outbreak area covers three countries. In this area, Bausch says, people identify more with their ethnic group than with their country; borders aren’t much of a dividing line in everyday life. If you live in the area you might cross a border to go to the market or attend school, and you may speak your local language rather than the national language of French (Guinea) or English (Liberia and Sierra Leone).

This all adds up to a situation where a health worker in one country can’t just call up the other side to say hey, this patient had contacts on your side of the border, can you follow up? Even when language is not a barrier, there may not be a phone line to deliver that message nor the organization and resources to do the job.

As the virus spreads–it’s now in Nigeria–socio-economic factors like these will likely determine where an outbreak will catch fire and where it will fizzle out. “I think a military analogy is appropriate here,” Bausch says. “How many fronts can you fight on?”

Correction: A previous version of this post stated that Liberia and Sierra Leone were “embroiled in civil war.” This isn’t currently true, and I’ve corrected the statement above. Thanks to Adia Benton for pointing out the error.

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