Tracking a Serial Killer: Could Ebola Mutate to Become More Deadly?

Why we need to terminate Ebola 2014 before the virus learns too much about us.

Forty years ago, Ebola was just the name of a river. It was a small waterway of no particularly sinister character that flowed through northern Zaire, not far from the village hospital where the first known outbreak of a new viral disease had been centered. That river gave its name to the new virus, and now "Ebola" is a global byword for ugly death, misery, and fear of contagion.

The 2014 epidemic of Ebola virus disease in West Africa is unprecedented in scope, and much attention has been focused, rightly, on how it has gotten so badly out of control.

Behind that question are three others, less obvious, more complicated, and crucial to seeing Ebola in a broader context: Where did the virus come from? Where is it going? What's next? We do well to consider these questions even as we react to the daily headlines, urge our leaders to take more deeply committed action, and support the organizations (such as Doctors Without Borders) that are fighting the epidemic so courageously in West Africa.

Where Did It Come From?

The outbreak began in early December, in a village called Meliandou, southeastern Guinea, not far from the borders with both Liberia and Sierra Leone. The first known case was a two-year-old child who died, after fever and vomiting and passing black stool, on December 6. The child's mother died a week later, then a sister and a grandmother, all with symptoms that included fever, vomiting, and diarrhea. Then, by way of caregiving visits or attendance at funerals, the outbreak spread to other villages.

It wasn't until March, three months later, that local officials alerted the Guinean Ministry of Health about these clusters of a strange, lethal disease in the countryside. By then, human-to-human transmission had started to multiply the case count. But tracing linked cases raises the question of ultimate origin. How did that first child get sick?

Ebola virus is a zoonosis, meaning an animal infection transmissible to humans. The animal in which a zoonosis lives its customary existence, discreetly, over the long term, and without causing symptoms, is called a reservoir host. The reservoir host of Ebola virus is still unknown—even after 38 years of efforts to identify it, since the original 1976 outbreak—although one or more kinds of fruit bat, including the hammer-headed bat, are suspects. There are hammer-headed bats in southeastern Guinea. It's possible that somebody killed one for food and brought it to Meliandou, where the child became infected either by direct contact with the bat or by virus passed on the hands of an adult.

Why are these facts and suppositions significant? Because they remind us that Ebola virus abides endemically in the forests of equatorial Africa. It will never be eradicated as long as those forests exist, unless the reservoir host itself is eradicated (not recommended) or cured of the viral infection (not likely possible). The virus may retire into its hiding place for years at a time, but eventually it will return, as a result of some disruptive contact by humans with the reservoir host. Then it will spill over into us again. All thinking and planning about how to defend against Ebola virus disease in the future needs to take account of that reality.

Another puzzling fact about origins is that the West Africa epidemic involves a species of ebolavirus (that's the label for the group, which includes five species) previously known only from outbreaks in the Democratic Republic of the Congo and its close neighbors.

A different species has emerged in Ivory Coast, another West African country, just east of Guinea and Liberia. According to a study published in Science in late August by Stephen K. Gire of Harvard and a long list of co-authors, the virus in West Africa seems to have diverged from its lineage in Central Africa just within the past decade. It somehow leapfrogged over or around the Ivory Coast ebolavirus in order to situate itself in southeastern Guinea. That suggests the unnerving prospect that the Central African ebolavirus (the only one strictly known as Ebola virus) is expanding its range, either by infecting new populations of reservoir hosts or by migrations of those host animals.

One way or another, it has been on the move.

Where Is It Going?

The virus has also traveled within living human bodies. We know that it went from Liberia to Dallas within the late Thomas Eric Duncan, from Liberia to Nigeria by way of the late Patrick Sawyer, and from Sierra Leone to Spain by way of two Spanish missionary priests, both also now deceased, who were evacuated for treatment.

And it has been carried to Omaha, Atlanta, London, Paris, Hamburg, Frankfurt, and Oslo within infected people, mostly health and aid workers brought home to be treated.

But just as worrisome as the virus's geographic spread is its journey across the evolutionary landscape. Is it mutating in ways that could make it more dangerous to humans? Is there any chance that it might become transmissible through the air, like the flu, the SARS virus, or a common cold?

