Next year was supposed to be the end of the line for Guinea worm.
The epic, decades-long campaign against the parasite — which humans and animals can contract from drinking water and which, about a year later, emerges as a worm up to 3 feet long from painful lesions on the feet or legs — has been one of the big success stories in modern global health. In the 1980s, more than 3 million people in sub-Saharan Africa and South Asia contracted Guinea worm annually. Last year, that number was down to 28.
Former President Jimmy Carter, who turned 95 this week and whose nonprofit Carter Center has led much of the Guinea worm eradication effort, has said that he would "like for the last Guinea worm to die before I do."
But that goal moved further out of reach this week, when the World Health Organization quietly revealed that it has moved its expected Guinea worm eradication date, which had been 2020, ahead a decade, to 2030. The change was first reported in Nature.
Over the past few years, the eradication effort has faced a series of setbacks. Last year, South Sudan, one of the countries hit hardest by the parasite, declared victory over it. But only a few months later a new outbreak surfaced there.
In 2013, researchers began to notice that in Chad, Guinea worm was proliferating among dogs — including some cases in which a single dog could carry more than 60 worms. The number of known infected dogs in Chad is rising, from a few hundred cases at first to as many as 2,000 this year. The parasite also seems to be spreading among baboons in Ethiopia.
The outlook is also spotty on the human front. Last year, Guinea worm was detected for the first time in a patient in Angola. And longstanding civil conflicts in Mali and Sudan have hampered researchers' efforts to track infections there.
WHO's date change is a response to all these developments, according to Ashok Moloo, a spokesperson for the agency's Department of Control of Neglected Tropical Diseases. But it also hinges on a change in scientists' definition of what it means for Guinea worm to be "eradicated."
That happened in April, at a meeting in Ethiopia of the International Commission for the Certification of Dracunculiasis Eradication. The independent advisory panel, which includes leading epidemiologists from around the world, gauges progress toward Guinea worm eradication. The full conclusions of that meeting aren't public, but at least one consequential decision was reached, according to Joel Breman, an infectious disease researcher and the president of the American Society of Tropical Medicine and Hygiene, who led the meeting.
"We redefined eradication as elimination of transmission in animals as well as in humans," Breman says. "We're not exactly sure when the last dog or other animal will give up their worms. So that means there will be this delay."
Breman says epidemiologists, ecologists, geneticists, veterinarians and other scientists are working overtime to better understand how Guinea worm can spread between animals and people — and the more evidence they collect, the grimmer the outlook. They suspect that infected dogs can spread the parasite when they step through streams that people collect drinking water from; Guinea worm may also spread through fish people eat.
"At points it was suspected, perhaps hoped, that the cycle of infection in animals was separate from that in people," Robbie McDonald, an ecologist at the University of Exeter who was not a member of the WHO advisory commission, told NPR in an email. "It is now clear that dogs are a maintenance host and that the worms in people are the same as in dogs. Therefore, eradication requires elimination in dogs as well as in people and this will take longer to do, and moreover, longer to demonstrate."
This isn't the first time Guinea worm has defied its internationally designated expiration date: The first deadline was 1991 — then 2009, then 2015, then 2020. If and when Guinea worm is eventually eradicated, it would become only the second human disease in history to face its demise. Smallpox was the first, eradicated in 1980.
Jessica Fairley, a tropical disease epidemiologist at Emory University, told NPR in an email that the date change wasn't a big surprise.
"In the realm of disease control, targets are generally getting more realistic (and thus pushed later and later), and so it is with guinea worm," she said. "I am confident that some of the challenges, like conflict areas and the canine reservoir, will be overcome in the next decade. It is better to be realistic and deliberate."
Emily Staub, a spokesperson for the Carter Center, didn't respond to a question about how Carter has reacted to the date change, saying in an email only that "the goal is to end human suffering from Guinea worm disease as quickly as possible."
WHO remains optimistic that Guinea worm will be a thing of the past, someday. "Eradication of dracunculiasis [Guinea worm's scientific name] will happen," Moloo says, "despite emerging challenges."
Will it take a full decade? Breman hopes not.
"What we're doing here is under promising," he says, "and hoping we can overachieve."