HELMINTHS Anisakid Larvae

I Causative Agent and Disease cycle is completed when ingested by the The larval form (third stage juvenile) final host, usually a marine mammal but of several nematode species within the sometimes a bird or fish. Incidental para- subfamily Anisakinae are found coiled sitism of a human host usually results in the flesh and viscera of parasitized in re-encystment of the juvenile worm. fish. Common genera include Anisakis, The nematode matures in the gut of the Paranisakis, Porrocaecum, Pseudoter- marine mammal host and releases eggs ranova and Contracaecum. The larvae into the sea to continue the cycle. Some are relatively non-pathogenic to the fish anisakid larvae can also be transmitted host, although visceral adhesions from from fish to fish through predation. migrating larval worms do occur. V. Diagnosis II. Host Species Presumptive diagnosis is made by Anisakid larvae are common in visual examination of the body cavity, marine and anadromous fishes worldwide organs and flesh of the fish for typi- and have also been reported in squid cal coiled worms. Examination under and cuttlefish. In Alaska, among other a dissecting microscope can verify the fish species, these worms are commonly identity of the larval nematodes based on found in Pacific salmon and cod, walleye morphological characteristics. PCR has pollock, Pacific halibut and Pacific her- been useful in confirming species that ring (herring worm). has resulted in changing taxonomy.

III. Clinical Signs VI. Prognosis for Host Parasitized fish contain reddish or Prognosis for the fish host is good. In white, tightly coiled larval worms found most cases the worms are well tolerated most commonly in skeletal muscle and and there have been few reported cases visceral organs. These areas may exhibit of fish mortality due to juvenile anisakid mild inflammation, encapsulation and/or parasitism. granuloma formation. Visceral adhesion may occur in fish when many juvenile VII. Human Health Signifcance worms are present in the visceral cavity. Anisakiasis in humans can be ac- This condition causes production of quired by eating viable worms in raw or fibrous connective tissue by the fish host partially cooked fish. The Food and Drug in response to irritation from migrating Administration recommends cooking worms. fish at 67ºC for 5 minutes or freezing at -20ºC for at least 7 d before eating to kill IV. Transmission juvenile anisakid worms. Anisakid worms have a complex life cycle involving several hosts. Eggs eliminated in the feces from the final host hatch in the sea where the larvae are consumed by crustaceans (usually Euphausids), which in turn are eaten by fish. The larva burrows into the gut or flesh of the fish and encysts until its life

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Anisakis Life Cycle

Humans contract by consumption of raw or undercooked Marine mammal

fish (abnormal host) final host for the adult nematode Anisakis

Eggs and second stage larva

Fish and Squid

Free swimming larva ingested by crustaceans (Euphausids)

Anisakis third stage juvenile worms tightly coiled in liver.

Anisakis third stage juvenile worm being pulled from salmonid muscle.

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