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Unseen Dangers in Every Catch
Most Americans view freshwater fishing as a peaceful pastime or a source of local food. But a new study out of Southern California has uncovered a disturbing reality: more than 90% of popular freshwater game fish in the region carry invasive parasites capable of infecting humans.
Published in the Journal of Infectious Diseases, the study by researchers at UC San Diego’s Scripps Institution of Oceanography suggests that a dangerous public health threat has quietly spread throughout U.S. waters—yet remains largely unknown to the public, the medical community, and even health officials.
The primary culprits are two species of flatworm parasites—Haplorchis pumilio and Centrocestus formosanus—which typically cause gastrointestinal distress, fatigue, and weight loss in humans.
In severe or prolonged infections, these parasites have been linked to more serious outcomes, including strokes and heart attacks. The parasites have long been a problem in parts of Southeast Asia, but their presence in American waters reveals a disturbing case of biological invasion and neglect.
From Snail to Fish to You
These parasites arrived in the U.S. by hitching a ride in an uninvited guest: the red-rimmed melania snail, also known as the Malaysian trumpet snail. This invasive snail has now spread to at least 17 states and Puerto Rico. The trematodes complete their life cycle by first infecting the snail, then freshwater fish, and finally a warm-blooded host—such as a bird or human—when the fish is consumed.
Ryan Hechinger, an ecologist and parasitologist who led the study, points out that Americans rarely associate freshwater fish with parasitic infection. “That’s part of the problem,” he says. “We’re not looking for them, we’re not reporting them, and people don’t even realize they’re at risk.”
The risk is real. In San Diego County, the research team examined 84 fish across seven common species—like bluegill and largemouth bass—from five popular fishing spots. They found that 93% were infected with Haplorchis pumilio, and at some sites, Centrocestus formosanus was present in over 90% of fish. Some fish harbored thousands of parasites.
Raw Traditions, Rising Risk
Though these parasites can be killed through proper cooking or by freezing the fish for at least a week (as per FDA guidelines), many Americans are bypassing these safety steps. A companion survey of YouTube videos showed that out of 125 popular fishing and cooking videos—totaling nearly 5 million views—65% failed to mention the need for cooking or freezing the fish. That suggests a widespread trend of eating freshwater fish raw or undercooked, unknowingly increasing the risk of infection.
Emma Palmer, a co-author of the study and marine scientist at the Smithsonian Environmental Research Center, points to these videos as a signal of cultural habits shifting toward risky culinary practices.
“Five million views isn’t just about interest,” she notes. “It reflects behavior. People are watching how others prepare their catch and mimicking that, often without understanding the risks.”
Why Haven’t We Heard About This?
Despite widespread parasite presence in fish and invasive snails, there have been no officially reported human infections in the U.S. But that doesn’t mean infections aren’t occurring—it just means they’re not being tracked.
“There’s no requirement for doctors to report these types of infections,” Hechinger explains. “And most physicians wouldn’t even think to test for trematodes if a patient comes in complaining of digestive issues.”
As a result, infections may be misdiagnosed or go completely undetected. That creates a dangerous feedback loop: without data, public health agencies don’t raise alarms, and without warnings, the public doesn’t change behavior.
The researchers are now calling on public health officials to include trematode infections on the list of reportable diseases. Doing so would help generate real data, improve medical awareness, and offer an early warning system before this silent threat becomes a more visible crisis.
Parasites in the Shadows
This discovery highlights a broader, long-standing issue: America has underestimated the danger posed by parasitic diseases. While often associated with developing nations, parasitic infections affect millions within the U.S.—especially among vulnerable and underserved communities.
Consider Chagas disease, a parasitic illness afflicting up to 300,000 Americans, mostly immigrants from Latin America. Or toxocariasis, an infection spread through contaminated soil or pet feces, which has left many children with permanent vision loss. Even trichomoniasis, a sexually transmitted parasite, disproportionately affects older Black women in the U.S. and is linked to severe reproductive complications.
Infections like cysticercosis, caused by the pork tapeworm Taenia solium, can trigger seizures and hospitalize thousands annually. Yet none of these diseases dominate headlines. Instead, they remain in the margins of the medical system, ignored in public discourse and poorly understood even by clinicians.
A Call to Awareness and Action
The forgotten parasites—both those in freshwater fish and those silently impacting millions of Americans—reveal a blind spot in U.S. public health priorities. Our tendency to dismiss parasites as a third-world issue has left us unprepared for threats that are now homegrown.
But awareness is a powerful tool. Freezing or properly cooking fish is a simple step that can neutralize the risk. Better education campaigns, medical training, and disease reporting protocols can prevent outbreaks and ensure early treatment. And most importantly, understanding the life cycles and transmission routes of these parasites empowers individuals to protect themselves and their communities.
The research from Southern California is a wake-up call. As Hechinger said, “This kind of research is essential for identifying new public health threats.” But it’s only the first step. Now it’s time for policy, medicine, and the public to catch up—before the forgotten parasites make themselves known in ways we can no longer ignore.
