Taeniasis

  Classification   History Clinical Presentation Transmission Reservoir/Vector/Incubation Period Morphology Life Cycle Diagnostic Tests Management and Therapy Epidemiology Country Information Public Health and Prevention Strategies

 

 

Taeniasis is a tapeworm (cestode) infection acquired by the ingestion of raw or undercooked meat of infected animals.  Although many species exist, two species, Taenia saginata and Taenia solium, cause pathology in humans.  T. saginata is associated with the ingestion of the worm's larval form found in infected beef while T. solium is associated with that of infected pork.

 

       

 

T. saginata is also commonly known as beef tapeworm.  T. solium is similarly referred to as pork tapeworm.

 

Both species are worldwide in distribution-- approximately 100 million cases of infection world-wide annually.  Approximately 50 million cases of these cases are T. saginata while the other 50 million are T. solium related. Incidence of infection of T.solium is also based on other factors such as religious inhibitions on eating pork, inspection of pork before distribution for human consumption, and high degrees of sanitation which limits exposure of the intermediate hosts (such as pigs and cattle) to human feces.

 

Classification

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Cestodes

Order: Cyclophyllidea

Family: Taeniidae

Genus: Taenia

Species: saginata/ solium

 

Other members of Taenia that exist but humans are dead end hosts include:

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History of Discovery

Researchers believe that about 2 million years ago, African hominids (our early ancestors), who scavenged for food or preyed on antelope and other bovids, were exposed to tapeworm colonization.  These worms were using hyena and large cats as definitive hosts and bovids as intermediate hosts. This occurred before the origin of modern humans and substantially earlier than the domestication of swine and cattle and the development of agriculture.  The conclusion was inferred from an examination of host and parasites evolutionary histories and from evidence for the rate of molecular evolution between species of Taenia.

 

Rather than humans' acquiring Taenia from cattle and pigs, researchers believe man gave tapeworms to these domestic animals, since the association between Taenia and hominids was established before the domestication of these food animals. It was not until about 10,000 years ago, with the development of agriculture, that cattle, swine, and companion carnivores became intermediate hosts. DNA analysis of the worms also suggest that prehumans acquired these tapeworms before cattle and swine.

 

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Clinical Presentation

People infected with adult taenia often are asymptomatic.  Infected people may become aware of infection by noticing proglottid segments of the tapeworm  in their feces. Symptoms of infection, if any, are general: nausea, intestinal upset, vague abdominal symptoms such as hunger pains, diarrhea and/or constipation, or chronic indigestion.  Increased eosinophils may be a sign of infection.

A more severe form of taeniasis, cystercercosis, can occur upon ingestion of T. solium eggs found in the feces of infected humans.  These eggs hatch in the small intestine and migrate to various tissues of the body and form cysts.  T. saginata rarely causes cystercercosis.

The pathology associated with cystercercosis depends on which organs are infected and the number of cysticerci.  An infection consisting of a few small cysticerci in the liver or muscles would likely result in no overt pathology and go unnoticed.  Those that form in voluntary muscle tend to be asymptomatic, but may cause myositis, with accompanying fever and eosinophilia.  On the other hand, a few cysticerci, if located in a particularly "sensitive" area of the body, might result in irreparable damage.  For instance, a cysticercus in the eye might lead to blindness, a cysticercus in the spinal cord could lead to paralysis, or a cysticercus in the brain (neurocysticercosis) could lead to traumatic neurological damage or epileptic seizures. For this reason, cysticerci gather more attention when they occur in the central nervous system or the eye rather than when they develop in voluntary muscles.

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Transmission

In humans, it is the ingestion of under-cooked beef (T. saginata) or pork (T. solium) containing the larval cysts.

Intermediate hosts, such as cows and pigs, are infected with the tapeworm when they come into contact with the worm's eggs located in the feces of infected humans. 

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Reservoir

In addition to humans, T. Saginata also persists and has a developmental stages in the cow.  T. solium persists and has developmental stages in pigs.

