Life cycle of Taenia solium
The infection caused by the complex Taeniasis/cysticercosis in humans represents a rather peculiar phylogenetic encounter between Taenia solium, the most evolved cestode parasite (order cyclophyllidea, family taeniidae) (Willms, 1992) and human, the most evolved mammal (order primates, family hominidae). Both protagonists are endowed for survival with sophisticated biological mechanisms. In fact, rather than being another infection, cysticercosis is the transplant of the embryo of Taenia solium, into the tissues of the intermediate host (pork and humans) where it hatches in the intestine and is transported, by the blood stream, to tissues to become a larva. For the cycle to be completed a human must ingest, undisturbed and intact, a cysticercus whose size is between 0.5 and 1 cm diameter. The unique source of this larva is undercooked pork meat; it is truly amazing to observe that the cyst survives not only the cooking process but also the powerful masticatory movements and the fast intestinal passage. Once within the digestive tract, the metacestode (larva), measuring between 1 and 2 mm evaginates from the cyst and strongly attaches itself to the intestinal wall with the aid of its four suckers and a double chain of hooks. Two months later, a cestode measuring 2–4 metres long has developed; thereafter, every day, a few mature progglotides spontaneously detach, each containing several thousand fertilized eggs, which in turn will pollute areas with deficient sanitary installations and inadequate disposal of human feces. When contaminated food is ingested either by pigs or humans, cysticercosis develops in these new hosts. In a strict sense, only cysticercosis in pigs and taeniasis in humans are favourable for the perpetuation of the parasite; cysticercosis in humans is a failed attempt, since cannibalism would be the only possibility for a cysticercus in human muscle or brain to develop to the cestode stage in the intestine of another human. Nonetheless, neurocysticercosis (NCC) is the most frequent and severe parasitic disease of the nervous system of humans.
The presence of a whole cyst larva within the brain provokes a far more complicated immune response of rejection or tolerance than would be the case for other less complex infectious agents (Del Bruto et al., 1998).