• Helminth, Cestode (tapeworm); several other species, but D. latum is the most common species infecting humans.
• The bodies of tapeworms are flat and ribbon-like with a head (scolex) containing organs of attachment. Unlike the other tapeworms, the scolex of D. latum does not contain suckers and hooklets, but instead has a long, lateral, sucking groove which enables the tapeworm to attach to the intestinal mucosa.
• The individual segments of the tapeworm are called proglottids. The distal proglottids are more mature, almost completely occupied by a uterus filled with eggs.
• Hermaphroditic (male and female reproductive organs in each mature proglottid).
• Longest tapeworm found in humans (20-30 feet in length).
• Transmission: ingestion of infected raw or undercooked freshwater fish.
• Immature eggs are released in feces, embryonate in freshwater, then release a free-swimming larval form called a coracidia which are ingested by small crustaceans (first intermediate host). The larval form in the crustacean develops into a procercoid larval form which when ingested by small freshwater fish (second intermediate host) migrate into the fish flesh and develop into a plerocercoid larva. Small fish (minnows) are ingested by larger predator fish (trout, salmon, pike, perch, etc) and the plerocercoid larva migrate to the muscle.
• The plerocercoid larva is the infectious form for humans (definitive host).
• Adult worms live in the human small intestine and eggs and proglottids are passed in the stool.
• Infections occur worldwide, particularly in cool lake regions where eating raw, smoked, salted, or pickled fish is popular (northern Europe, Russia and Asia).
• Humans become infected when eating raw or undercooked fish containing D. latum larval forms.
• Most cases are asymptomatic.
• Nonspecific GI symptoms: diarrhea, abdominal pain, vomiting, weight loss, fatigue.
• Rare (0.1-2%): severe vitamin B12 deficiency due to competition between the host and the worm for dietary B12. Individuals may develop megaloblastic anemia and neurological manifestations.
• Complications: may include intestinal obstruction or gallbladder disease caused by migrating proglottids.
• Microscopic stool exam for characteristic eggs and/or proglottids.
• The eggs are oval (45 x 90 um) with an operculum (lid) on one end and a knob on the opposite end.
• Typical proglottids are wider than they are long, containing a central rosette uterine structure.
Therapy: Praziquantel is the treatment of choice
Prevention and control:
• Prevention of water contamination by controlling the disposal of human sewage into freshwater lakes.
• Do not consume raw or undercooked freshwater fish (cook adequately or freeze).
• Prompt treatment of infections.