At a Glance
- A study from UC San Diego found that most freshwater fish in Southern California are infected with parasitic flatworms that can cause serious health issues if consumed raw or undercooked.
- These parasites originate from invasive snails and are transmitted to humans through fish, posing an unrecognized risk to those who regularly eat freshwater catches.
- Researchers discovered that up to 93 percent of sampled fish carried Haplorchis pumilio, with some fish harboring thousands of individual parasites.
- Many popular YouTube videos on raw fish preparation failed to mention the importance of proper cooking or freezing, potentially increasing public exposure to these harmful parasites.
- Although thorough cooking or freezing can kill the parasites, repeated exposure may lead to long-term health consequences, prompting a call for increased awareness and safer food practices.
A new study from researchers at UC San Diego’s Scripps Institution of Oceanography has revealed a surprising health risk from eating freshwater fish in Southern California. The study, published in the Journal of Infectious Diseases, discovered that the most popular freshwater fish in the region carry parasitic flatworms called trematodes. These parasites, which are transmitted through fish, can cause gastrointestinal issues, weight loss, and, in rare cases, heart attacks or strokes in humans.
The parasites, Haplorchis pumilio and Centrocestus formosanus, are carried by an invasive snail species, Melanoides tuberculata, which has spread across 17 U.S. states and Puerto Rico. The trematodes’ life cycle begins with the snails, which infect fish. Then, humans or other animals can become infected by eating the contaminated fish. This discovery is a wake-up call for people who consume freshwater fish, particularly those who eat them raw or undercooked.
The team found that nearly 93% of fish from seven species caught in San Diego County were infected with the H. pumilio parasite, with some individual fish hosting thousands of parasites. The second parasite, C. formosanus, was found in 91% of the fish at certain fishing spots. The researchers also discovered that many YouTube videos featuring raw fish preparation did not mention the importance of cooking or freezing the fish to avoid the parasites, potentially increasing the risk of infection.
The good news is that cooking or freezing the fish kills the parasites, reducing the chances of infection. However, repeated exposure to these parasites could lead to more severe health issues. The researchers hope their findings will raise awareness among the public, medical professionals, and public health officials, urging better fish preparation practices to protect against this under-recognized health risk.
References
- Palmer, E. M., Metz, D. C. G., & Hechinger, R. F. (2025). Further evidence for plausible transmission of fishborne trematodiases in the united states: Game fish carry human-infectious trematodes and are eaten raw. The Journal of Infectious Diseases, jiaf180. https://doi.org/10.1093/infdis/jiaf180
- University of California-San Diego. (2025, June 3). Most freshwater game fish in Southern California carry invasive parasites capable of infecting humans, study finds. Phys.Org; University of California-San Diego. https://phys.org/news/2025-06-freshwater-game-fish-southern-california.html
