Cysticercosis
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Pigs normally serve as the intermediate host for the pork tapeworm, Taenia solium, and humans are infected with the adult stage of the tapeworm when they ingest an immature tapeworm (a cysticercus) in raw or undercooked pork.
However, if humans ingest eggs of T. solium, they can be infected with cysticerci, resulting in a condition known as cysticercosis. What might be the source of these eggs? Humans harbor the adult stage of this tapeworm, and it is the adult stage that produces eggs. Thus, many cases of cysticercosis probably result from a person ingesting eggs that are produced by a tapeworm living in his or her own intestinal tract.
Poor personal hygiene is one obvious way in which this could occur. It is also possible for the proglottids of T. solium to migrate anteriorly from the small intestine into the stomach and then back into the small intestine. Should this occur the eggs in the proglottids would hatch resulting in the potential for a massive infection of cysticerci.
People can also be infected via food contaminated with eggs, or via eggs present in a household or work environment. Since the tapeworm's proglottids can crawl out of the anus and contaminate clothing, furniture, etc., or drop to the ground, such contamination could occur in the absence of any visible source of "fecal" contamination.
Once the eggs hatch in the human's small intestine, the larvae penetrate the lining of the small intestine and enter the blood stream. From here the larvae can be distributed to any organ in the body. The larvae then grow into the metacestode stage, a cysticercus. Mature cysticerci can range in size from 5 mm in diameter up to 20 cm (almost 8 inches!) in diameter.
The pathology associated with cysticercosis depends on which organs are infected and the number of cysticerci. An infection consisting of a few small cysticerci in the liver or muscles would likely result in no overt pathology and go unnoticed. In fact, many cases of human cysticercosis are discovered only during routine autopsies. On the other hand, even a few cysticerci (perhaps only one), if located in a particularly "sensitive" area of the body, might result in irreparable damage.
For example, a cysticercus in the eye might lead to blindness, a cysticercus in the spinal cord could lead to paralysis, or a cysticercus in the brain (neurocysticercosis) could lead to traumatic neurological damage. Thus, even though infections with adult T. solium are rarely a problem, treatment of such infections is absolutely essential.
Historically, diagnosis of cysticercosis has been difficult. However, there are now several immunological tests available that will detect the presence of cysticerci, and improved imaging techniques such as CAT and MRI can be very useful in detecting cysticerci in various organs.