A recent study by the University of California, San Diego, has revealed that over 90% of the most commonly caught freshwater fish in Southern California are infected with parasites that can pose serious health risks to humans. These include two species of trematode worms: Haplorchis pumilio and Centrocestus formosanus. These parasites can cause gastrointestinal issues, fatigue, weight loss, and in severe cases, heart attacks or strokes.

Researchers analyzed 84 fish from seven species such as largemouth bass and bluegill captured at five popular fishing locations in San Diego County. They found that 93% were infected with Haplorchis pumilio, with some fish harboring thousands of parasites. Centrocestus formosanus was detected in 91% of fish from two of the five sites.
 

These trematodes were likely introduced to the U.S. through the invasive freshwater snail Melanoides tuberculata, which serves as an intermediate host. This snail has spread across 17 U.S. states and Puerto Rico, facilitating the parasites’ expansion in freshwater ecosystems.

While no confirmed human infections have been reported in the U.S., experts caution this may be due to a lack of awareness and absence of mandatory reporting. The infection risk can be mitigated by thoroughly cooking the fish or freezing it for at least one week before eating raw, according to FDA guidelines.
 

The researchers recommend adding fish-borne trematode infections to the list of diseases that doctors must report to public health agencies. They plan to share their findings with county health departments in Southern California to raise awareness about this emerging public health threat.

This discovery underscores the need for continuous monitoring and public education around the health risks of consuming freshwater fish especially in areas where invasive parasites are becoming established.