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.

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,

The parasites are introduced species from elsewhere, and Americans are perhaps not aware of the risk, the authors wrote.