To fully understand the life cycle of this parasite, it is essential to start by following some fundamental steps that first lead us to the adult form of the cestode (3-5 meters in length), which is the form capable of replicating and releasing eggs, residing in the intestines of the infected human. In this part of the body, the parasite feeds by absorbing nutrients present in the intestinal lumen through osmosis. This cestode releases 3 to 4 proglottids (gravid segments) into the intestines, which are expelled externally with the feces of the infected person. Each proglottid can contain up to 250,000 eggs, which can survive in the environment for several months. The cycle requires the presence of an intermediate host, specifically a host that allows the larval forms of the cestode to develop, which in this case are represented by pigs. Pigs become infected by consuming food contaminated with fecal material containing eggs, or by ingesting cestode eggs excreted with the feces of an infected human that have contaminated the environment and come into contact with the pig.
Once ingested by the pig (intermediate host), the eggs hatch, releasing an oncosphere, or the premature larval form, which, through the bloodstream, reaches the muscles (especially the masticatory muscles), the tongue, and the heart of the animal, developing into the secondary larval form known as the cysticerco. The cysticerco can remain viable for several years in these body areas of the pig. The cycle concludes when a human ingests these larvae contained in undercooked or even raw infected pork (if thoroughly cooked, the larvae die): the parasite, upon reaching the human intestine, anchors itself and, as mentioned earlier, grows to restart the cycle. This is the biological characteristic of T. solium; however, it is rare for humans to become infected by ingesting eggs (which are usually not infectious for humans) rather than larvae as per the expected life cycle. In this case, the human acts as an intermediate host by developing cysticerci within muscles, the tongue, and, in more pathological forms, in the brain. In humans, the larvae can rarely cause serious sequelae if they localize in the brain, leading to neurocysticercosis.
The parasite's life cycle is completed, as in infection by the tapeworm, when the human ingests undercooked pork containing cysticerci. The cysts evaginate and attach to the small intestine with their scoleces.
Although humans are generally considered definitive hosts, consuming infected meat and harboring adult tapeworms in the intestine while passing eggs through feces, sometimes a cysticerco (or a larva also referred to as "vesicular worm") can develop in humans and, through such behaviors, become an intermediate host. This occurs if the eggs reach the stomach, usually due to dirty hands, but also through vomiting. Cysticerci can often lodge in the central nervous system, where they can cause severe neurological issues such as epilepsy and even death. The presence of cysticerci in a person's body is referred to as cysticercosis.
