KAKALE MASSA, Chad — Martoussia, the celebrity of the moment in this remote fishing village, pants heavily under the awning where he lies chained. Still, he remains calm and sweet-tempered as the crowd presses in.
Children gawk as volunteers in white surgical gloves ease a foot-long Guinea worm from the dog’s leg and American scientists quiz his owner, a fisherman, about how many worms Martoussia has had.
The village chief, Moussa Kaye, 87, is asked the last time one of his people had a worm. “Not since 40 years ago,” he says.
In this arid central African country, the long global struggle to eliminate a horrifying human parasite has encountered a serious setback: dogs. They are being infected with Guinea worms, and no one knows how.
Scientists are desperate to solve the puzzle. If the answer isn’t found soon, or if the worms begin to spread widely into other species — a handful already have been found in cats and even baboons — then 32 years of work to end the scourge may crumble, said Mark L. Eberhard, a parasitologist at the Centers for Disease Control and Prevention.
Once a pathogen runs wild in an animal population, there is little chance it can be wiped out. “An animal reservoir is the kiss of death for eradication,” Dr. Eberhard said.
It has happened before. In the 1930s, the drive to eradicate yellow fever died when scientists realized monkeys carried the virus.
This setback has come just as the decades-long campaign edges tantalizingly close to victory.
In 1986, when the Carter Center — the global health philanthropy in Atlanta founded by President Jimmy Carter — launched the eradication drive, an estimated 3.5 million people in 21 countries had worms.
Last year, only 30 human cases were found: half in Chad and half in Ethiopia.
But six years ago, here on the hot, dusty banks of the Chari River, the worms mysteriously began emerging from dogs. Last year, over 800 Chadean dogs had them.
Dogs cannot infect people directly, but they may carry the worms into ponds from which people drink, which is how humans are normally infected.
“They haven’t caused a big human outbreak yet, knock wood, but that’s my nightmare,” said Ernesto Ruiz-Tiben, who directs the Carter Center’s campaign.
To prevent that, Chad is paying villagers to tether dogs like Martoussia until all their worms wriggle out. The reward is $20 cash, plus a stout chain with two locks. (Dogs chew through ropes or are freed by children who take pity on them.)
The reward is $100 to humans with worms. To generate publicity, the cash is handed out at ceremonies held in the weekly roadside markets where villagers gather to barter meager fish hauls for goods like plastic buckets or quart bottles of gasoline.
At one such ceremony in Dangabol, in southeast Chad, Dr. Hubert Zirimwabagabo, who heads the Carter Center’s work in the country, played a quiz game with the audience, handing out bars of soap as prizes.
Asked what caused worms, one winner shouted, to general laughter: “Drinking bad water — and speaking ill of others.”
Then Dr. Zirimwabagabo asked local officials to present $100 to each of three women who had worms, reported them, and kept them away from drinking water. The officials obliged with grand ceremony, to loud ululations. People here may not see that much cash in a year.
The worms are synonymous with excruciating pain. The disease’s formal name is dracunculiasis: “affliction with little dragons.”
It is an ancient horror. Some scholars think Guinea worms may have been the fiery serpents said in the Bible to have attacked the Israelites in the desert. At one time, they contaminated many Middle East oases.
After the larvae are ingested, they work their way from the intestine to just under the skin, where they mate and grow.
Ultimately, the female exudes acid from her head, creating a painful blister, usually on the leg or foot, but sometimes even in eye sockets or on genitals. When the blister pops, she emerges — a yard-long uterus as thin and translucent as a Thai noodle.
Because the worm must be wound out on a stick, an inch or so a day, some say she inspired the rod of Asclepius, the ancient symbol of medicine: a snake twisted around a stick.
The agony inevitably drives the victim to cooling water, where the female releases her microscopic larvae. To continue the life cycle, they must be consumed by tiny aquatic creatures called copepods.
After decades of backbreaking work, dracunculiasis is one of two human diseases on the brink of eradication. The other is polio, which persists only in Pakistan and Afghanistan. (The only disease eradicated in humans is smallpox.)
Last year, worms were found in humans only in Chad and Ethiopia, and in nine dogs and one cat in Mali.
Ethiopia’s outbreak may have ended: the cases were all in laborers on one farm where the contaminated pond has been treated. Experts hope the Malian animal cases were dead ends.
That leaves Chad, where the worm appears to be making a last stand.
The country is a strange mix of disciplined soldiers and neglected civilians. In the multinational battle against the terrorist group Boko Haram, Chad’s army is a feared force. The gendarmes watching its main highways are alert and ride new motorcycles.
But the highways are badly potholed, though tolls are collected every few miles. Roadside markets are often made of sticks and thatch instead of bricks and tin. Even the tomb of François Tombalbaye, the nation’s first president, is just a car-sized lump of tile ringed by barbed wire — not a statue in sight.
The worm-ridden areas lie at the end of this cascade of neglect.
The Chari River is wide but shallow, flowing sluggishly toward Lake Chad. The larvae need stagnant water, so the worst-affected villages are those beside muddy pools left during the dry season.
Reaching these places means bouncing for hours down sandy riverbeds, or even piling motorcycles into a pirogue, crossing the river and riding them in.
In the village of Tarangara, a large pond was nearly fished out. Recently six men sieving it for an hour with a 100-foot net came up with only two dozen four-inch creatures that elsewhere would be called crappies and catfish.
Seven dogs here had worms last year. Scientists initially assumed they were infected after drinking water.
But human and dog cases are almost never found in the same villages. Also, dogs drink loudly, lapping the water with their tongues, which is thought to scare away the infectious copepods.
