For the past few years, the world has been on the edge of one of the biggest medical triumphs of modern history: Wiping out a horrific parasite from the face of the Earth.
In the early '80s, there were 3.2 million cases of Guinea worm — a 2-foot-long worm that emerges slowly — and excruciatingly — from a blister on the skin.
A massive campaign, led by former President Jimmy Carter, has eradicated the worm from all but four countries. And this year, there have been only seven cases, the Carter Center reports.
"I'd like for the last Guinea worm to die before I do," President Carter told reporters last year.
But a surprising wrinkle has cropped up: The worm has found a new way to hide and thrive.
Back in 2013, something strange started happening in Chad: Dogs were showing up with Guinea worm emerging from their legs. Many dogs. With many, many Guinea worms.
"A 2-year-old female dog had a record 62 Guinea worms emerge from it," says David Molyneux, at the Liverpool School of Medicine, who has worked on stopping Guinea worm for 40 years.
"That's a huge infection in an animal. I mean I've never heard of that before. That's extraordinary," he says.
So many worms in one dog means one thing: "There's an awful lot of Guinea worms out there," Molyneux says.
This year there have been more than 600 dogs infected with Guinea worm, scattered across a large part of Chad, which is nearly twice the size of Texas.
"This is a massive challenge," Molyneux says. "It's going to be a longer haul [to eradication] than we anticipated."
Other scientists agree.
For more than a hundred years, scientists thought they had Guinea worm figured out. They thought the critter needed people to survive and reproduce. So if you got rid of cases in people, the worm would go extinct.
But now it looks like Guinea worm might not need people, says Mark Eberhard, a scientist with the Centers for Disease Control and Prevention who just got back from Chad to assess the dog situation. "It looks like dogs can drive infections, too."
And so to eradicate Guinea worm, the world has to wipe out the worm not only in people, but also in dogs.
And that, right now, is a big challenge, Eberhard says.
To keep dogs from spreading Guinea worm, they have to be tied up for two weeks, while the worm or worms come out of their legs. Otherwise the dogs could contaminate nearby water and spread the parasite to an entire village.
But dogs in Chad aren't like dogs here in the U.S. They're not pets. They don't come inside or sleep on the bed.
Families keep them to protect their crops from baboons and other critters. "They are, in most instances, no more or no less part of the household as are the chickens, the goats and the cattle," Eberhard says. "Many of them roam free, night and day."
So the Carter Center is paying people $20 to tie up infected dogs. The agency is also giving them collars and chains. "People seem to be using them," Eberhard says.
But there's another big problem: No one knows how the dogs are getting infected.
At first they thought dogs were eating infected fish guts. Now they think it might be frogs.
An international team of scientists has started using GPS collars to track the dogs and radio isotope labeling to figure out what food is left on their whiskers.
Right now, Eberhard says, euthanizing the dogs is "off the table."
"That's quite a drastic intervention," he says. "More than 10,000 dogs would need to be culled. That would be the largest culling project for dogs ever."
Despite these huge challenges, the Carter Center is undeterred about eradication.
"This problem in dogs is new and unprecedented," says the Carter Center's Dr. Donald Hopkins, who has led the effort to eradicate Guinea worm. "I'm confident we're going to do it. We've have already seen 17 of 21 countries get rid of Guinea worm, and it's stayed away there."
That confidence comes with some serious credentials: Hopkins was key in helping to eradicate smallpox — the only human disease to be wiped out, so far.
ARI SHAPIRO, HOST:
For the past few years, the world has been on the edge of one of the biggest medical triumphs of modern history. A global effort has nearly wiped out the Guinea worm, a horrific parasite. Where there were millions of cases in the 1980s, there were only seven this year. If those cases dropped to zero, Guinea worm would become only the second human disease ever eradicated. But a surprising wrinkle has cropped up. Guinea worm, it seems, has found a new way to hide and thrive. NPR's Michaeleen Doucleff reports.
MICHAELEEN DOUCLEFF, BYLINE: The last remaining cases are in Africa. If you have Guinea worm, it's excruciatingly painful. The worm can be three feet long, and it slowly emerges from your skin. Sometimes it can take weeks for the worm to come out. David Molyneux is at the Liverpool School of Tropical Medicine. He's been working on eradicating Guinea worm for more than 40 years. He says for decades, things were going swimmingly. Cases had fallen off drastically from millions to just a handful. Then, in 2013...
DAVID MOLYNEUX: Something strange was happening.
DOUCLEFF: In rural parts of Chad, dogs started showing up with worms coming out of their legs - many dogs with many, many worms.
MOLYNEUX: A female dog had a record 62 Guinea worms emerge from it.
DOUCLEFF: Most infections only involve one worm, maybe two or three - but 62?
MOLYNEUX: Now, I mean, that's a huge infection in an animal. I mean, I've never heard of that before. That's extraordinary.
DOUCLEFF: Molyneux says such a huge infection means one thing.
MOLYNEUX: It means there's an awful lot of Guinea worm out there.
DOUCLEFF: An awful lot of Guinea worm spreading across Chad. This year, there have been more than 600 dogs infected, spread over a huge part of the country.
MOLYNEUX: My concern is that it's going to be a longer haul, I think, than we anticipated, and it's a massive challenge.
DOUCLEFF: You see, for more than 100 years, scientists thought they had the Guinea worm figured out. They thought the critter needed people to survive. So if you got rid of it in people, poof, the worm is gone, extinct. But now it looks like Guinea worm can also live in dogs. Dr. Don Hopkins is with the Carter Center, which has led the eradication effort. He says to wipe Guinea worm off the planet...
DON HOPKINS: We have to get rid of this worm in dogs, as well as the few remaining worms in humans before you can safely say that this disease has been eradicated.
DOUCLEFF: And stopping Guinea worm in dogs isn't easy. Hopkins says it's actually harder than with people. To keep dogs from spreading Guinea worm, they have to be tied up for two weeks while the worms come out of their legs. And dogs in Chad aren't like dogs here in the U.S. They're not pets. They don't come inside or sleep on the bed.
HOPKINS: People use them in households to help protect their crops from baboons and things like that.
DOUCLEFF: So they roam free day and night. The Carter Center has started paying people $20 to tie up their infected dogs. But there's another problem - no one knows how the dogs are getting infected. At first, they thought dogs were eating infected fish guts. Now they think it might be frogs. Scientists at the Centers for Disease Control and Prevention have started using GPS collars to track the dogs and radio isotope labeling to figure out what food is left on their whiskers. Despite all of this, Hopkins isn't deterred about eradication.
HOPKINS: I'm confident we're going to do it and we'll see. But we have seen already 17 of these 21 countries get rid of Guinea worm and have it stay away.
DOUCLEFF: And that confidence comes with some serious credentials. Dr. Hopkins knows a thing or two about ending diseases. He was key in eradicating smallpox, so far the only human disease to be wiped out. Michaeleen Doucleff, NPR News. Transcript provided by NPR, Copyright NPR.