A Mystery Worm Infection in Vietnam Shows a New Species Can Live in Humans

A Mystery Worm Infection in Vietnam Shows a New Species Can Live in Humans
The man sought treatment for his illness at a hospital in Hanoi. (Photo: Linh Pham, Getty Images)

The mystery over a worm infestation found in a Vietnamese man last year has been solved, though not without adding another creepy wrinkle to the tale. The worms inside the man weren’t the Guinea worm (Dracunculus medinensis), a now-rare parasite on the brink of eradication, as originally thought. Instead, they were a related and unknown species of Dracunculus likely native to the area. Whether this other worm poses any current or future threat to humans is still unclear.

In June 2020, media outlets in Vietnam reported on the strange case of a 23-year-old man who had visited a hospital with abscesses along his limbs and neck. Doctors eventually discovered and pulled out five adult worms, each around 1 to 2 feet long, from the man’s wounds, as well as larvae. The man was given antiparasitics and no further infestations were reported.

At the time, a doctor on the case was quoted as saying that the man seemed to have Guinea worm disease, which would have been an alarming discovery for several reasons. In the 1980s, millions of people throughout Asia and Africa contracted these excruciating and sometimes permanently debilitating infections every year. But decades of gruelling public health work have driven the worm to near extinction save in a few select regions of Africa. (In 2020, only 27 cases were reported in total.)

Despite some setbacks lately, it’s hoped that by 2030, Guinea worm will be the second human pathogen ever fully wiped out, following smallpox. So to find the Guinea worm thousands of miles away from its last known vestiges, and in an area where it’s never been discovered, would have raised some serious concerns.

Outside experts who spoke to Gizmodo at the time were sceptical that this really was Guinea worm. Adam Weiss, director of the Guinea Worm Eradication Program at the Carter Centre, a human rights organisation founded by former U.S. president Jimmy Carter, even mentioned the distinct possibility that the man’s infestation was caused by another species of Dracunculus. And that theory was right on the money.

A Guinea worm being extracted from a child's foot at a containment centre in Savelugu, Ghana in a March 2007 photo. The worm found in the Vietnam case was a related species, possibly native to reptiles. (Photo: Olivier Asselin, AP) A Guinea worm being extracted from a child’s foot at a containment centre in Savelugu, Ghana in a March 2007 photo. The worm found in the Vietnam case was a related species, possibly native to reptiles. (Photo: Olivier Asselin, AP)

Earlier this March, the doctors behind the case published their report in the International Journal of Infectious Diseases. Following the man’s treatment, they sent off samples of the worm to the U.S. Centres for Disease Control and Prevention for genetic testing, which confirmed that it wasn’t D. medinensis. But beyond that, the mystery deepens.

Like many parasites, different members of Dracunculus infest different primary species. D. medinensis is the only known worm to specifically hunger for human hosts, for instance, and there are other species that go after mammals and reptiles. The doctors concluded that the man’s worms bore a closer physical resemblance to reptile-loving worms than to any human and mammal worm.

But only a few Dracunculus species have had their genes studied up close, and the analysed worm didn’t match up to any of these species. It’s possible that the worm could belong to a species already discovered but not genetically sequenced, but for now, its identity is still a question mark.

Another lingering thread left to be pulled at is whether this incident was a one-off event or the first signs of an emerging disease in the area. Dracunculus worms have their preferences, but they do occasionally jump the species barrier — an ability that’s actually stymied the Guinea worm eradication effort in recent years, as the worms have started to infest dogs in certain areas. And there have been suspected cases of other non-Guinea but Dracunculus human infestations sporadically reported in parts of Asia over the years.

So just because this unknown worm may not usually pick on humans, that doesn’t preclude the possibility that it someday could. We also don’t know how the worms even ended up in the man’s body, though the leading theories are that he either drank water contaminated with infected copepods (small crustaceans that are part of the worms’ life cycle) or ate contaminated raw fish.

“The natural hosts and route of exposure of this presumably zoonotic Dracunculus [species] have yet to be resolved and warrant further investigation and surveillance for similar cases in humans and animals in the region,” the authors wrote.

In an accompanying editorial, infectious disease researchers Martin Grobusch and Thomas Hänscheid laid out the numerous unanswered questions and cautioned against being too optimistic that this case was a rare event, at least not without more investigation.

“However, it does not appear to be too far-fetched to propose that we might be dealing with one of nature’s curiosities rather than the advent of a novel emerging helminth pathogenic to humans,” they wrote.