 

Vector

None

 

Incubation Period

It takes about 5 to 12 weeks for the worm to mature into adulthood in the human intestine. Usually only a single worm is present at at time. However, multiple worms have been known to inhabit the human body.   T. solium may survive up to 25 years or more.

 

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Morphology

T. saginata

Larvae - Cysticerci are approximately 7.5 - 10mm wide by 4 - 6 mm in length and are are found in the cow after ingestion of the worm eggs.

 

Adults - The adult tapeworms have an average length of ~ 5 meters, consisting of approximately 1000 proglottids, but may grow up to 25 meters in length. Thus the adult form of T. saginata may be larger than that of T. solium.

 tenia1.jpg (81012 bytes) The scolex has four suckers but no hooks, which is found on Taenia solium.  The scolex in this tapeworm is slightly larger than that of T. solium, at approximately 2mm in diameter.  Gravid proglottids are shed either in the feces or leave the anus on their own. When the proglottids reach the external environment desiccation occurs and the eggs are released when the proglottid bursts.

 

Eggs - similar  to T. solium eggs. (picture on right)

 

T. solium

 

Egg: spherical, 6-hooked tapeworm larvae with egg shell.

 

Larvae - These small cysticerci are approximately 6- 18mm wide by 4 - 6mm in length when found in the muscles or subcutaneous tissues of their intermediate host (generally, pigs). The cysticerci may however be found in other tissues such as those of the central nervous system where they may grow much larger, up to several cm in diameter.

 

Adults - The adult tapeworms have an average length of ~3 meters, but can grow up to 8 meters in length.  The worm contains between 800 and 1000 proglottids.  The gravid proglottids (right) is approximately 12mm long.  T. saginata has 15 to 20 branches on each side, while T. solium has 7 to 13.  taenia.jpg (18k)The scolex (left) in this tapeworm may be differentiated from T. saginata as it is equipped with a double crown at approx. 30 hooks.

 

Images obtained from:

www.medonline.com.br/med_ed/ med7/galeri7a.htm

www.ulb.ac.be/sciences/biodic/ EImCestodes.html

http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Taeniasis_il.asp?body=S-Z/Taeniasis/body_Taeniasis_il3.htm

research.amnh.org/exhibitions/ epidemic/taenia.htm

 

 

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Life Cycle

The life cycles of T. saginata and T. solium are very similar.  Life cycle differences include possible autoinfection route in T. solium, and the different intermediate hosts for each parasite.  

 

T. saginata

Definitive host (sexual reproduction): human.

Intermediate host: cattle

 

1.)Feces of humans contain the eggs of the tapeworm.

2.)Egg is ingested by cattle. 

3.)Eggs hatch to release hexacynth (six-hooked) larvae in small intestine.

4.)Larvae migrate through the intestinal and enters the blood, lymph system.

5.) Larvae is carried to tissues such as heart and other muscles to develop cysticercus.

6.) Man is infected by ingesting uncooked meat containing cysticercus.

7.) Once ingested, the scolex of parasite attaches to the intestinal wall and grow into a mature tapeworm which sheds eggs in the feces of the infected human.

 

T. solium

Definitive host: human.

Intermediate host: pig (accidental host: human)

 

1.) Man ingests the larvae by eating uncooked contaminated pork containing the larvae in cysticerci.

2.) Larvae develops into adult form (only in humans)-- the tapeworm.

3.) Tapeworm attaches to the intestinal lining of human and sheds its eggs in human feces.

4.) Pigs come into contact with human feces and ingest the tapeworm eggs that contain the eggs. 

5.) Eggs penetrate the small intestine of the pig, enter the hepatic portal vein, enter the general circulation

6.) Eggs migrate to the skeletal or cardiac muscle where they form cysticerci.

 

Auto-infection route: Sometimes an infected human may ingest the eggs found in its feces. If this is the case, then cysticerci may develop throughout the tissues of the  body, but particularly the brain as there is an affinity for the brain. Neurocysticercosis may result.

