Most folks casting lines in Southern California’s freshwater lakes probably aren’t picturing parasites. They’re picturing lunch. A sizzling fillet. Maybe a splash of lemon. What they’re not picturing is thousands of flatworms tucked into their catch, each one a microscopic stowaway with a taste for the human gut.
But according to a recent study published in the Journal of Infectious Diseases, that’s the new reality. Over 90% of common freshwater fish sampled in San Diego County tested positive for trematodes tiny sluglike parasites that can turn a peaceful meal into a medical mystery.
And unless you’re freezing or fully cooking your catch, you’re at risk.
“Americans usually don’t think about parasites in their local fish. It’s just not something we grew up worrying about,” said Ryan Hechinger, senior author of the study and a biologist at Scripps Institution of Oceanography
Imported invasion
These parasites aren’t native troublemakers. They hitched a ride, likely decades ago, through an uninvited guest: the Malaysian trumpet snail (Melanoides tuberculata). Introduced through aquariums or the exotic pet trade, this invasive snail is a well-documented host for parasitic hitchhikers.
Once here, it found California’s warm freshwater lakes, streams, and reservoirs cozy enough to call home.
And it brought company.
The trematodes, Haplorchis pumilio and Centrocestus formosanus originated in Southeast Asia, where infections are more commonly linked to eating raw or undercooked fish, veggies, or crustaceans. Their life cycle is the stuff of biological drama: they start in snails, then infect fish, and eventually end up in birds or humans who eat the fish.
Hechinger and his team have now tracked these parasites and their snail accomplices throughout Southern California, including Los Angeles, Riverside, San Bernardino, San Diego, Imperial, and Orange counties.
Last summer and fall, researchers partnered with the California Department of Fish and Wildlife to analyze fish from five popular fishing spots in San Diego County. They focused on familiar species like largemouth bass and bluegill.
The results? Startling.
- 93% of all fish carried H. pumilio with some harboring thousands of parasites.
- C. formosanus showed up in 91% of fish at two sites.
- The parasites lodged in the fins, gills, and more worryingly, muscle tissues and connective flesh places even a good filet knife might not fully avoid.
This means even filleting off the head, fins, or gills doesn’t guarantee safety. Cross-contamination during prep adds another layer of risk.
Cooking is your best defense
If you’re wondering how to avoid turning your next catch into a GI issue or worse, a few basic rules go a long way:
- Cook thoroughly. Heat kills trematodes.
- Freeze first. If you plan to eat your fish raw (think sushi-style), make sure it’s been frozen properly. That kills most parasites.
- But freezing isn’t foolproof, cooking is the gold standard
Unfortunately, fish prep culture may be part of the problem. In a side study, researchers reviewed 125 YouTube fishing videos with more than 5 million views. Nearly two-thirds didn’t even mention safe prep techniques no freezing, no proper cooking, just a quick clean and onto the plate
The bigger picture
This isn’t just a tale about parasites. It’s about an ecosystem shift. Invasive species are changing what it means to fish locally, and human habits haven’t caught up. Trematodes don’t care about state lines or fishing licenses. They follow the food chain snail to fish to us.
And if we’re not careful, the next “local catch of the day” might come with more than just lemon and dill.
