The life cycle of T. solium is represented graphically below. It consists of six main steps:

Figure courtesy of the CDC's Division of Parasitic Diseases. [C]

STEP 1. Infected humans (definitive host) excrete the eggs or gravid proglottids in their feces, passing the parasite from the gastrointestinal tract onto nearby vegetation. In egg or gravid proglottid form, T. solium is able to remain viable anywhere from days to months. T. solium can be diagnosed at this point in the life cycle.

Note: Autoinfection can also occur at this point in the life-cycle via fecal-oral contamination. In this case, eggs or gravid proglottids re-enter the body through the mouth and often travel to the central nervous system (CNS), the muscles or the eye, where they develop into cysticerci. The presence of cysticerci in these locations leads to the pathogenesis of cysticercosis (neurocysticercosis in the CNS). [4,5]

STEP 2. Pigs (intermediate host) acquire infection by eating and digesting the eggs or gravid proglottids along with the parasitized vegetation.

STEP 3. The eggs or gravid proglottids migrate to the pig's intestine and as oncospheres, break through the intestinal wall. Then, via the circulatory system, they embed themselves in the muscles of the pig and develop into cysticerci (the infective form of T. solium). Cysticerci have the ability to persist in the muscle for many years.

STEP 4. Humans acquire the infection by eating the undercooked or raw flesh of an infected animal.

STEP 5,6. Cystercerci migrate to the small intestine of the human host and develop into their adult tapeworm form normally within two months. By attaching to the intestinal wall with their scolices (hooked structures), these adult tapeworms may persist for long periods of time, even years.