Parasites linked to fish consumption appearing in Southern California fishing areas

Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.

Parasites linked to fish consumption appearing in Southern California fishing areas

A species of snail that carries trematodes, flatworms that can infect humans, is common in California, according to research from the Scripps Institution of Oceanography at the University of California, San Diego.

In the lakes and reservoirs of Southern California, the researchers also discovered three types of flatworms that are harmful to humans in snails. The flatworms send swimming stages from the snails’ bodies at a specific stage of development. Then some of them burrow in fish tissue. People run the risk of contracting diseases if the fish are caught and consumed either raw or undercooked, with symptoms ranging from gastrointestinal pain to perhaps even brain hemorrhage.

Ryan Hechinger, a biologist at Scripps Oceanography and the study’s principal investigator, explained that the work began with a chance finding made by graduate student Dan Metz at a reservoir in San Diego where fishing is popular. The snails were probably introduced as an invasive species to the United States through the aquarium trade up to 60 years ago. However, the team was baffled by how many they were in the local waters.

The researchers said that the existence of the snail in California wasn’t even really acknowledged, let alone the fact that it was prevalent and carried parasites that might infect humans.

The report was published in the Journal of Infectious Diseases on November 8. The team discovered they were also carrying eight different species of trematodes in addition to the snails. There are three that are known to be infectious, and two of them are acknowledged as significant yet underappreciated dangers to human health in other parts of the world. The remaining five are probably not dangerous to people but were presumably brought over from Asia. Their presence raises the possibility that nearby waterways contain more trematode species. Around the world, the snail is known to harbor up to 35 different trematode species, 11 of which can infect humans.

Hechinger stated that there’s every reason to believe and preliminary evidence to support the trematodes’ presence in the tissue of a variety of Southern California fish, including those that are frequently targeted for fishing. But according to Food and Drug Administration (FDA) recommendations, properly cooking or freezing fish intended to be consumed raw for one week will destroy the trematodes.

The researchers said that there is no need to panic. They said that the most crucial thing is for people to understand that there are parasites in the area and in the fish we consume. They added that we can take the easy precautions to prevent getting sick from them. Additionally, when diagnosing ill patients, doctors might take this parasite infection risk into consideration.

The study was funded by the National Institutes of Health to determine the parasites’ prevalence in Southern California. The research team is currently working to gather evidence of the parasites’ prevalence in edible freshwater fish from Southern California, including largemouth bass, bluegill, and channel catfish.


Daniel C G Metz, Andrew V Turner, Alexandria P Nelson, Ryan F Hechinger, Potential for Emergence of Foodborne Trematodiases Transmitted by an Introduced Snail (Melanoides tuberculata) in California and Elsewhere in the United States, The Journal of Infectious Diseases, 2022;, jiac413,

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