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.

More than 90% of popular freshwater game fish in Southern California contained an introduced parasite capable of infecting humans, according to a new study from researchers at UC San Diego’s Scripps Institution of Oceanography.

Southern Californians have been warned that more than 90 percent of popular game fish have been found to contain invasive, parasitic worms that can infect humans. Two species of the parasitic flatworms known as "trematodes" were found infecting five species of freshwater fish from San Diego County in a study by researchers from University of California,