Parasites
Parasites (in the larval stage) found in uncooked or undercooked, unfrozen seafood can pose serious health risks. Among parasites, nematodes or roundworms (Anisakis spp., Pseudoterranova spp., Eustrongylides spp., and Gnathostoma spp.), cestodes or tapeworms (Diphyllobothrium spp.), and trematodes or flukes (Chlonorchis sinensis, Opisthorchis spp., Heterophyes spp., Metagonimus spp., Nanophyetes salminicola, and Paragonimus spp.) are of significant concern in seafood.
Products implicated in human infections include ceviche (fish and spices marinated in lime juice); lomi lomi (salmon marinated in lemon juice, onion, and tomato); poisson cru (fish marinated in citrus juice, onion, tomato, and coconut milk); herring roe; sashimi (raw fish slices); sushi (raw fish with rice and other ingredients); green herring (lightly brined herring); drunken crabs (crabs marinated in wine and pepper); cold-smoked fish; and undercooked grilled fish.
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Nematodes
Anisakiasis is a disease caused by ingesting larvae of the nematodes (roundworms) Anisakis simplex and Pseudoterranova decipiens. The adult stages of A. simplex or P. decipiens live in the stomach of marine mammals, where they embed in the mucosa in clusters.
Eggs produced by adult females are passed in feces, hatch, and yield second-stage larvae. Upon ingestion by crustaceans, third-stage larvae develop and become infectious to fish and squid. After ingestion by these hosts, the larvae migrate from the intestines to the peritoneal cavity, and upon the host’s death, to muscle tissues.
Through predation, larvae transfer from fish to fish until ingested by a marine mammal, where they develop into adults, thus completing the cycle. Humans become infected by eating raw or undercooked marine fish.
After ingestion, the anisakid larvae penetrate the gastric and intestinal mucosa, causing symptoms of anisakiasis, such as violent abdominal pain, nausea, and vomiting within hours. If larvae pass into the bowel, severe eosinophilic granulomatous responses may mimic Crohn’s disease after 1-2 weeks. Anisakiasis is common where raw fish is consumed, especially in Japan, South America, and the Netherlands.
Eustrongylides spp.
The larvae of Eustrongylides spp. are large, bright red nematodes measuring 25-150 mm in length and 2 mm in diameter. They are commonly found in freshwater, brackish, and marine fish. The larvae mature in wading birds like herons, egrets, and flamingos.
If consumed in raw or undercooked fish, the larvae can attach to the gut wall, causing severe pain. Infections are extremely rare and have been linked to consuming live minnows and sashimi.
Gnathostoma spinigerum
Gnathostoma spinigerum infects vertebrate animals. In its natural hosts (cats, dogs, and wild animals), adult worms reside in tumors in the gastric wall. Immature eggs passed in feces hatch in water, releasing first-stage larvae.
After ingestion by crustaceans, the larvae develop into L2. When crustaceans are eaten by fish, frogs, or snakes, the larvae develop into L3, which infects humans through the consumption of undercooked fish or poultry.
Infections cause migration of immature worms (L3s) through the body, leading to painful swellings in the skin or serious issues such as eosinophilic meningitis. Gnathostoma spinigerum is prevalent in Asia, especially in Thailand and Japan.
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Cestodes (Tapeworms)
Diphyllobothrium latum, the fish or broad tapeworm, is the largest human tapeworm. Several other species (D. pacificum, D. cordatum, D. ursi, etc.) have also been reported to infect humans. The adult tapeworm resides in the small intestine, attaching to the mucosa, and can grow over 10 meters long.
Humans acquire the infection by consuming raw or undercooked freshwater fish. Diphyllobothriasis can last decades, with symptoms such as abdominal discomfort, diarrhea, vomiting, and weight loss. In severe cases, vitamin B12 deficiency and anemia can occur. Diphyllobothriasis is most common in regions with lakes and rivers, including the Northern Hemisphere, Uganda, and Chile.
Trematodes (Flukes)
Clonorchis sinensis, the Chinese or oriental liver fluke, resides in small to medium-sized biliary ducts. Humans become infected by eating undercooked, salted, pickled, or smoked freshwater fish. The adult flukes can survive 20 to 25 years in humans, with manifestations such as abdominal pain, nausea, and diarrhea.
Long-term infections can lead to serious complications like cholangitis, pancreatitis, and cholangiocarcinoma. Opisthorchiasis, caused by Opisthorchis viverrini and O. felineus, is also associated with consuming undercooked fish. These infections can present similar symptoms, with severe cases leading to bile duct complications or even cholangiocarcinoma.
Control Measures
Proper handling and processing of fish are crucial to minimizing the risk of parasite infection. Heating fish sufficiently to kill bacterial pathogens also kills parasites. Freezing fish at -20ºC (-4ºF) or below for seven days or at -35ºC (-31ºF) for 15 hours effectively eliminates parasites.
Trimming belly flaps or physically removing parasites can reduce the number of parasites but does not completely eliminate the hazard. Adhering to FDA guidelines and implementing HACCP controls can help ensure the safety of fish consumed raw.
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Frequently Asked Questions
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