(As far back as the 1860s, the first biologist to describe the worm’s life cycle noted that some wolves got infected, Dr. Ruiz-Tiben said.)
So a new theory emerged: experts surmised that fish small enough to eat copepods, like those caught in Tarangara’s pond, were to blame. Villagers gutting them tossed the entrails to their dogs.
The government asked villagers to stop doing that, but dog infections persisted. Scientists aren’t sure whether that is because villagers aren’t cooperating or fish guts were never the problem.
In Kakale Massa, Martoussia’s owner freely admitted tossing him bits. “He likes carp best,” he said.
Here in Tarangara, Bernadette Tamal, 38, had her dog Dounia chained to a post while its worms came out. Like every other dog around here, Dounia was small, thin, sharp-snouted, and mottled brown and white.
As she cleaned okra leaves, Mrs. Tamal said she never gave Dounia entrails and pointed to a hole in the ground covered by a broken gourd.
Dr. Zirimwabagabo checked: It was brimming with fresh fish guts.
But Mrs. Tamal admitted she didn’t know what else Dounia ate as she wandered about. Many villagers said their dogs got mostly leftover boule, the local staple made of pounded millet or sorghum with fish sauce.
Protein analysis of dog whiskers, Dr. Eberhard said, showed that most dogs lived on boule and human feces, along with some extra protein, though it was unclear what.
Scientists have not found worms in fish flesh. “We’ve tested dozens, and only one, a primitive catfish, had L3 in its tissues,” said Dr. Ruiz-Tiben, referring to the infective third stage of the larvae.
So a new suspect has emerged: frogs. Tadpoles eat lots of copepods, which may survive in flesh as they grow.
And worms tend to occur in groups that eat frogs: children who hunt them with slingshots, adults “who aren’t 100 percent mentally or have alcohol problems,” Dr. Ruiz-Tiben said. And dogs — who often accompany children to the local pond.
Because cooking kills worm larvae, Dr. Ruiz-Tiben suspects the children and impaired adults are eating the frogs raw or barely cooked over grass fires — and then tossing bits to the dogs.
So Dr. Eberhard and Christopher A. Cleveland, a parasitology student from the University of Georgia, spent 10 days here recently asking people about frogs. Mr. Cleveland has found similar worms in American raccoons and otters, both of which eat frogs.
Some Chadeans insist that neither they nor their dogs ever eat frogs.
“By tradition, we don’t,” said Alfonso Mende, 24, a guinea-worm worker in a village called Marabodokouya I, which is inhabited by the Sara-Kaba tribe. “They are poison — we will die.”
That’s a common misconception, Mr. Cleveland said. The local toads are poisonous, but some of the bullfrogs are edible and can weigh up to three pounds, yielding lots of protein. Other tribes, including the Massas in Kakale Massa, happily eat them, he said.
Recently the Carter Center’s office in Sarh, a provincial capital in southern Chad, filled with an aquatic pungency as Dr. Eberhard and Mr. Cleveland gutted and skinned 13 frogs.
They ran the carcasses through a meat grinder, strained their flesh through mosquito netting, then used microscopes to scan the exudate for larvae, which they sucked into tiny tubes to carry to Atlanta for DNA testing. (Many larvae look alike, so Guinea worm larvae must be confirmed genetically.)
There are an estimated 60,000 dogs in the 1,800 villages that the Carter Center considers susceptible, and what to do about them has been heavily debated.
A deworming medicine tested on many dogs here is not working. Dogs drink anywhere, so boring more wells will not help. (Chad has a national well-drilling program, but it often misses remote villages.)
Killing all the dogs has been discussed but, thus far, rejected.
(Culling poultry to stop bird flu is routine. In 2001, Britain killed six million cows and sheep to stop a foot-and-mouth disease outbreak.)
But people are obviously attached to their dogs, and here they are needed for hunting and to protect huts against thieves, crops against baboons, and livestock against hyenas.
Culling “would be a big challenge,” said Dr. Philippe Tchindebet Ouakou, Chad’s national coordinator for Guinea worm eradication. “I would put emphasis on education for behavior change instead.”
It could also cause an outcry, the Center fears.
“In the end, it’s the government’s program and we can’t stop them,” Dr. Ruiz-Tiben said. “But the Carter Center and the donors want nothing to do with dog killing.”
At the same time, from the former president on down, the center is frustrated that eradication is taking so long. It is now overtly using shame as a tactic.
Its updates include cartoons of an international foot race. In the latest, runners representing South Sudan and Mali have broken the tape, while Ethiopia and Chad are straggling.
“They need to put more skin in the game,” Dr. Ruiz-Tiben said. He described Chad’s President Idriss Déby, who took power in a 1990 coup, as “passive” about the effort.
“He says the right things on the phone to President Carter, but doesn’t visit the villages to talk to people and make the local governors do more. They’re happy for us to do the work, but they don’t contribute money.”
Bureaucrats are paralyzed by fear of losing their jobs, and simple tasks like license plates for the center’s vehicles take months.
Local officials say they are aware that the world is watching.
“Yes, I feel pressured,” said Youssouf Mbodou Mbami, the newly appointed governor of Moyen-Chari province, where many of Chad’s worm cases are. “I might not have chosen this job because this problem persists, but I won’t hesitate to help.”
Dr. Ouakou said the government is keenly aware that national pride is on the line. But “since the dogs and even cats have entered the dance, it is difficult to predict when we will have mastered the situation,” he said.
President Déby wants to finally end the plague, Dr. Ouakou said — but has not, he conceded, given him a deadline.
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