Taenia solium, the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses humans (Homo sapiens) as its definitive host and pigs and boars (family Suidae) as the intermediate or secondary hosts. It is transmitted to pigs through human feces that contain the parasite eggs and contaminate their fodder. Pigs ingest the eggs, which develop into larvae, then into oncospheres, and ultimately into infective tapeworm cysts, called cysticerci. Humans acquire the cysts through consumption of uncooked or undercooked pork and the cysts grow into adult worms in the small intestine. There are two forms of human infection. One is "primary hosting", called taeniasis, and is due to eating undercooked pork that contains the cysts, resulting in adult worms in the intestines. This form generally is without symptoms; the infected person does not know they have tapeworms. This form is easily treated with anthelmintic medications which eliminate the tapeworm. The other form, "secondary hosting", called cysticercosis, is due to eating food, or drinking water, contaminated with faeces from someone infected by the adult worms, thus ingesting the tapeworm eggs, instead of the cysts. The eggs go on to develop cysts primarily in the muscles, and usually with no symptoms. However, some people have obvious symptoms, the most harmful and chronic form of which is when the cysts form in the brain. Treatment of this form is more difficult but possible. The adult worm has a flat, ribbon-like body which is white and measures 2 to 3 metres (6.6 to 9.8 ft) long, or more. Its tiny attachment, the scolex, contains suckers and a rostellum as organs of attachment that attach to the wall of the small intestine. The main body, consists of a chain of segments known as proglottids. Each proglottid is little more than a self-sustainable, very lightly ingestive, self-contained reproductive unit since tapeworms are hermaphrodites. Human primary hosting is best diagnosed by microscopy of eggs in faeces, often triggered by spotting shed segments. In secondary hosting, imaging techniques such as computed tomography and nuclear magnetic resonance are often employed. Blood samples can also be tested using antibody reaction of enzyme-linked immunosorbent assay. T. solium deeply affects developing countries, especially in rural settings where pigs roam free, as clinical manifestations are highly dependent on the number, size, and location of the parasites as well as the host's immune and inflammatory response.
Description Adult T. solium is a triploblastic acoelomate, having no body cavity. It is normally 2 to 3 metres (6.6 to 9.8 ft) in length, but can become much larger, sometimes over 8 metres (26 ft) long. It is white in colour and flattened into a ribbon-like body. The anterior end is a knob-like attachment organ (sometimes mistakenly referred to as a "head") called a scolex, 1 mm in diameter. The scolex bears four radially arranged suckers that surround the rostellum. These are the organs of adhesive attachment to the intestinal wall of the host. The rostellum is armed with two rows of proteinaceous spiny hooks. Its 22 to 32 rostellar hooks can be differentiated into short (130 μm) and long (180 μm) types. After a short neck is the elongated body, the strobila. The entire body is covered by a covering called a tegument, which is an absorptive layer consisting of a mat of minute specialised microvilli called microtriches. The strobila is divided into segments called proglottids, 800 to 900 in number. Body growth starts from the neck region, so the oldest proglottids are at the posterior end. Thus, the three distinct proglottids are immature proglottids towards the neck, mature proglottids in the middle, and gravid proglottids at the posterior end. A hermaphroditic species, each mature proglottid contains a set of male and female reproductive systems with numerous testes and an ovary with three lobes. The uterus is branched with a characteristic 7 to 13 branches per side. The cirrus, a sex organ at the terminus of the vas deferens, and vagina open into a common genital pore or atrium. The oldest gravid proglottids are full of fertilised eggs, Each fertilised egg is spherical and measures 35 to 42 μm in diameter. If released early enough in the digestive tract and not passed, fertilised eggs can mature using upper tract digestive enzymes. The tiny oncosphere larvae, activated by exposure to host enzymes and bile salts, penetrate the intestinal wall and migrate in the blood stream or lymphatics to reach sites where they can develop into cysticerci. These have three morphologically distinct types. The common one is the ordinary "cellulose" cysticercus, which has a fluid-filled bladder 0.5 to 1.5 cm (1⁄4 to 1⁄2 in) in length and an invaginated scolex. The intermediate form has a scolex. The "racemose" has no evident scolex, but is believed to be larger. They can be 20 cm (7.9 in) in length and have 60 ml (2 fl. oz.) of fluid, and 13% of patients with neurocysticercosis can have all three types in the brain.
Life cycle
The life cycle of T. solium is indirect as it passes through pigs and boars, commonly Sus domesticus due to their association with people, as intermediate hosts, into humans, as definitive hosts. In humans the infection can be relatively short or long lasting, and in the latter case if reaching the brain can last for life. From humans, the eggs are released in the environment where they await ingestion by another host. In the secondary host, the eggs develop into oncospheres which bore through the intestinal wall and migrate to other parts of the body where the cysticerci form. The cysticerci can survive for several years in the animal.
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