Parasitology 101 is an educational blog that that can be used as a study guide for microbiology, infectious disease and medical technology students. The “bullet-point” format keeps the information concise and to the point.

Diphyllobothrium latum egg/CDC
Diphyllobothrium latum egg/CDC

General Information

  • Also known as “Fish Tapeworm” and “Broad Tapeworm”
  • Largest human tapeworm, strobila can reach 30-45 ft in length
  • Worldwide, particularly in cool lake regions of the Northern hemisphere (North America, Europe and Asia) and South America
Morphology (adults)
  • 30-45 ft in length, 1 cm in width
  • Scolex (head) spatulate with two slit-like grooves (bothria)
  • 3000 proglottids, each with “rosette-shaped” uterus.
  • Proglottids wider than long
Morphology (eggs)
  • Oval or ellipsoidal, 75 um x 45 um
  • Operculated
  • Undeveloped embryo (coracidium)
  • Small, terminal abopercular knob
Life Cycle
  • Adult in small intestine
  • Immature eggs shed from gravid proglottids, passed in feces
  • Eggs develop in fresh water> develop into coracidium
  • Coracidium emerge from egg, ingested by freshwater crustacean (copepod, Cyclops sp.) First intermediate host
  • Develops into procercoid larva
  • Infected crustacean ingested by small freshwater fish (minnows for example) (second intermediate host)> develops into plerocercoid larva (sparganum)
  • Infected small fish ingested by larger “predatory fish” (perch, trout, etc)> plerocercoid larva released and penetrates muscles of predator fish
  • Humans infected when consuming raw or undercooked infected fish
  • Plerocercoid larva develops to adult in small intestine
  • From beginning of infection> approximately 6 weeks until eggs passed in feces
  • Adult can live for up to 20 years, each adult can shed 1 million eggs/day
  • Usually asymptomatic, patient becomes aware when worm segments passed in stool
  • Clinical manifestations may include pernicious anemia due to vitamin B12 deficiency
  • Abdominal discomfort, diarrhea, vomited, weight loss
  • Heavy infections can result in intestinal obstruction


  • Microscopic identification of eggs in stool sample. Examination of characteristic proglottids also useful.
  • Praziquantel or nicolsamide
  • Freshwater transmission dependent on available human/mammal definitive host, suitable intermediate hosts, consumption of raw or undercooked freshwater fish
  • Pollution of freshwater with infected human feces
  • Thorough cooking of freshwater fish, or
  • Freezing fish for 24 hours at -18C (0F), or
  • Irradiation
D. latum gravid proglottids/CDC