"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

May 10, 2024

Dracunculus insignis

The Guinea Worm, Dracunculus medinensis, is an agonising parasite for those who have to endure its wrath. The female worm can grow up to 80 cm long and when it comes time for it to release its offspring, it does so by poking its body partially out of the host's arms or legs, all while causing a fiery pain that forces the host to immerse their limbs into the water, allowing the worm to release its larvae. This parasite has afflicted humans since antiquity, with description of pathologies and treatment associated with the worm dating from ancient Egypt, and depiction of the parasite in a 15th century painting

In the modern era, the Guinea worm has been the subject of an eradication effort by the World Health Organization (WHO) since the 1980s. An obituary was even written about this parasite in 2013. But while this campaign has been largely successful, the effort to completely eradicate the Guinea worm has hit an obstacle in some regions as the worm has taken to using dogs as alternative hosts in place of humans.

Left: Large bundle of Dracunculus insignis in the paws of a river otter (Lontra canadensis), Right: A Dracunculus worm being removed from a river otter (Lontra canadensis)

But aside from the infamous Guinea worm, there are many other species of Dracunculus out there which are found in a wide range of animals, many of which are actually reptiles. Of those, Dracunculus insignis is considered the most important because in addition to parasitising many species of wildlife,
it can also parasitise cats and dogs. The female worm can grow to 30 cm long, and about 300 days after the initial infection, the mature worm - now loaded with larvae - will migrate to the extremities and exit through a lesion, to explosively release a load of baby worms to begin the cycle anew.

This study looked at Dracunculus worms in river otters from North America. The worms the researchers examined came from various sources, including wildlife parasite surveys, as well as dead otters which were obtained from trappers. In addition, they also collected some worms from an otter in Florida that was recovering in a rehabilitation centre after being struck by a car. During its stay in rehab, worms started emerging on their own out of the otter's body. It was just one thing after another for that unlucky otter.

The worms dwelled in swollen abscesses under the skin on the otter's back, and examination of dead otters obtained from trappers revealed that some of the worms were also located in swellings deep in the limb joints or in the otter's paws, particularly D. insignis. In total, the researchers found four different Dracunculus species in the otters - alongside D. insignis, there was also D. lutrae, as well as two other unique lineages of Dracunculus, one of which was first discovered in a Virginia opossum. It seems that otters are just a cornucopia of different Dracunculus species, some of which are currently undiscovered. Just last year, another newly found species of Dracunculus - D. jaguape - was described from neotropical otters (Lontra longicaudis).

Like other Dracunculus, those worms have larvae that stowaway in tiny crustaceans called copepods where they moult and grow. But unlike the Guinea worm which usually infect people when they drink from stagnant water that contains parasitised copepods, in order to get inside otters, the larval Dracunculus would need to take a detour up the food chain into larger aquatic animals such as a fish or amphibians which are on these otters' menu. Incidentally, that is also the suspected route through which the Guinea worm is infecting dogs in places like Chad, because in contrast to how humans drink water, the way that dogs lap water with their tongue means they are unlikely to end up swallowing infected copepods.

While most of the research on Dracunculus have focused specifically on the Guinea worm and its "classical" route of transmission through infected copepods, this has blinded us to the other potential ways that these parasites can circulate in the environment. Understanding how D. insignis and other wildlife-borne Dracunculus complete their life cycles can provide insight into the different ways that these parasites reach their hosts, which in turn can help us better understand how to control the Guinea worm in affected communities.