Two species of fish-borne parasites have been found in freshwater fish routinely caught for consumption. Researchers say people could be eating the fish in ways that are conducive to transmission.

The analysis of freshwater fish collected at five fishing sites in San Diego County, CA, was published in the Journal of Infectious Diseases and showed that two species of trematodes, a type of flatworm, were prevalent in seven fish species. The infected fish species are found across the United States.

One of the trematode varieties was found to have infected fish from all testing sites with 93 percent of the fish tested being infected. The other type of parasite was found in fish at two of the testing sites and had an overall prevalence of 89 percent.  

Researchers with the Scripps Institution of Oceanography at the University of California-San Diego discovered that fish species found to be infected with the parasites included largemouth bass, bluegill, black crappie and common carp.

An invasive snail has been found to be a host for the parasites, which cause human illness. Trematodes have historically infected people in Southeast Asia and the Western Pacific and local transmission wasn’t a concern in the United States until the arrival of the freshwater snail that serves as a host. The snail is now found in at least 17 states and Puerto Rico. Previous research by the same team found that the snail and the two trematodes have become widespread at freshwater fishing sites in California. 

“There is a plausible risk of transmission of H pumilio and C formosanus anywher people eat freshwater fish from wher the M tuberculata snail is present,” the researchers wrote.

In the initial stage of their lifecycle, trematodes infect the snails, then exit the infected snails and move on to a second intermediate host — fish. The adult trematodes then infect hosts, like birds or people, that eat the infected fish. If ingested by people, trematodes infect the small intestine, causing gastrointestinal symptoms that range from mild diarrhea to weight loss and lethargy.

Risky consumption
The research team reviewed 125 videos posted on YouTube from 2008 through 2024 that depicted the catch and consumption of the type of freshwater fish known to host H pumilio and C formosanus. They found that 65 percent depicted the consumption of raw, unfrozen fish, which could facilitate transmission and infection. The videos were viewed more than 5 million times.

The study authors say public health officials should consider steps to increase awareness of trematode infection and the measures that people can take to reduce the risk, like properly cooking or freezing fish before consumption, which kills the parasites.

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