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Most Americans' experience with plague is limited to history books. In the 14th century, it famously wiped out half of Europe's population. But right now, the bacteria is quietly ravaging wildlife in parts of the American West.
NPR's Elizabeth Shogren has the story.
(SOUNDBITE OF A PRAIRIE DOG)
ELIZABETH SHOGREN, BYLINE: This is what it sounds like if you walk through a prairie dog town in South Dakota's Badlands National Park.
(SOUNDBITE OF A PRAIRIE DOG)
SHOGREN: A chorus of prairie dogs telling each other to watch out for threats. But just down the road, as Dean Biggins hikes into what use to be a large prairie dog town, all he hears is an occasional call of the meadowlark.
(SOUNDBITE OF A MEADOWLARK)
DEAN BIGGINS: Several years ago, plague moved in with a vengeance and really pretty much wiped out the prairie dog.
SHOGREN: Biggins is a biologist for the U.S. Geological Survey. He first saw plague in action in Meeteetse, Wyoming in 1985, when he watched colony after colony of prairie dogs turn into ghost towns.
BIGGINS: Really a devastating feeling to have watched it and just basically be helpless to do very much about it.
SHOGREN: When plague wipes out a prairie dog town, it dramatically changes the ecosystem. It takes away food for predators like hawks and coyotes, and the intricate system of burrows and underground tunnels created by all those prairie dogs and used by many other critters collapses.
Back then, Biggins says scientists believed plague had a predictable MO.
BIGGINS: Plague would come from this unknown source and invade quickly into a prairie dog town, explode and kill almost all the prairie dogs, and then disappear back into its reservoir form where it existed without damaging anything too much.
SHOGREN: But it was Biggins' work with another animal that gave him clues that scientists were wrong about plague. In the late 1990s, Biggins was running a captive breeding program for endangered black-footed ferrets in Colorado. All of the sudden, ferrets started getting very sick.
BIGGINS: It was horrifying. I mean, absolutely horrifying. We actually did not know it was plague then. We suspected because of the symptoms. They tried to drink water, couldn't drink. They bled from the nose, so they were hemorrhaging inside.
SHOGREN: It turns out keepers had mistakenly fed the ferrets chunks of prairie dog meat infected with plague.
BIGGINS: I think we lost something like 26 ferrets to plague. So it was a real kick in the head to me.
SHOGREN: But it gave Biggins an idea. Maybe plague was the reason why ferrets weren't thriving in the wild. He started looking at the animals in Montana. He did not find a big outbreak killing ferrets. But he did find plague. He found it in coyotes and badgers. Plague doesn't make these animals sick. But when they've been exposed, it shows up in antibodies in their blood.
BIGGINS: If these animals can find something with plague out there, the ferret does the same thing. The ferret is going to die.
SHOGREN: Maybe plague was killing ferrets so fast that there was no trace left for researchers to see. So they decided to vaccinate ferrets to test their hunch.
BIGGINS: Turned out that the vaccinated ferrets survived at a rate of about 240 percent better than non-vaccinated ferrets, so there was our answer.
SHOGREN: They were right. Plague was active. That told Biggins that plague had a different MO. It didn't go on a mad killing spree, mostly in prairie dogs, and then go dormant.
BIGGINS: Now we recognize that the disease is out there killing mainly mammals every year.
SHOGREN: Since then, Biggins has confirmed his theory in field experiments on other small mammals. Plague is killing various kinds of mice and ground squirrels in New Mexico and Mexican wood rats in Colorado.
BIGGINS: The threat is to the ecosystems of the West. I think we could be having basically a Black Death type of episode occurring rather continuously in the United States that we haven't even recognized.
SHOGREN: Some experts say there isn't enough evidence to quantify plague's death toll. There aren't enough scientists studying these small animals in the West. That's because there aren't that many people getting plague, only seven reports each year in the U.S. Few people live where outbreaks occur.
Biologist Travis Livieri contracts with the federal government to capture ferrets and give them checkups. He's watched plague ravage populations of rare black-footed ferrets. Now, when he when he goes to scientific meetings, he warns his colleagues.
TRAVIS LIVIERI: Is everything, quote, unquote, "normal" with your favorite species? If it isn't, consider that plague might be there. It might be affecting your species in ways that you couldn't even imagine.
SHOGREN: In nearby Badlands National Park, biologist Dean Biggins' team is looking for mice. They've checked tens of thousands of traps over five months. But they've caught relatively few mice.
DIANNA KREJSA: We have a closed box.
SHOGREN: Researcher Dianna Krejsa looks into the trap.
KREJSA: It's a false positive, is that what we call it. It's closed but nobody is in there.
SHOGREN: Biggins is pretty sure he knows why this gorgeous habitat is so sterile. The plague that swept through here several years ago, wiping out prairie dogs, is still busy killing.
Elizabeth Shogren, NPR News. Transcript provided by NPR, Copyright NPR.