Researchers issue warning after discovering game fish infected with parasite in California: 'These parasites are here in the US'

If you're eating freshwater fish in Southern California, you'll want to take extra precautions since fish have become infected with parasites. 

A study by the University of California, San Diego found that over 90% of game fish studied had been infected with a parasite. 

What's happening?

The study, published in the Journal of Infectious Diseases, found "two species of [parasitic] flatworms called trematodes," which typically cause weight loss, lethargy, or gastrointestinal problems when humans get infected. In rare cases, it can cause heart attacks or strokes. 

Haplorchis pumilio and Centrocestus formosanus were the "two species of parasitic trematodes," identified, per UC San Diego. These parasites are typically found in Southeast Asia, but it is likely that an aquatic invasive snail, known as the Malaysian trumpet snail or red-rimmed melania, brought them to the U.S. over 10 years ago.

The researchers examined 84 different species of fish in 2023 from five fishing sites in San Diego County, including bluegill and largemouth; 93% were infected with the Haplorchis pumilio parasite, and "Centrocestus formosanus, was found at two of the five locations where it occurred in 91% of the fish," per UC San Diego. 

UC San Diego's Scripps Institution of Oceanography ecologist and parasitologist, and senior author of the study, Ryan Hechinger, said, "These parasites are here in the U.S., and they're infecting fish that people are eating." 

Why are these parasites concerning?

These parasites, which are appearing in freshwater fish, serve as an example of why invasive species can be dangerous. While not all non-native species are harmful, those that are can cause damage to the ecosystem and pose health risks, as in this case. 

The Asian tiger mosquito is another invasive species that can carry disease. It entered the U.S. in the mid-1980s, and some harbored the West Nile virus. The mosquito also invaded Europe in 2024, carrying the dengue fever virus. 

Invasive species also wreak havoc on the economy, causing damage to crops, livestock, and fish, costing the U.S. $20 billion a year for cleanup. 

Additionally, they are harmful to the environment because they take resources from native species, sometimes causing them to go extinct. 

How often do you worry about the quality of your drinking water? Never Sometimes Often Always Click your choice to see results and speak your mind.

What's being done about the parasite?

Hechinger said there is "no need to panic," because you can thoroughly cook or freeze your fish "for at least one week" to kill the parasites, if you plan to eat it raw, per Food and Drug Administration guidelines, as reported by UC San Diego. 

There have been no reported cases yet, but researchers want the parasite to be added to the diseases that public health officials are required to report. 

Hechinger said, "This kind of research is so important to identifying new public health threats, and it wouldn't have been possible without NIH funding." 

Invasive species such as this aquatic snail that is spreading parasites are one of the critical climate issues that are vital to understand to protect the public from these threats.

Join our free newsletter for good news and useful tips, and don't miss this cool list of easy ways to help yourself while helping the planet.