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To crack Ebola’s code, scientists search for elusive animal host

June 4, 2015 at 6:35 PM EDT
The deadly Ebola virus normally spreads among animals but occasionally spills over to humans, to dire effect. To understand how such diseases make that jump, scientists must find the animal host. But the hunt for live samples of Ebola in animals has never turned up a smoking gun. Science correspondent Miles O’Brien follows epidemiologists in Sierra Leone on their hunt for deadly diseases.

JUDY WOODRUFF: Tonight, we launch a series of reports on the Ebola crisis.

Science correspondent Miles O’Brien traveled to West Africa to examine how the deadly virus took hold.

LINA MOSES, Tulane University: Kenema became the epicenter of the outbreak in July.

MILES O’BRIEN: Lina Moses is back at it, on the trail of a killer virus near Kenema, Sierra Leone. She is still haunted by memories of the worst days of the Ebola epidemic last year.

LINA MOSES: We didn’t go searching for Ebola. Ebola came to us. It came with a vengeance.

MILES O’BRIEN: She took us to the remote villages of Kpalu.

LINA MOSES: This is the first time we have trapped for a while since the Ebola outbreak started.

MILES O’BRIEN: An epidemiologist and disease ecologist with Tulane University’s Viral Hemorrhagic Fever Program, Lina leads a team focused primarily on a virus with Ebola-like symptoms, Lassa fever. Lassa and Ebola are so-called zoonotic diseases caused by viruses, parasites or bacteria that are normally spread among animals, but occasionally spill over to humans, often causing severe disease.

Understanding how these viruses make the jump into humans is at the core of her research.

LINA MOSES: Any time you’re looking at zoonotic disease, you have to start looking at the animal that carries it, the animal that maintains it in nature, and that’s the only way you can start to control it.

MILES O’BRIEN: We are hoping to trap the Lassa virus inside its animal host, or reservoir, a common African rat.

LINA MOSES: We estimate that 80 percent of all Lassa fever cases come directly from rodent to human. So, really, what we’re trying to do is break that rodent-to-human relationship.

MILES O’BRIEN: Lina’s team baits two dozen traps with peanut butter, oats, palm oil and dried fish, and leaves them overnight. And while we all sleep, our night-vision camera catches a curious rat known as Mastomys natalensis, the reservoir species for Lassa fever. This one didn’t take the bait.

But another one does, as we discover on our return the next morning. James Koninga quickly suits up for a field necropsy.

He works inside a roped-off area, a safe distance from the center of the village. He is a seasoned a hand at this dangerous task. There is good reason for the caution. Ebola may get all the media attention, but Lassa fever is nearly as deadly, in this part of Sierra Leone, a fatality rate of nearly 70 percent.

JAMES KONINGA: It’s the most dangerous part of it. I have to be very careful.

MILES O’BRIEN: What is the risk here if the rat has the Lassa fever virus? How dangerous or potentially dangerous is it?

LINA MOSES: Well, it’s similar to — I liken it to health care workers in an Ebola treatment center. They get urinated on. They could get bit if the rodent is not properly anesthetized. So, the risks of actually getting — getting infected from are Lassa just as high.

MILES O’BRIEN: Koninga has been doing this since the 1970s. For the past 20 years, he has helped in the frustrating hunt for the Ebola virus as well.

JAMES KONINGA: We collected up to 3,000 animals, birds, rodents, bats, you name them, even domesticated animals.

MILES O’BRIEN: They even looked at mosquitoes and bedbugs. All that, and there’s never been a smoking gun, never any sign of the live virus.

But there is a prime suspect: fruit bats. That’s because researchers found Ebola antibodies in several bat species, meaning they must have been infected with the virus or some version of the virus. Fruit bats also are host to Marburg virus, in the same family as Ebola. The first person known to die of Ebola in this epidemic, the so-called index case, in December 2013, was a young Guinean boy who played beneath this tree, which was infested with bats.

It has since burned, making it impossible to know for sure if this was where and how he got sick. Researchers know much less about Ebola than Lassa fever.

LINA MOSES: You get hot spots. So you will get villages that have a lot of cases for maybe one or two years, and then they quiet down.

MILES O’BRIEN: If you ever saw Ebola in a rat here, that would be significant, yes.

LINA MOSES: I would be nervous, yes, definitely nervous.

MILES O’BRIEN: Historically, Ebola has been a cold case investigation for epidemiologists. It appeared and vanished, sometimes for years, all in very remote places, until this outbreak.

This time, better roads and deeper human encroachment into the bush sent Ebola on a dangerous journey, into teeming places dense with potential victims. This was the first time Ebola came to cities, and that was a big part of the problem. This is the Kroo Bay slum in Freetown, Sierra Leone. There’s no electricity, no running water, and you really don’t have to use your imagination to understand that Ebola here could quickly spread out of control.

But is it simply demographics that made the outbreak so deadly? Or is it biology as well?

DR. PARDIS SABETI, Broad Institute: There are likely some versions of Ebola that don’t make you sick at all. There are some versions that probably don’t infect humans at all. And there are some that may infect humans, but not spread the way this does.

MILES O’BRIEN: Computational genomicist Pardis Sabeti is a Harvard professor in the vanguard of efforts to better understand Ebola by studying its genetic characteristics.

She led the study that made the genetic link between the index case to the thousands that followed, all from human-to-human contact. It’s a family tree of virus cousins, but with 99 distinct Ebola genomes, hundreds of mutations, making them different.

DR. PARDIS SABETI: They are replicating through each transmission, every eight-day cycle, as a new set of viruses that are going to be able to released to another individual. And so we are just talking about a much faster time period in which evolution can happen. It’s changing, not just between individuals, but within individuals.

It starts replicating and it’s making lots of copies of itself and, every once in a while, will make a mistake and these mutations emerge. And so we do have to pay attention, because we are not always dealing with a single entity. We are dealing with a changing entity.

MILES O’BRIEN: Back in Sierra Leone, Lina Moses is wondering what it will look like next time. While the epidemic was unprecedented, Ebola, as it turns out, wasn’t a new visitor to West Africa. Researchers recently took a fresh look at old blood samples from patients here with previously unknown diseases, and they found Ebola antibodies.

LINA MOSES: So, it looks like this is not the first introduction of Ebola into Sierra Leone and into this region, and it’s not going to be the last.

MILES O’BRIEN: At the government hospital where Lina is based, they are getting ready for the next time, building a U.S. Pentagon funded-research facility and isolation ward. Beside it is the grave of one of Sierra Leone’s leading epidemiologists, who died here last year treating Ebola patients.

LINA MOSES: It was pretty awful, actually. There were — people who really scared. What they call IPC, infection prevention and control, wasn’t in place to any measure that would protect people, and a lot of people got sick.

MILES O’BRIEN: Friends? How many did you lose?

LINA MOSES: The people that I worked with for several years, we lost nine of our staff, most of them very good and personal friends of mine. So…

MILES O’BRIEN: In Freetown, the main cemetery is now filled, too many fresh graves. The crisis may have waned, but the virus lives on, a threat that can’t be easily buried.

Miles O’Brien, the PBS NewsHour, Freetown, Sierra Leone.

GWEN IFILL: Tune in tomorrow night, as Miles explores how health workers are trying to develop a rapid test for Ebola.