Diphyllobothrium (fish tapeworm)

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Epidemiology

widely distributed in temperate zones
- especially northern Europe, Scandinavia, Russia & Japan
- in North America Canada, north central states, Pacific coast states & Alaska
definitive hosts include piscivorous mammals & possibly birds
eggs passed unembryonated in the feces must reach a fresh water stream or lake to continue development
after several weeks of embryonation in fresh water, a ciliated larval form with 6 hooks (coracidium) hatches & is ingested by a copepod
the coracidium develops into a procercoid larva which is infective for a 2nd intermediate host, a fish
in fish, the procercoid migrates into the tissues & develops into a plerocercoid larva
plerocercoids may be passed up the food chain unchanged & accumulate in larger fish
human acquire Diphyllobothrium infection by ingestion of raw or inadequately cooked fish (containing infectious plerocercoids) that have spent at least part of their life in fresh water

diagnosis is made by finding brown, oval-shaped operculate eggs in feces
eggs measure 58-76 by 40-52 um
a length of strobila or intact worm passed in the feces allows identification of species
- scolex is elongated with longitudinal grooves (bothria) which replace suckers
- gravid proglottids are wider than they are long with genital pores located midventrally adjacent to a rosette-shaped uterus
serum vitamin B12

praziquantel 10-20 mg/kg PO once
niclosamide 2 g PO once
vitamin B12 IM if B12-deficient
prevention
- heating fish to > 54 degrees C for 5 minutes
- freezing to < -18 degrees C for 24 hours

↑ Clinical Diagnosis & Management by Laboratory Methods, 19th edition, J.B. Henry (ed), W.B. Saunders Co., Philadelphia, PA. 1996, pg 1289
↑ Harrison's Principles of Internal Medicine, 13th ed. Isselbacher et al (eds), McGraw-Hill Inc. NY, 1994, pg 933
↑ Grimm L What's Eating You: 12 Common Intestinal Parasites. Medscape. November 25, 2019 https://reference.medscape.com/slideshow/intestinal-parasites-6010996