The eradication of Guinea worm, thought to have been within grasp, is now at least several years away because of a major setback in the North African country of Chad, according to global health experts and others.
With human cases tumbling in recent years — only 11 so far in 2016, down from an estimated 3.5 million in 21 countries in 1986 — it had been assumed that the eradication effort was on the verge of consigning Guinea worm to the annals of history.
But the parasite, which can be over a yard long and burrows out of an infected person’s body, appears there to have found a niche in a new host: hundreds of dogs.
It’s not clear why Guinea worm has leapfrogged from people to dogs in Chad. CSI-type sleuthing — including the use of GPS tracking to map the dogs’ movements and whisker analysis to determine what they are eating — is being used to try to decipher how canine infection is occurring.
Most Guinea worm experts believe eradication of the disease is still achievable. But even the most optimistic among them acknowledge it’s going to take more time than they had expected.
“At least five, 10 years, I would say. Let’s say at least five years,” Dr. Dirk Engels, director of the World Health Organization’s department of neglected tropical diseases, told STAT.
Only two diseases have ever been eradicated — smallpox and rinderpest, an economically devastating disease that plagued cattle and other ruminants.
Like those diseases, Guinea worm has long been a scourge, both for those infected and the communities in which they live.
The emergence of the worm or worms — in the past, people were often infected with multiple worms — is debilitating. Crops can go untended because people are in too much pain to work, a fact that explains why in Mali, Guinea worm is known as the “disease of the empty granary.”
Former President Jimmy Carter, 91, whose foundation has made eradication a top priority, has said he hopes to live to see the worm that has been in his sights for so long wiped out.
Others who shared those hopes are now more pessimistic that eradication will happen in the immediate future.
David Molyneux, a professor at the Liverpool School of Tropical Medicine and a member of the WHO’s International Commission on the Eradication of Guinea Worm, said he was less hopeful than he was just a few years ago.
“The problem is we had an expectation that the endgame would be difficult,” he said. “But we didn’t anticipate it would be this difficult.”
The problem in Chad, first spotted in 2012, has expanded with each passing year. From January to May alone, 498 infected dogs were identified, an increase of 150 percent over the same period in 2015. It’s estimated that in 2016 the number will top 600.
Why do the dogs complicate matters so greatly?
It is hard enough to break the cycle of Guinea worm transmission when there are just infected people involved. But ensuring infected dogs — which roam freely — don’t infest shared water sources of Guinea worm larvae is a far trickier endeavor.
The existence of Guinea worms in any species means they are more likely to remain in the environment — and to ultimately infect humans, although through a circuitous route.
Guinea worms have a complex lifecycle that requires them to make contact with water to produce offspring. The larvae released by adult females are consumed by minute crustaceans called copepods. And people who drink unfiltered water in places where Guinea worms are found — currently only Mali, Chad, South Sudan, and Ethiopia — ingest the infected copepods.
In humans, stomach acids free the larvae from their crustacean carriers, and they burrow through stomach and intestinal walls and mature in the abdominal cavity. Once they are adults, the worms mate.
The males die and the females start to move through tissue seeking a way to exit the body, often through an excruciatingly painful blister that forms on a leg.
The process is slow. It can take weeks for a worm to emerge from the human body. The burning sensation the emerging worm creates often drives those who are infected to submerge the affected area in water — which is just what the worm needs to start the cycle anew.
The whole cycle takes roughly a year, which adds to the complexity of controlling the parasite. If health officials put in place new containment measures, it won’t be clear for another year if or how well they work.
Dr. Donald Hopkins, who has been with the Carter Center’s Guinea Worm Eradication Program from the start, said in the past rare sightings of infected dogs were reported. But health experts were not expecting to see the scale of the problem being observed in Chad.
“We’d gone so long and not seen anything like this number of dogs infected,” he said.
Health officials, he said, are making some progress in containing the spread of the worms in dogs. Small cash rewards offered for reporting a human case of Guinea worm infection have been extended to encourage reporting of dog cases as well.
When an infected person is identified, health authorities try to isolate the person for the duration of the worm’s exit. They are cared for — an assisted extraction can substantially reduce the time it takes for a worm to emerge — but they are kept away from water sources.
People who own dogs that are shedding a Guinea worm are paid the equivalent of $20 to tether the dog while the worm emerges, a faster process in canines than in humans.
The latest monthly update from the Carter Center reported that in Chad, 81 percent of the 498 dogs with Guinea worms identified by the end of May had been contained. But that means about 95 dogs were allowed to roam free, likely depositing Guinea worm larvae back into a water source.
It isn’t clear how dogs are getting infected to begin with — one of the mysteries a team of researchers from University of Exeter in England is trying to solve.
The scientists, who were in Chad earlier this summer, put GPS collars on about 150 dogs to chart their movements. Whisker samples were taken from the dogs; they will be analyzed for clues to the dogs’ diets.
The goal is to figure out what the dogs eat and where they drink, said team leader Robbie McDonald, a zoologist who studies free-ranging animals. Though dogs in Chad have owners, they are allowed to roam and some scavenge for food.
McDonald said the group is analyzing the data they gathered now and expect to share their findings later this year.
Many experts believe that the infection process in dogs may be a little different than what occurs in people. Dogs from affected villages — which rely on fishing from Chad’s Chari River — were observed eating the eviscerated guts of freshly caught fish. Maybe the fish guts were infested with larvae-laced copepods?
The fishermen were urged to bury the guts, and most now appear to do so. But dogs still seem to be getting infected.
Mark Eberhard, a Guinea worm expert from the Centers for Disease Control and Prevention, believes tadpoles and frogs may be harboring infected copepods, serving as a delivery mechanism.
Researchers have shown in the past tadpoles and frogs can be infected with Guinea worm larvae, but that kind of experimental work isn’t proof it actually happens in nature.
But in an article published online in the journal Emerging Infectious Diseases on Thursday, Eberhard and colleagues reported finding Guinea worm larvae in a frog caught in Chad.
Molyneux, who is not convinced there is an intermediate host in the dog infection cycle, said targeting the infected copepods with an effective insecticide has to be central to the response.
“If we aggressively go for controlling the copepods, neither dogs nor humans can be infected and neither can the fish be infected,” he said. He admitted, though, there are potential environmental downsides and, even if there weren’t, the task is a large one.
Molyneux also noted the problem isn’t just in Chad. Smaller numbers of infected dogs have been reported in Mali, Ethiopia, and South Sudan.
“The question is: Is the parasite jumping across hosts as it reaches its extinction?” he said.
“There are question marks about the role of dogs in these other countries emerging. It’s not just in Chad that one has one’s concerns.”
Eberhard is more sanguine.
“There’s nothing about the infection in dogs that’s supernatural. It will respond to the same interventions that have worked on the human side,” he insisted.
“It’s certainly going to add a year or two or three to the timeline. But at this point, I’m not anywhere close to thinking that we’re not going to be successful.”
Is the cash incentive sufficient to encourage people to bring the parasite to the dog?
To be honest, I don’t think so！
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