The Guinea worm (Dracunculus medinensis) is one of humankind’s oldest foes. The parasite is transmitted by ingesting worm larvae in contaminated drinking water. The worms may have been the “fiery serpents” mentioned in the Bible, and evidence of the parasite has even been found in Egyptian mummies.
After ingestion, the larvae burrow through the stomach and into the abdominal cavity. There they mate, after which the females migrate toward the skin’s surface, exiting through a painful, burning eruption. Sufferers attempt to lessen the pain by cooling the infected region in a pool of water—often a common water source for the local community—which inadvertently continues the worm’s life cycle.
But soon the Guinea worm’s reign of human suffering may finally be at an end. The parasite is poised to become only the second human pathogen eradicated. (The first was smallpox, declared eradicated in 1980.) In the past 30 years, cases of Guinea worm have fallen from approximately 3.5 million infections per year to a mere 19 cases in three countries in 2016. This dramatic decline is due largely to efforts by the Carter Center, which began their efforts to control Dracunculus in 1986.
Despite all of the progress, there’s one last serious hurdle to overcome: other animals spreading Guinea worm. Recent work has shown that dogs can be infected with Guinea worm, and may be contributing to the maintenance of the worm in nature and its spread to humans in Chad, where more than half of 2016’s cases have occurred (11 cases in all, with an additional three in Ethiopia and five in South Sudan). Two new papers suggest additional animal carriers—frogs and fish—may also be adding to the problem.
Reducing Guinea worm infections has so far focused on preventing the transmission cycle by eliminating consumption of contaminated water. To do this, scientists have used a multi-pronged attack: They educate people about how Guinea worm is transmitted; instruct them not to enter bodies of water if they have a worm emerging from their body; and provide filters for drinking water to keep copepods (and thus the Guinea worm larvae) from being ingested. Teams have also provided clean water sources in some cases, such as new wells, and treated water sources with larvicide to kill any existing worm larvae.
The strategy requires close surveillance of affected areas to determine if there are new Guinea worm cases. This approach has been extremely successful but was stymied in Chad by the finding of dogs infected with Guinea worm. Infected dogs could release the worms into water sources, re-contaminating the drinking water, and starting the cycle in humans all over again. And according to Donald Hopkins, special advisor for Guinea worm eradication with the Carter Center, people may have also become infected directly by eating undercooked fish or frogs which were harboring Guinea worm larvae. The worm could then carry out its life cycle within humans, much as if they were ingested with water. These reservoirs also worsen ongoing infections in dogs, which re-establish Guinea worms in the water supplies. Dogs could also eat infected fish or frogs directly from water sources, or ingest entrails from fish discarded by fishermen after gutting.
Several interventions in Chad have been established to prevent Guinea worm transmission. Parasitologist Mark Eberhard, a co-author on the two new studies identifying Guinea worm infections in frogs and fish, tells mental_floss that teams in the country implemented new health messages recently, telling individuals “to cook your food well and to bury entrails, and not let dogs eat fish or other animal viscera. However, such attempts to change behavior are not only difficult but take some time to penetrate fully at the community level.”
Hopkins tells mental_floss that another step they’re taking is educating communities about dog infections. They’re working to stop that cycle by advising people to take two important steps: in addition to burying entrails to make sure dogs can’t get to them, they suggest tethering dogs that have an emergent worm so they can’t contaminate the water. Both tactics appear successful, according to Hopkins. “We can inspect a sample of communities in endemic areas to see whether or not they’re burying fish entrails,” he says. “The program has shown them to dig deep holes and put covers on them, so these dogs cannot dig these fish entrails up. That’s doing very well. Our samples have been showing now for more than a year and a half that more than 80 percent of individual samples in these communities are burying these entrails.”
Similarly, education on dog Guinea worms is improving. “We’re working with communities that, as soon as they see a worm coming out of a dog, they tether the dog. The country has also offered reward equivalent to $20 USD for reporting infected dogs to the program and tethering infected dogs,” Hopkins says. The reward covers the cost of feeding the dog other food besides the possibly infected leftover entrails. “The latest figures on dogs so far this year show that 77 percent of those dogs have been tethered.”
In previous countries where Guinea worm has been eliminated, once human infections had ceased, a three-year monitoring period was enacted to be sure the worm was truly gone. Eberhard notes that “no country certified has had reinfection.” With Chad, that waiting period will have to see not only no new infections in humans, but also in dogs. Hopkins says the program had previously seen sporadic infections in dogs “in Mali, Ghana, India, Pakistan, and a few other countries, but the infections in dogs died out once transmission in humans was stopped. In Chad, the difference is that we have more dogs than people infected.” Still, Hopkins anticipates that the interventions put into place will work to break that cycle and finally end the parasite’s presence in Chad.
The final stretch of this eradication campaign may be one of the most trying. Like polio—another infection nearing eradication, which re-emerged in Nigeria on the Chad border in 2016 after a two-year absence—careful surveillance and close contact with the affected populations are key to finding new cases. We are close to the end, but the final push will require vigilance and quick responses to stop any new infections from spreading.
November 30, 2016 – 2:30pm