It was long believed that the Dracunculus medinensis parasite, which causes guinea worm disease in humans, has a fairly simple life cycle:

1. A person drinks water contaminated with copepods, or water fleas, that are infected with the nematode parasite’s larvae.

2. Next, in the human stomach, the larvae are freed and migrate through the body.

3. Once they mate, the fertilized female starts looking for exit points, usually the legs.

4. When she starts to emerge, trying to get back into water, the infected person suffers pain, fevers, and sometimes even permanent damage to affected limbs.

5. If he or she steps into water, be it a pond or a river, the whole process starts over.

The new study, published today in Current Biology by scientists from the University of Exeter, UK, Chad Guinea Worm Eradication Programme and the University of Torino, Italy, paints a more complicated picture, however: the nematode parasite’s life cycle can also include dogs feasting on fish that carry the parasite’s larvae.

Dr David Molyneux

“This study provides the strongest evidence yet that there is an alternative transmission cycle to the classic one,” said David Molyneux, PhD, parasitologist at the Liverpool School of Tropical Medicine, in an interview with Medscape Medical News.

First hints that the long-established version of Dracunculus medinensis’s life cycle might not be strictly correct emerged in 2012, when Chad began signaling dracunculiasis infections in dogs. Experts believe this discovery was most likely caused by increased surveillance and not by a novel host switch — a notion supported by a recent genetic study. In 2020, five African countries reported as many as 1593 animal infections with Dracunculus medinensis, mostly in dogs.

How the dogs were getting infected, however, remained largely unknown — until now. Robbie McDonald, PhD, animal ecologist at the University of Exeter, who led the Current Biology study, and his colleagues tracked close to 300 dogs along the Chari River in Chad with GPS loggers and analyzed stable isotope signatures in the animals’ whiskers to reconstruct their diets. They found out that in the hot-dry season, when local people fish the most, dogs that had greater contribution of fish to their diets were the ones most likely to be infected with guinea worm. For every additional 1% of fish in diet, the relative risk of infection rose by 28%.

Fish have previously been found to be paratenic hosts of the guinea worm — they can transport copepods infected with the larvae of Dracunculus medinensis without being part of the parasite’s life cycle. This, of course, doesn’t preclude the possibility of dogs catching the disease directly by drinking water contaminated with water fleas. “We also found there is evidence in this study of the ‘classical’ pathway. It is not yet possible to say which is the more important,” McDonald said in an interview with Medscape Medical News.

Zoonotic transmission of dracunculiasis is the second, alongside wars and political instability, obstacle to eradication of the guinea worm disease — a goal currently set for 2030. With support of several international organizations and NGOs, governments of endemic countries have been fighting to get rid of dracunculiasis since the 1980s. It’s a story of great success. In 1986, there were an estimated 3.5 million cases of guinea worm disease worldwide. Between January 1 and November 30, 2021, there were a mere 14 cases in just four countries: Chad, Ethiopia, Mali, and South Sudan.

The successful eradication campaigns were based on provision of safe water, education, surveillance systems, isolation of cases, and treatment of waters infected with the parasite with Abate, a potent larvicide. Now, however, it appears that extra measures will be needed, “targeted at reducing dog exposure to fish remains during the seasonal fisheries,” McDonald said.

The last stretch of guinea worm eradication campaign seems to be the hardest, and the 2030 eradication goal seems unlikely to be met. New findings on how dracunculiasis is transmitted will help us fight the disease in the long term but make the short-term objectives more complicated. “This new transmission epidemiology will increase the challenges of eradication,” Molyneux said.

The authors have disclosed no relevant financial relationships.

Curr Biol. Published online December 14, 2021. Full text

Marta Zaraska is a science journalist based in France and the author of “Growing Young: How Friendship, Optimism and Kindness Can Help You Live to 100.”

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