A Dangerous Parasite Has Quietly Spread in US Freshwater Fish, Study Finds

Freshwater fish enthusiasts should be extra careful the next time they fry up their favorite catch. 

In early June, researchers from UC San Diego's Scripps Institution of Oceanography released a new analysis published in the Journal of Infectious Diseases and funded by the National Institutes of Health (NIH), showing that 90% of freshwater game fish in Southern California contained an “introduced parasite capable of infecting humans.”  

According to the researchers, they identified two species of flatworms called trematodes that can cause gastrointestinal distress in humans, as well as other symptoms like weight loss and lethargy. And in rare cases, they are even capable of causing strokes and heart attacks.

“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” Ryan Hechinger, an ecologist and parasitologist at Scripps and the study’s senior author, shared in a statement. “But these trematodes have now been widely introduced in the U.S., and that means that doctors and the public should be aware.” 

The two parasites are typically found in Southeast Asia, the researchers explained, but they probably migrated to the U.S. around 10 years ago inside the body of the invasive Malaysian trumpet snail, which is now present in 17 states and Puerto Rico. The researchers noted that the parasite typically completes its life cycle in three stages: first in the snail, then in a fish, and finally in a warm-blooded animal, such as a bird or human, after they consume the fish.

The researchers observed that the Haplorchis pumilio parasite was present in 93% of the fish they collected and examined from five popular fishing spots around San Diego County. The fish included largemouth bass and bluegill. Some of the fish, they noted, carried “thousands of the parasites.” The second parasite, Centrocestus formosanus, was detected at two of the five locations and in 91% of the fish.

“These parasites are here in the U.S., and they’re infecting fish that people are eating,” Hechinger added. “We hope this study can help make public health officials, doctors, and the public more aware.”

But the researchers want to make one thing clear: If you cook your fish properly, you should be safe. “Fully cooking fish or freezing any intended to be eaten raw for at least one week should kill the trematodes,” they noted in their release, citing the Food and Drug Administration guidelines.

However, the researchers also conducted a social media survey and found that most Americans who eat freshwater fish probably aren't taking the proper precautions. They analyzed 125 YouTube videos, which had nearly 5 million views combined, about cooking and preparing freshwater fish, and discovered that 65% didn't mention the proper preparations.

“Nearly 5 million views show there is widespread interest and possibly a widespread practice of folks eating freshwater fish raw,” Emma Palmer, a marine scientist at the Smithsonian Environmental Research Center, added. 

The good news is, Hechinger says, “There haven’t been any reported cases of these parasites infecting Americans,” but he added that this may be because “nobody is looking for cases and doctors aren’t required to report them.”

So, the team is suggesting that trematode infection be included in the list of diseases doctors are required to report to public health officials to ensure proper tracking. But for now, just make sure to follow the correct procedures the next time you cook freshwater fish, just in case.