Understanding Parasites in Seafood

What You Need to Know

Every living creature on our planet, including aquatic species, can host parasites. These organisms occur just as commonly in fish as pests do in fruits and vegetables. Two main categories of parasites may affect humans through contaminated food or water: parasitic worms and single-celled protozoa.

Parasitic worms encompass several types: roundworms (nematodes), tapeworms (cestodes), and flukes (trematodes). These organisms range dramatically in size from nearly microscopic to several feet long. Protozoa are microscopic single-cell organisms invisible to the naked eye.

Similar to how consuming raw or undercooked meat carries risks, eating uncooked, lightly preserved, pickled, or cold-smoked seafood presents comparable hazards. However, parasites pose no health threat when fish is thoroughly cooked.

Risk Factors and Prevention

Consumers should be particularly cautious when consuming raw or minimally processed seafood preparations such as sashimi, sushi, ceviche, and gravlax. For these dishes, it's essential to use commercially frozen fish. Alternatively, you can freeze fish to an internal temperature of -4°F for a minimum of 7 days to eliminate any present parasites. Note that standard home freezers typically operate between 0°F and 10°F, which may not reach temperatures cold enough to destroy all parasites effectively.

Parasites in their larval stage found in raw, undercooked, or improperly frozen seafood can present human health concerns. Among various parasites, specific types are most concerning in seafood: nematodes or roundworms (including Anisakis spp., Pseudoterranova spp., Eustrongylides spp. and Gnathostoma spp.), cestodes or tapeworms (Diphyllobothrium spp.), and trematodes or flukes (including Chlonorchis sinensis, Opisthorchis spp., Heterophyes spp., Metagonimus spp., Nanophyetes salminicola and Paragonimus spp.).

High-Risk Seafood Preparations

Certain seafood preparations have been associated with human infections:

• Ceviche (seafood marinated in citrus juice with spices)
• Lomi lomi (salmon cured in lemon juice with onions and tomatoes)
• Poisson cru (fish marinated in citrus, onion, tomato, and coconut milk)
• Herring roe
• Sashimi (thinly sliced raw fish)
• Sushi (raw fish combined with rice and other ingredients)
• Green herring (lightly salted herring)
• Drunken crabs (crabs marinated in wine and spices)
• Cold-smoked fish (such as lox)
• Insufficiently cooked grilled fish

Effective Control Methods

Proper cooking (reaching 145°F for 15 seconds) of raw fish that effectively destroys bacterial pathogens will also eliminate parasites.

Several factors influence how effectively freezing destroys parasites: the freezing temperature, duration needed to freeze the fish tissue completely, length of time the fish remains frozen, the fish's fat content, and the specific parasite type. Temperature, storage duration, and parasite species appear to be the most critical factors. For instance, tapeworms are more vulnerable to freezing than roundworms, while flukes demonstrate greater resistance than roundworms.

The FDA recommends specific freezing protocols: freezing and maintaining at -4°F (-20°C) or colder for 7 days total, or freezing at -31°F (-35°C) or below until completely solid and maintaining at -31°F (-35°C) or lower for 15 hours, or freezing at -31°F (-35°C) or below until solid and storing at -4°F (-20°C) or colder for 24 hours. These conditions are sufficient to destroy parasites. The FDA's Food Code provides these freezing requirements to retailers offering fish for raw consumption.

While trimming belly flaps or candling fish to physically remove visible parasites can reduce parasite numbers, these methods don't completely eliminate the hazard or minimize it to acceptable safety levels.

Important perspective: The health risk from parasites is considerably lower than risks from bacterial contamination and improper seafood handling.

Additional Information and Resources