There are a lot of gross parasitic diseases. A prime example is dracunculiasis, otherwise known as Guinea worm (Dracunculus medinensis) infection. People get infected by drinking water or eating undercooked fish contaminated with “water fleas” that are infected with Guinea worm larvae. Over a year or so, the worm grows silently in the body, with females reaching 60-100 cm in length. Fertilized female worms work their way to the skin surface, usually in the lower leg, cause a small blister. There, they expel larvae into water, where they can then infect someone else. The adult worms continue to emerge over the course of a few weeks, a very painful and troubling process for the infected person (see life cycle diagram below).
The image (left) depicts the emergence of a female Guinea worm from the lower leg of a person. Once the worm emerges from the wound, it can only be pulled out a few centimeters each day and wrapped around a small stick, or piece of gauze. Sometimes the worm can be pulled out completely within a few days, but the process often takes weeks. (Image source: CDC Public Health Image Library #8211)
Significant efforts have been underway to eradicate Guinea worm, and they’ve made a lot of progress. Human cases had dropped dramatically, from millions to handfuls. In 2014, only 126 cases were identified (down from 3.5 million in 1986); cases were from Chad, Ethiopia, Mali and South Sudan. Guinea worm was thought to have one key property necessary for successful eradication of disease: a single definitive host species. It was believed that the parasite needed humans to complete its life cycle, so if the disease could be eradicated in people, the parasite would be eradicated completely.
Unfortunately, that doesn’t appear to be the case. Guinea worm is now being found in dogs. Guinea worm infections in dogs were first identified in Chad a couple years ago (Eberhard et al, Am J Trop Med Hyg 2014), and the numbers seem to be increasing. 600 infected dogs have been identified in Chad so far this year, so canine cases have now surpassed human cases, and it’s logical to assume that this represents just a small fraction of the true number of infected dogs. This complicates matters greatly, since it’s now evident that eradicating Guinea worm not only requires elimination of the parasite in people, but also in dogs (and possibly other species). That’s challenging, since keeping infected dogs tied up and away from water sources (which they can contaminate with the parasite larvae) for a couple weeks as the worm makes its way out of the body isn’t easy to do. The Carter Foundation is actually paying people to do just that, which actually seems to be helping.
Whether dogs are a source of Guinea worm outside of Chad, and how much they contribute to human infection needs to be studied. Methods to prevent and treat infections in dogs need to be investigated too. At best, this will complicate and delay eradication of Guinea worm. At worst, it will prevent eradication altogether. Efforts to reduce exposure of people through access to uncontaminated drinking water will still be effective, but the more infected individuals (canine or human), the more surface water contamination there is. Exposure to people via surface water is hard to completely eliminate, so if it remains in dogs, human infections will continue to occur.
This is just one more example of how parasites (like bacteria and viruses) often find ways to confound our thinking.