Nearly every single freshwater fish in Southern California may be riddled with parasites that could cause serious health issues in humans.

However, the scientists who found the parasites said the threat could endanger millions throughout the US, after finding dozens of popular YouTube videos that were promoting misinformation about raw fish.

Researchers from the University of California-San Diego revealed that a staggering 93 percent of the 84 freshwater game fish they examined carried two species of trematodes - parasitic worms that infect people who eat raw or undercooked fish.

These freshwater species included bluegills, largemouth bass, green sunfish, bluegill-green sunfish hybrids, redear sunfish, black crappies, and the common carp. 

The two species of flatworms discovered (Haplorchis pumilio and Centrocestus formosanus) typically cause stomach problems, weight loss, and fatigue in people.

However, the research team warned that particularly severe infections can lead to heart attacks of strokes.

The researchers added that Americans don't often think about freshwater fish having parasites, but the outbreak has already been found in Texas, Florida, and Utah as well.

Ryan Hechinger, an ecologist at UC San Diego's Scripps Institution of Oceanography said: 'These parasites are here in the US, and they're infecting fish that people are eating.'

The study authors fear that millions of people could be at risk of contamination, based on the amount of interest in eating raw fish on social media and platforms like YouTube.

During their study, published in the Journal of Infectious Diseases, Hechinger's team also looked at 125 popular YouTube videos which featured people eating raw freshwater fish.

Nearly two-thirds (65 percent) did not mention the proper cooking or freezing methods for these fish, which the team believes is promoting bad habits and will likely lead to the spread of dangerous parasites among seafood lovers.

According to the study authors, many videos also spread false information, like claiming that marinating fish in citrus juice or choosing 'healthy-looking' fish eliminates the risk of parasites, which isn't true.

The fish are typically found in warm, shallow waters with vegetation nearby. However, this has also become a thriving habitat for the invasive snail Melanoides tuberculata, which carries the trematodes spreading to local fish. 

Freshwater game fish include multiple species that live in lakes, rivers, or reservoirs. They're popular targets for recreational fishing, often because anglers consider them fun to catch or good to eat.

Fully cooking any fish caught in these locations will eliminate the parasites, according to guidelines from the US Food and Drug Administration (FDA).

For those who eat raw fish, in items like sushi, they need to freeze their catch for at least a week to kill the trematodes.

The parasites were mostly in the fins and gills, though H. pumilio was often found in the muscle tissue near fin bases, which could end up in fish fillets. 

Overall, researchers found 78 of the 84 fish they examined are carrying the parasitic worms.

Other popular freshwater game fish that could be at risk from the snail's parasites include catfish, trout, salmon, and perch.

While there hasn't been widespread outbreak of trematode infections reported in California or elsewhere, Hechinger said that doesn't mean many people aren't getting sick.

'Nobody is looking for cases and doctors aren't required to report them,' he explained.

Hechinger added that his team believes fish-borne trematode infections should be added to the list of diseases doctors have to report to public health officials. 

The FDA explains that improper cooking of fish means it hasn't reached an internal temperature of at least 145°F.

For those freezing their fish to eat it raw, it needs to reach a temperature of -4°F for seven days or be flash-frozen at -31°F for at least 15 hours.

When fish is improperly cooked or frozen, the trematodes can survive and then infect the human body when eaten.

Once swallowed, these parasites enter your stomach and not even the stomach's acids can kill the larvae, which are protected by a cyst-like covering.

In the small intestine, digestive enzymes break down this covering and release the young worms into the gastrointestinal tract.

Over the next few days to weeks, the juvenile worms grow into adults, which are about one to two millimeters long.

As the worms multiply and attach themselves to the small intestine, they irritate the intestinal lining, leading to symptoms like stomach pain, diarrhea, nausea, and indigestion.

After eating infected fish, most symptoms of H. pumilio or C. formosanus typically start within one to two weeks. If left untreated, the more severe symptoms of a chronic infection could start to show one to three months after eating the fish.

Once doctors diagnose someone with an infection, the standard treatment is an anti-parasitic medication called Praziquantel. The infections typically clear up within days after taking the drug.