The parasites are called trematodes. Two species of the flatworms were discovered in California’s freshwater fish, according to a study published Tuesday in the Journal of Infectious Diseases. The tiny, flattened and sluglike creatures can cause gastrointestinal problems, weight loss and lethargy when a person eats an infected fish.

Researchers at UC San Diego’s Scripps Institute of Oceanography say they found two types of parasitic flat worms, or trematodes, in more than 90% of the region’s freshwater fish.

Nearly all freshwater game fish at popular recreational fishing areas in Southern California have been exposed to invasive parasites with the ability to infect humans, a new study published in the