Diphyllobothrium latum

D. latum egg/CDC

General Information

• Also known as "Fish Tapeworm" and "Broad Tapeworm"
• Largest human tapeworm, strobila can reach 30-45 ft in length

Geography

• 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

Pathology

• 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

Diagnosis

• Microscopic identification of eggs in stool sample. Examination of characteristic proglottids also useful.

Treatment

• Praziquantel or nicolsamide

Epidemiology

• 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

Prevention

• Thorough cooking of freshwater fish, or
• Freezing fish for 24 hours at -18C (0F), or
• Irradiation

D. latum gravid proglottids/CDC