Swamp eels are a delicacy in Asian cuisine and can be exported and purchased live in markets around the world. The eels must be eaten with caution, as they can be infected with nematodes from the genus Gnathostoma, with some pretty hefty consequences. The U.S. Geological Survey recently sampled eels from Asian markets as well as some that were caught in the wild. The study found that over a quarter of the eels purchased at market were infected with the parasite. The full results of the investigation were published in the journal Emerging Infectious Diseases.

So what happens if you ingest a worm? Luckily, the nematodes can only cause problem in humans if they are ingested during advanced third-stage larvae (AL3), which is a relatively short window. On the other hand, if they are eaten during this timeframe, it’s very possible for the host to develop gnathostomiasis, which is not good news. Shortly after the worm is ingested, fever and pain arise as the worms burrow out of the intestines and head toward the subcutaneous tissue, which is just under the skin. There, the worms cause painful swelling and edema which can continue on and off for up to a month. Sometimes, the worms will burrow into organs or central nervous tissue, which can cause meningitis, encephalitis, and other disorders with potentially fatal results. Surgical removal of the worms is the best treatment, though there are some medications available if the worm is located in an inoperable area.

Though the eels (and the worms) are native to Asia, they are legally transported to the U.S. to be sold in markets. After the eels have been purchased live at market, they can either be cut up at the market or at home. However, some of the eels have been introduced into American freshwaters as part of a cultural ceremony. While they haven’t out-competed native fish like other invasive species, they do present the hazard of infecting other fish that share the water, including other species that are more commonly eaten by humans.

Obviously, this is cause for some concern. The USGS purchased eels from five different markets located in Manhattan’s Chinatown, Atlanta, and Orlando. Additional eels were caught and represent 4 out of 5 known introduced swamp eel populations. Among the 67 eels caught in the wild, three of them were infected with multiple AL3 nematodes. The greater worry comes from those purchased in the markets. A total of 47 purchased eels were analyzed, and 13 of them had a total of 36 AL3 worms.

Most of the eels sold in these markets are imported legally from Bangladesh. Though the eels had much higher instances of parasitic infection, gnathostomiasis is relatively rare in humans in that region, though fairly common in dogs. Though the species identified in the study have not been confirmed to cause disease, they are most likely able to. For those who do consume swamp eel, the best way to protect against infection is to make sure that it has been well-cooked. They also recommend that the same precautions are taken when eating freshwater fish, shellfish, poultry, and frogs.

Gnathostoma species identified during the study. Credit: Rebecca A. Cole et al.