Taenia solium

Taenia solium
Scolex of Taenia solium
Scientific classification
Kingdom:	Animalia
Phylum:	Platyhelminthes
Class:	Cestoda
Order:	Cyclophyllidea
Family:	Taeniidae
Genus:	Taenia
Species:	T. solium
Binomial name
Taenia solium Linnaeus, 1758

Taenia solium, also called the pork tapeworm, is a cyclophyllid cestode in the family Taeniidae. It infects pigs and humans in Asia, Africa, South America, parts of Southern Europe, and pockets of North America. Like all cyclophyllid cestodes, T. solium has four suckers on its scolex ("head"). T. solium also has two rows of hooks.

Description

T. solium has a very similar life cycle to Taenia saginata,. Cysticerci have three morphologically distinct types. The common one is the ordinary "cellulose" cysticercus which has a fluid filled bladder that is 0.5 cm to 1.5 cm in length and an invaginated scolex. The intermediate form has a scolex while the "racemose" has no evident scolex but are believed to be larger and much more dangerous. They are 20 cm in length and have 60 ml of fluid and 13% of patients might have all three types in the brain. Humans usually infect through eating infected pork, fostering adult tapeworms in the intestine, and passing eggs through feces, but autoinfection is also possible. In that case, a cysticercus (a larva sometimes called a "bladder worm") develops in the human and the human acts like an intermediate host. This happens if eggs get to the stomach, usually as a result of contaminated hands, but also due to retroperistalsis. Cysticerci often occur in the central nervous system, which can cause major neurological problems like hydrocephalus, paraplegy, meningitis, convulsions and even death. The condition of having cysticerci in one's body is called cysticercosis.

Eggs can be diagnosed only to the family (biology) level, but if a proglottid's uterus is stained with India ink, the number of visible uterine branches can help identify the species: unlike the Taenia saginata uteri, T. solium uteri have only five to ten uterine branches on each side.

Infection with T. solium adults is treated with niclosamide, which is one of the most popular drugs for adult tapeworm infections, as well as for fluke infections. As cysticercosis is a major risk, it is important to wash one's hands before eating and to suppress vomiting if a patient may be infected with T. solium. If neurocysticercosis occurs the drug of choice is either albendazole or praziquantel. These drugs damage the parasites skin internally causing it to disintegrate and is then removed by the host's immune system.

Infection may be prevented with proper disposal of human feces around pigs, cooking meat thoroughly, and/or freezing the meat at -10^oC for 5 days. Most cases occur because infected food handlers contaminate the food.

Life cycle

Life cycle of T. solium. Click the image to see it full-size

This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier. Pigs and humans become infected by ingesting eggs or gravid proglottids. Humans are infected either by ingestion of food contaminated with feces containing eggs, or by autoinfection. In the latter case, a human infected with adult T. solium can ingest eggs produced by that tapeworm, either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis. Once eggs are ingested, oncospheres hatch in the intestine, invade the intestinal wall, and migrate to striated muscles, as well as the brain, liver, and other tissues, where they develop into cysticerci. In humans, cysts can cause serious sequelae if they localize in the brain, resulting in neurocysticercosis. The parasite life cycle is completed, resulting in human tapeworm infection, when humans ingest undercooked pork containing cysticerci. Cysts evaginate and attach to the small intestine by their scolex. Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years.

Pathogenesis

Ingestion of T. solium eggs or proglottid rupture within the host intestine can cause larvae to migrate into host tissue and cause cysticercosis. This is the most frequent and severe disease caused by T. solium. In symptomatic cases, a wide spectrum of symptoms may be expressed including headaches, dizziness and occasional seizures. In more severe cases, dementia or hypertension due to perturbation of the normal circulation of cerebrospinal fluid can occur. The severity of cysticercosis depends on location, size and number of parasite larvae in tissues, as well as the host immune response. Other symptoms include sensory deficits, involuntary movements and brain system dysfunction. In children ocular location of cysts is more common than cystation in other locations of the body. If a person is heavily infected with T. solium, it can lead to neurocysticercosis which can lead to epilepsy, seizures, lesions in the brain, blindness and tumors like growths. This kind of patients will also show the low level of eosinophils when they run the blood test.

Diagnosis

Diagnosis requires biopsy of the infected tissue and examination of feces. T.solium eggs and proglottids found in feces diagnoses taeniasis and not cysticercosis. Cysticercosis is diagnosed primarily on confirming the prescence of hooks on the scolex of T.solium. Radiological test such as X-ray, CT scans and MRIs can also be used to detect diseases. X-rays are used to identify calcified larvae in the subcutaneous and muscle tissues and CT scans and MRI’s are used to find lesions in the brain.

Treatment

PZQ (praziquantel) is the drug of choice for the treatment of T. solium infection. For cysticercosis, one can be treated with Albendazole combining with steroid to reduce the inflammation. In some cases like tumors in the brain, treating with drugs is not enough and they need to be surgically removed as necessary.

Prevention and Control

The best way to control of getting tapeworms is by eating fully cooked pork. Moreover, high level of personal hygiene and prevention of fecal contamination with pig foods also play major roles in prevention of getting the parasites.

Epidemiology

Stained T. solium proglottid.

Taenia solium is found worldwide, however, it has shown to be more common in cosmopolitan areas. Because pigs are intermediate hosts of the parasite, completion of the life cycle occurs in regions where humans live in close contact with pigs and eat undercooked pork. Cysticercosis is often seen in areas where poor hygiene allows for contamination of food, soil or water supplies. Prevalence rates in the United States have shown that immigrants from Mexico, Central and South America and Southeast Asia account for most of the domestic cases of cysticercosis. Taeniasis and cysticercosis are very rare in predominantly Muslim countries, as Islam forbids the consumption of pork. It is important to note that human cysticercosis is acquired by ingesting T. solium eggs shed in the feces of a human T. solium tapeworm carrier via gravid proglottids, and thus can occur in populations that neither eat pork nor share environments with pigs, although, as stated, the completion of the life cycle can occur only where humans live in close contact with pigs and eat pork.

See also

• Cysticercosis cutis
• List of parasites

 This article incorporates public domain material from websites or documents of the Centers for Disease Control and Prevention.

External links

• [1] Taenia solium Genome Project - UNAM
• [Dooley, J.F. 1980.] Health precautions in Mexico. Letter to the editor. J.A.M.A 243:1524.
• [Flisser, A. 1988. Neurocysticercosis in Mexico. Parasitol. Today4:131-137.
• [Rabiela, M.T.,A. Rivas, and A.Flisser. 1989. Morphological types of Taenia solium cysticerci. Parasitol. Today5:357-359.
• [Siutto, Edda. 2000. Taenia solium disease in humans and pigs: an ancient parasitosis disease rooted in developing countries and emerging as a major health problem of global dimensions. Microbes and Infection: 1875-1890.
• [http://www.dpd.cdc.gov/dpdx/HTML/Cysticercosis.htm
• [http://www.stanford.edu/class/humbio103/ParaSites2001/taeniasis/solium2.html

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