HELMINTHS Diphyllobothrium

I. Causative Agent and Disease predatory that become paratenic Six of diphyllobothrid ces- hosts when other infested fish are eaten. todes (tapeworm) occur in Alaska, all of The life cycle is complete when the fish which use fish as a second intermediate host is eaten by a or bird defini- and/or as a paratenic host. Three species tive host where the worm becomes an of larval Diphyllobothrium that most egg-producing adult. commonly occur in Alaskan salmonid include D. ditremum, dendriticum V. Diagnosis and nihonkaiense. The cestode larvae Diagnosis is made by visual identi- can be found free in the visceral cavity fication of the cestode during necropsy or encysted in the viscera or muscle tis- of a parasitized fish. Plerocercoid stages sues. of Diphyllobothrium have a compressed scolex with characteristic bothria or II. Host Species grooves. The body is usually slightly Planktivorous and carnivorous fresh- wrinkled, suggesting segmentation. PCR water fishes are potential hosts in North/ has been useful in confirming species South America and Eurasia including that has resulted in changing . salmonids, whitefish, , northern pike, sticklebacks, burbot, and blackfish. VI. Prognosis for Host Prognosis for the host is good pro- III. Clinical Signs vided the infestation is low and there are The larval Diphyllobothrium can not other stressors involved. Juvenile fish be found (sometimes encysted) in the are more adversely affected than older muscles, viscera and connective tis- fish and can die from severe plerocercoid sues of the fish host causing adhesions, infestations. hemorrhaging (particularly the liver) and ascitic fluid resulting in abdominal dis- VII. Human Health Signifcance tension. Severe infestations in juvenile Species of this cestode group can fish can cause mortality. successfully parasitize humans. Most human infestations are accidental since IV. Transmission the natural hosts are fish eating birds Infestation of the fish host is part of and . Infestation in man occurs a 3-host life cycle for this parasite. Adult by ingestion of raw or lightly smoked worms are found in the fish that contain viable plerocercoid of definitive hosts that are fish eating larvae. The Center for Disease Control birds or mammals (including humans). recommends cooking fish at 67ºC for 5 Eggs from adult worms are released into minutes or freezing fish at -20ºC for at the water with feces where they develop least 7 d to kill worm parasites before into a free swimming coracidium larval ingestion. stage that is ingested by , the first intermediate host. The procercoid develops in the and, when eaten by the fish second intermediate host, develops into the plerocercoid stage. Plerocercoids re-encyst near the gut of

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Diphyllobothrium Life Cycle

Humans parasitized by consumption of raw or undercooked fish Fish eating birds and mamals are final hosts for adult cestode Diphyllobothrium

Eggs

Piscivorous fish Ciliated larvae (paratenic host) (coracidium)

Crustacean ingested by second Procercoid larvae in first intermediate host (fish) where intermediate host, larvae develop into plerocercoids a copepod

Left: Subsurface white cysts (center) in brook liver with plerocercoids of Diphyllobothrium sp.; Right: Diphyllobothrium sp. plerocercoid with wrinkled body suggesting segmentation.

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Bothria (grooves) in plerocercoid scolex characteristic of Encysted plerocercoids in viscera of Diphyllobothrium sp. (black arrows).

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