New study finds dangerous parasites in 93% of Southern California fish

California – A new study has found that more than 90 percent of popular freshwater game fish sampled from Southern California waters are infected with invasive parasitic worms capable of infecting humans, raising concerns among scientists about an emerging and largely unrecognized public health risk in the United States.

The research, published Tuesday in the Journal of Infectious Diseases, reveals that two species of flatworms, or trematodes, have become widespread in Southern California fish populations. These parasites, Haplorchis pumilio and Centrocestus formosanus, are not native to North America. Scientists believe the red-rimmed melania, an invasive freshwater snail from Southeast Asia, introduced the parasites. According to the journal, the snail has spread to at least 17 states and Puerto Rico. These snails serve as a key host in the parasite’s life cycle.

Researchers at the Scripps Institution of Oceanography at UC San Diego examined 84 fish from seven commonly caught species, including largemouth bass and bluegill, collected from five fishing locations across San Diego County. Their findings were striking: 93 percent of the fish carried Haplorchis pumilio. At the same time, Centrocestus formosanus was present in fish at two of the five sites, infecting 91 percent of samples from those locations.

The life cycle of these parasites highlights the risks posed to humans. After infecting the snails, the trematodes move into fish muscle tissue. If those fish are consumed raw or undercooked, the parasites can infect a human or other warm-blooded animal, such as birds. While many infections may result in mild symptoms like stomach discomfort, the parasites are capable of causing serious complications, including weight loss, lethargy, and—in rare cases—stroke or heart attack.

Though the risk is avoidable through proper cooking or freezing procedures, researchers found troubling signs that people do not always take such precautions. The study included a review of 125 popular YouTube videos featuring freshwater fish preparation, which collectively garnered nearly five million views. In 65 percent of the videos, there was no mention of cooking or freezing guidelines to reduce the risk of infection.

Despite the lack of confirmed human infections in the U.S., scientists emphasized that absence of evidence does not equal absence of disease. These parasites are not currently on the list of notifiable conditions that doctors are required to report, meaning potential cases may go unrecognized or misdiagnosed. With many physicians unfamiliar with trematode-related illnesses, symptoms such as gastrointestinal distress could be attributed to other, more common causes.

The study’s authors plan to share their findings with local and state public health agencies and are advocating for trematode infections to be added to the list of reportable diseases. They hope greater awareness will lead to more accurate diagnoses, improved surveillance, and more precise guidance for the public.

While the immediate risk remains relatively low—especially for those who cook their catch thoroughly—the presence of these parasites across a broad swath of California’s freshwater ecosystem signals a new reality. Once considered a concern limited to parts of Asia and Africa, foodborne parasitic infections now warrant closer scrutiny within the United States. As with many ecological disruptions, the presence of these flatworms serves as a reminder of the often unseen consequences of invasive species and the complex ways they can ripple through human health and behavior.