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What Are the Dangers of Nematodes in Consumed Fish?

Nematodes or roundworms contain approximately 500,000 species, of which an estimated 60 are parasitic to humans. Nematodes found in marine ecosystems complete part of their life cycle in the intestines and muscle tissue of fish such as salmon, herring and sardines. The Anisakis worm is the most common marine nematode to cause infection in humans with reported cases in Japan, France and California. People who consume raw or undercooked fish are susceptible to infection.

  1. Marine Nematode Life Cycle

    • The nematode life cycle does not occur in one single host. Nematode eggs are excreted in the feces of marine mammals such as whales and dolphins. These eggs develop into larvae, which are then eaten by crustaceans. Here they develop further, and when fish consume crustaceans, nematode larvae penetrate the fish muscle and continue to grow. Some nematodes do not mature into adulthood in their fish host, but rather in marine mammals after the fish is eaten.

    Marine Nematodes in Humans

    • Humans are paratenic hosts, as they are not necessary for the nematode life cycle to complete, but are a source of infection if consumed. As parasites do not self-reproduce, the accumulation of enough parasitic nematodes to cause infection occurs only after repeated exposure to infectious larvae. For fish nematodes, this predominantly affects people who regularly consume contaminated raw fish. Humans do not seem to develop protective immunity against nematode infections.

    The Anisakis Worm

    • The Anisakis worm is generally transmitted via ingestion of foods such as sushi, salmon, smoked herring and pickled anchovies. The parasite survives for only a few weeks in the gastric wall of the small intestine, leading to tissue damage. Symptoms such as abdominal pain, diarrhea, vomiting and nausea occur from an hour up to two weeks after ingestion, and can persist for several months. The Anisakis worm causes the disease anisakiasis, which is an allergic reaction in the gastrointestinal tract.

    Precautions

    • Nematodes survive in unfrozen fish so the Food and Drug Administration advises that fish to be consumed raw should be frozen at -20 degrees Celsius or below for seven days, or -35 degrees Celsius for 15 hours. Salting and marinating does little to kill parasites, but cooking at 60 degrees Celsius and above is effective. Also, farmed fish such as salmon are safer to consume than wild fish.

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