Although Ebola becoming airborne is the ultimate disease nightmare, that seems to be almost vanishingly improbable, for reasons well put in a recent article in the Washington Post by Laurie Garrett, a senior fellow for global health at the Council on Foreign Relations. What is now a fluid-borne virus attaching itself to cells lining the circulatory system can't easily change into one that targets the tiny air sacs in the lungs.

"That's a genetic leap in the realm of science fiction," Garrett wrote.

The virus probably will not go airborne, but it could conceivably increase its Darwinian fitness in other ways, becoming more subtle and elusive.

The genetic study by Gire and his colleagues (five of whom were dead of Ebola by the time their study appeared) found 341 mutations as of late August, some of which are significant enough to change the bug's functional identity. The higher the case count in West Africa goes, the more chances for further mutations, and therefore the greater possibility that the virus might adapt somehow to become more transmissible-perhaps by becoming less pathogenic, sickening or killing its victims more slowly and thereby leaving them more time to infect others.

That's why, the Gire group wrote, we need to stop this thing everywhere as soon as possible. Future spillovers of Ebola are bound to occur, but those freshly emerged strains of the virus, direct from the reservoir host, won't contain any adaptive mutations that the West Africa strain is acquiring now.

We need to terminate Ebola 2014 before the virus learns too much about us.

What's Next?

No one knows, of course, how much worse the epidemic in West Africa will get. The U.S. Centers for Disease Control and Prevention issued a report, in late September, projecting that under the worst-case scenario there could be 1.4 million cases by early next year. The World Health Organization said Tuesday that new cases could rise to 10,000 per week by December, ten times the rate of the previous month. And the World Bank has warned that costs of the epidemic could reach $32.6 billion, which would be an economic catastrophe for the three West African countries that would compound their health catastrophes.

Will the epidemic spread more widely, igniting outbreaks in other parts of the world? We hope not. Will it turn up as additional cases, here and there, among people who have traveled from West Africa unaware, as Thomas Eric Duncan was reportedly unaware, that they were infected before boarding the airplane? Probably.

What's the best way to limit such occurrences? Rigorous screening at airports, quarantine for travelers who test positive, travel restrictions, or perhaps total bans on commercial flights arriving from Liberia, Guinea, and Sierra Leone-these measures should help. The most important and effective thing we can do, though, is to provide all possible assistance toward ending the outbreak where it began, in West Africa.

The world won't be free of Ebola 2014 until West Africa is free of it. Even severe restrictions, barring entry to anyone traveling from West Africa, would not make it impossible for the virus to get into America, or Europe, or wherever. To understand why, consider what I call the Nairobi Tabletop Scenario.

Imagine a doctor who departs from Monrovia, the capital of Liberia, feeling fine, on a flight to Nairobi, Kenya's capital, in East Africa. In transit he begins suffering a headache-nothing terrible yet, just discomfort, but it's the first hint of Ebola. At the Nairobi airport, in a café, the Liberian doctor coughs onto a table. Five minutes later, an American businessman touches that table. He rubs his eye. He departs to Singapore and spends three days there, in good health, discussing finance for his project in Kenya. Then he flies home to Los Angeles. To the screeners at LAX, he is an American businessman arriving from Singapore, with no history of recent travel in West Africa. But he's now infected with Ebola, carrying it into the United States.

How do you defend against the Nairobi Tabletop Scenario? By doing everything possible to end the epidemic in West Africa, and thereby to ensure that the Liberian doctor is healthy when he visits Nairobi.

Our safety against the menace of killer viruses can never be an absolute safety. There are too many of them, lurking within reservoir hosts amid distant forests or closer to home-viruses such as Nipah in Bangladesh, Marburg in Uganda, Lassa in West Africa, Sin Nombre virus in the American West, all the new influenzas coming out of southeastern Asia, plus many others that haven't yet been identified and named.

And there are too many of us humans, sharing the landscape with the reservoir hosts and with one another. We are too interconnected by air travel and transport. Viruses are simple organisms but well-adapted to the modern world. This year it's Ebola, devastating and scary. Next year it will be something else.

David Quammen is a contributing writer for National Geographic and author of Spillover and The Song of the Dodo, among other works. His new book Ebola: The Natural and Human History of a Deadly Virus will be published by W.W. Norton next week.

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