Eating freshwater fish raw can be risky and lead to some unpleasant side effects. There are a few main concerns with consuming raw freshwater fish, including parasites, bacteria, toxins, and nutritional differences compared to cooked fish.
Parasites
One of the biggest risks of eating freshwater fish raw is parasitic infection. Many parasites can be found in freshwater fish like trout, salmon, and tilapia. Common fish parasites include:
- Roundworms – Roundworm larvae can embed in fish tissue. Eating infected raw fish can lead to parasitic infection.
- Tapeworms – Tapeworm eggs or larvae may be present in raw freshwater fish. When ingested, the eggs hatch into larvae that can grow into large tapeworms.
- Flukes – Trematodes, a type of fluke worm, can infect freshwater fish. Consuming raw infected fish allows the parasite to move to the human intestinal tract.
These parasites can lead to symptoms like nausea, vomiting, diarrhea, abdominal pain, weight loss, weakness, and headache. In severe cases, freshwater fish parasites can lead to serious complications like digestive obstructions.
Bacteria
Freshwater fish harbor bacteria like Salmonella, Listeria, Vibrio, E. coli, and Clostridium botulinum. When fish are cooked, the high temperatures kill these dangerous bacteria. Eating raw fish leaves these bacteria alive, which allows them to infect your gastrointestinal tract when consumed.
Common symptoms of bacterial food poisoning include:
- Nausea
- Vomiting
- Abdominal cramps
- Diarrhea
- Fever
- Chills
- Headache
In rare cases, severe foodborne illness can lead to risky complications like meningitis, reactive arthritis, kidney failure, and even death.
Toxins
Freshwater fish can contain toxins like mercury, polychlorinated biphenyls (PCBs), and perfluorinated alkylate substances (PFAS):
- Mercury – Exposure to high levels of mercury toxins can damage the brain, kidneys, lungs, and other organs. Cooking helps reduce mercury levels in fish.
- PCBs – These industrial chemicals are carcinogenic toxins that accumulate in fish. PCBs can negatively impact the immune system and cause neurological problems when ingested.
- PFAS – PFAS are industrial pollutants that build up in freshwater fish. They can harm the liver, thyroid, and immune system with high exposure over time.
Toxins can accumulate in the tissues of freshwater fish over time. Cooking helps reduce the amounts present in fish before eating. With raw fish, any toxins present get directly ingested.
Nutritional Difference from Cooked Fish
Cooking fish not only kills bacteria and parasites, but also makes some nutrients more bioavailable for absorption:
- Protein – Cooking denatures fish proteins, making the proteins easier to digest and utilize in the body.
- Omega-3 fatty acids – Fatty acids can be better absorbed from cooked vs. raw fish.
- Vitamin D – Heat helps convert vitamin D to a more bioavailable form.
Other nutrients like vitamins B6, B12, and E are also more absorbable in cooked freshwater fish compared to raw. However, some heat-sensitive nutrients like vitamin C and thiamin are higher when fish is raw.
Risk Factors
Eating raw freshwater fish poses risks for most people. However, some groups are at even higher risk for complications:
- Children
- Older adults
- Pregnant women
- People with weakened immune systems
These populations are more vulnerable to severe bacterial infections, parasitic diseases, and heavy metal poisoning from contaminated freshwater fish.
Fish Preparation Techniques to Reduce Risk
There are some preparation methods that can help reduce (but not eliminate) the risks of raw freshwater fish:
- Frozen for 7+ days – Freezing at -4°F for over a week can kill parasites in fish meat.
- Salted or vinegar cured – Salting and pickling can reduce bacterial growth.
- High-grade sashimi – Sashimi-grade fish is inspected for parasites and frozen to kill bacteria.
However, these methods don’t remove all parasites and bacteria. Chemical pollutants like mercury and PCBs also remain.
Cases of Illness
There are documented cases of people becoming ill from consuming raw freshwater fish infected with parasites or bacteria:
- In 2021, five people got infected with Opisthorchis liver flukes after eating raw trout caught in Italy. They suffered from fever, abdominal pain, and digestive issues.
- In 2008 in Japan, a man had a 55 cm tapeworm removed from his gut after eating raw salmon. He experienced abdominal pain, nausea, and diarrhea.
- In 2020 in China, about 100 people got infected with Gnathostoma roundworms from eating raw or undercooked fish. Symptoms included skin rashes, fever, and stomach pain.
These examples illustrate why proper cooking of freshwater fish is recommended to prevent transmission of parasites and kill any harmful bacteria present.
Population at Highest Risk
Certain populations have an increased risk for complications if they consume raw freshwater fish. These high-risk groups include:
Children
Children are at higher risk for a few reasons:
- Weaker immune system – A child’s immune system is still developing, making them more prone to bacterial infections from raw fish.
- Higher parasite exposure – Kids often play in lakes and rivers, increasing exposure to waterborne parasites.
- Lower body weight – Toxins like mercury have a greater health impact with lower body weight.
Parasites, mercury poisoning, and digestive issues are key health concerns for children eating raw freshwater fish.
Older Adults
Older adults also face higher risks, including:
- Weakened immunity – The immune system weakens with age, making it harder to fight off pathogens from raw fish.
- Underlying conditions – Chronic diseases put seniors at increased complication risks with foodborne illness.
- Toxin sensitivity – The cumulative effects of toxins may be worse due to declining organ function.
Bacterial infections, viruses, parasites, and heavy metal accumulation are particular concerns for older adults eating raw freshwater fish.
Pregnant Women
Pregnant women need to take extra precautions with raw freshwater fish due to:
- Hormonal changes – Hormone fluctuations impact immunity and pathogen response during pregnancy.
- Morning sickness – Nausea and vomiting may increase the risk of dehydration or malnutrition from foodborne illness.
- Fetal development – Toxins like mercury can cross the placenta and negatively affect the developing fetus.
Miscarriage, preterm delivery, birth defects, and developmental disorders are potential risks if the mother contracts an illness from eating contaminated raw fish while pregnant.
Immunocompromised Individuals
Those with weakened immune systems also have heightened risks with raw freshwater fish, including people with:
- HIV/AIDS
- Cancer or cancer treatments
- Autoimmune disorders
- Organ transplants taking immunosuppressant medications
- Malnutrition
Their compromised immunity cannot fight off pathogens as effectively if they ingest contaminated raw fish. They are also prone to severe secondary infections and complications.
Recommended Safe Fish Consumption
To avoid the risks of raw freshwater fish, safe recommended consumption includes:
- Always cook fish to an internal temperature of at least 145°F to kill bacteria, viruses, and parasites.
- Avoid populations at high risk like children, older adults, pregnant women, and immunocompromised individuals from eating raw fish.
- Check local fish advisories for safe recommended amounts based on mercury or other toxin levels in local waterways.
- Freeze fish for at least 7 days at -4°F before preparing raw fish dishes to kill potential parasites.
- Only eat raw fish that has been handled, frozen, transported, and prepared in a safe manner as high-grade sushi or sashimi.
Properly cooking freshwater fish remains the safest preparation method. Consuming contaminated raw fish can lead to mild to severe gastrointestinal, neurological, and other symptoms depending on the pathogen, toxin, or complication.
Conclusion
Eating raw freshwater fish poses a few significant health risks, including infection from parasites, bacteria, and viruses. Toxins like mercury and PCBs can also accumulate in fish tissue. Cooked fish provides more bioavailable protein and nutrients compared to raw. While freezing, salt-curing, or pickling can help reduce risks, cooking fish thoroughly remains the recommended preparation method to prevent foodborne illness, especially for high-risk groups like children, seniors, pregnant women, and immunocompromised individuals.
