Medical Definition of fish tapeworm
: a large tapeworm of the genus Diphyllobothrium (D. latum) that as an adult infests the human intestine, is sometimes associated with a peculiar macrocytic anemia resembling pernicious anemia, and goes through its early stages in copepods of the genus Diaptomus and intermediate stages in freshwater fishes from which it is transmitted to humans or other fish-eating mammals when raw fish is eaten
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