Francesca Tomasi received her B.A. from the University of Chicago and currently does tuberculosis research.
A few months ago, we wrote about the eradication of a forgotten plague: Guinea worm disease, an age-old parasitic infection, is finally tantalizingly close to total global elimination. Thirty years ago, there were an estimated 3.5 million cases spanning 21 countries. So far in 2016, there have been 11 human cases. This is an incredible feat and one that suggests quite heavily that we are ready to close the books on Guinea worm disease forever. However, recent findings in the North African country of Chad (as well as a cluster of other nations) seem to be pulling the breaks on total global eradication – at least for a few more years. The problem at hand involves a so-called “spillover,” the adoption of a new host for the disease: dogs.
The Guinea worm parasite, Dracunculus medinensis, has historically been a human disease, exploiting our bodies to carry out its reproductive ages. That’s why a surge in canine cases has left people around the world bewildered. Researchers in Chad are now working to figure out how this species jump occurred. This likely harkens back to the One Health hypothesis, the idea that human, animal, and environmental health are a single, interconnected phenomenon as opposed to three discrete disciplines. Plus a species threatened with extinction is likely to develop new mechanisms of transmission in a last-ditch effort to preserve its spot on planet Earth.
Dogs were first found to harbor Guinea worm infections in 2012, and the numbers have been growing over the last four years. Recent estimates predict over 600 new canine infections in 2016, a number and situation that complicates eradication efforts.
For starters, dogs roam around freely and are indifferent to the public health practices imposed on humans. Clearly, this makes it impossible to track infections without the help of a third party, observant humans who report cases in real-time. It also makes it extremely difficult to keep infected dogs out of water sources, an essential environment for the Dracunculus life cycle. Thus, the persistence of Guinea worms in any nonhuman species facilitates the parasite’s environmental endurance, perpetuating the likelihood of human infection, and diluting chances of complete disease eradication.
The main idea behind the Guinea worm lifecycle is that Dracunculus parasites need water in order to proliferate. Adult females release a slew of tiny larvae in water, and aquatic copepods eat them up. These copepods – little crustaceans – are also extremely small and therefore often make their way unnoticed into people’s unfiltered drinking water. Thus, residents in Mali, Chad, South Sudan, and Ethiopia – the only remaining countries with cases of Guinea worm disease – consume contaminated water. Digestive stomach acids then break up the copepods, allowing larvae to break free and burrow into the abdominal cavity where they reach adulthood and reproductive maturity. At this point, females snake through the body, searching for an exit so they can find water to discharge their budding larvae. Most of the time, they force their way out by creating a blister. This strategy bodes well for the worm, because the burning symptoms that ensue from these painful cuts often tempt patients to seek solace in cool water – exactly what the mother worm, now about one meter long, needs for her babies to blossom.
Another tricky aspect is the fact that the lifecycle described above takes about a year, meaning that any efforts implemented today to thwart the spread of Guinea worm disease in dogs and humans will take at least another year to show any results. For now, however, public health authorities can turn to tried and true techniques to monitor and control infections. Such efforts include cash rewards for individuals who report human and dog cases, incentivizing an active community-level surveillance of the parasite. When infected people are identified, they are quarantined and cared for until the worm has made a complete exit, far away from any communal water source. That way, worms are one by one depleted without having a chance to release even more infectious larvae.
But what can you do about a wild dog with Guinea worm disease? Fortunately, the worm exit process seems to take less time in canines than in humans. Unfortunately, of course, you can’t ask a dog to sit still for hours or days at a time, so people have been financially incentivized (paid $20) to tether dogs far away from water while the Guinea worm burrows out of their body, a process that takes about two weeks.
Since canine Guinea worm disease was identified in 2012, data have shown promising results using these containment efforts. Approximately 81% of dogs with Guinea worm infections in Chad have so far been contained. Of course, on the other hand, 81 percent is not 100, meaning 19% of infected dogs (roughly one hundred of them) are still running around free, likely frolicking in fields and jumping into streams. As long as someone with Guinea worm disease deposits larvae into water, the parasite cannot be eradicated.
Let’s revisit the question that started it all – namely, why are dogs suddenly getting infected with Guinea worm? Scientists are working to answer this using tracking devices to log what the average canine in an affected region is eating. Most hypotheses posit some sort of new food source; for instance, dogs have been known to eat amphibians or fish, which might very well harbor infected copepods. In fact, a recently published paper by experts at the CDC describes an instance of Guinea worm larvae isolated from a frog in Chad.
As we have discussed, the approach to Guinea worm disease eradication, as with many infectious diseases, is parallel. Quarantining patients from water sources is one obvious measure that has done wonders in allaying the threat of Guinea worm disease in the majority of water sources across Africa. Assisted worm removal ensures proper disposal of an adult worm. Certain drugs have also been used to directly target the parasites. Now, the observations of novel Dracunculus host species call for an aggressive increase in one more particular measure: insecticides. Ultimately, most scientists can agree that copepods are the comestibles for all individuals involved (humans, frogs, and fish through contaminated water, and most likely dogs through contaminated fish). Targeting these crustaceans, therefore, should make it impossible for any host – existing or burgeoning – to provide Dracunculus medinensis a hospitable growth environment. In the end, what has worked for humans should also work for dogs, and any other potential new hosts for Guinea worms. So while it has been pushed back a few years while scientists race to quell the outbreak in dogs, the end of Guinea worm disease is still very much in sight.