Question
Life cycle of taenia solium.
Answer
Taenia solium (the pork tapeworm) has a life cycle involving humans as both definitive and (accidental) intermediate host, and pigs as the usual intermediate host.
Definitive host cycle (intestinal taeniasis): humans acquire infection by eating undercooked/raw pork containing cysticerci (the larval bladder-worm stage); in the human small intestine, the scolex evaginates and attaches to the intestinal mucosa via its hooks and suckers, developing into an adult tapeworm over ~2–3 months; the adult worm, consisting of a scolex and a chain of proglottids (segments), can grow to several metres in length; the terminal gravid proglottids (containing thousands of eggs) detach and are passed in stool, either intact or after rupturing to release eggs.
Intermediate host cycle (cysticercosis): when a pig ingests eggs/gravid proglottids from contaminated environment (faecal-oral transmission from human carriers), the eggs hatch in the pig intestine, releasing oncospheres that penetrate the gut wall, disseminate via the bloodstream, and encyst in muscle tissue as cysticerci — this completes the cycle when a human eats undercooked infected pork.
Human cysticercosis (the medically important complication): humans can also become an accidental intermediate host if they ingest T. solium eggs (rather than cysticerci) — via faecal-oral contamination (contaminated food/water, or autoinfection from a person who already harbours the adult intestinal tapeworm) — the ingested eggs hatch and release oncospheres that disseminate and encyst in human tissues, most importantly the central nervous system (neurocysticercosis), but also muscle, subcutaneous tissue, and eye.
Clinical significance: neurocysticercosis is a major cause of acquired epilepsy/seizures in many endemic regions worldwide, and can also cause raised intracranial pressure (from cysts obstructing CSF flow) and focal neurological deficits, depending on cyst location and the degree of inflammatory response as cysts degenerate.
Laboratory diagnosis: stool microscopy for eggs/proglottids (intestinal taeniasis, though T. solium eggs are morphologically indistinguishable from T. saginata eggs, requiring proglottid morphology or molecular methods for species distinction); imaging (CT/MRI) for neurocysticercosis, showing characteristic cystic lesions with or without a visible scolex; serology (ELISA, immunoblot) for cysticercosis.
Prevention: proper cooking of pork, sanitation to prevent faecal contamination of food/water and pig feed, treatment of human tapeworm carriers to prevent both further transmission and the risk of autoinfection/cysticercosis.