 

Summary of both routes (autoinfection not shown):

 

Life cycle of Taenia saginata & T. solium

http://www.dpd.cdc.gov/dpdx/HTML/Taeniasis.htm

 

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Diagnostic Tests

Taenia eggs and proglottids can be identified though microscopic identification. However, this technique is not possible during the first 3 months following infection, prior to the development of adult tapeworms. Microscopic examination of eggs does not distinguish between the two species. Species can be distinguished, however, by examining proglottids.  Also, immunological tests can detect the presence of cysticerci and improved imaging techniques such as CAT and MRI can be very useful in detecting cysticerci in various organs.

 

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Management and Therapy

The best way to prevent taeniasis is to make sure meat is cooked thoroughly.   Freezing to -5˚C for 4 days, -15˚C for 3 days, or -24˚C for 1 day kills the larvae as well.  As with most cestodes, treatment involves the use of Praziquantel.  Niclosamide is also effective.

 

For T. solium, extra care and hygiene (such as frequent hand washing) must be used to keep from ingesting eggs and developing cysticerci in the brain.  Surgery, praziquantel, and albendazole can be used to treat cysticerci.

 

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Epidemiology

The area most affected by taeniasis currently is Irian Jaya, Indonesia, the western half of New Guinea Island. In field surveys conducted in 2000 and 2001, researchers found that 5 (8.6%) of 58 local people and 7 (11%) of 64 local dogs living approximately 1 km from the local capital city, Wamena, in Jayawijaya District, harbored adult tapeworms and cysticerci of T. solium. Due to the prevalence of this tapeworm worldwide and increasing immigration and foreign travel, T. solium will likely continue to emerge as an important pathogen in the United States.

 

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Country Information

T. saginata infection is common in areas of the world where beef is commonly eaten and human sanitation is poor. It is commonly found throughout South America and in Africa but is found in North America as well.  T. saginata has worldwide distribution, but unlike T. solium infection with T. saginata is frequently encountered in the United States.

T. solium is more prevalent in poorer communities where humans live in close contact with pigs and eat undercooked pork, and is very rare in Muslim countries. Infection with T. solium is rarely encountered in the United States except areas of high immigration from Mexico, Latin America, the Iberian peninsula, the Slavic countries, Africa, India, Southeast Asia, and China.

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Public Health and Prevention Strategies

Photo of meat inspection.In the U.S., laws have been passed that requires meat inspection for cysticerci prior to meat being put on the market of human consumption. Routine serologic surveillance of cystercercosis and preventing cattle grazing in contaminated areas will help prevent worm spread. Adequate cooking of meat destroys the tapeworm larvae and will prevent infection by tapeworm. Good hygiene and hand washing after using the toilet will prevent self-infection in a person already infected with tapeworms in addition to contamination of foodstuffs by human feces.  Proper disposal of feces, to avoid contamination of food, soil, and water, is important as well.

 

Image: www.vdacs.state.va.us/ meat&poultry/  

 

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Useful Web Links

More information on Taeniasis:

http://www.tdh.state.tx.us/zoonosis/diseases/cystipam.pdf

http://www.cdc.gov/ncidod/dpd/parasites/taenia/default.htm

http://www.path.cam.ac.uk/~schisto/Tapes/Taenia.html

 

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References

Markell, Edward, David John, Wojciech Krotoski. Medical Parasitology. Philadelphia: W.B Saunders, 1999.

 

Warren, Kenneth. Immunology and Molecular Biology of Parasitic Infections. Boston: Blackwell Scientific, 1993.

 

Schantz PM. Taenia solium cysticercosis: an overview of global distribution and transmission. Chapter in Taenia Solium cysticercosis. From basic to clinical science. CABI Publishing 2002. pp. 63-74.

 

Townes JM, Hoffman CJ, Kohn MA. Neurocysticercosis in Oregon, 19952000. Emerg Infect Dis [serial online] 2004 March. 2004 May 25. Available from: http://www.cdc.gov/ncidod/EID/vol10no3/03-0542.htm

 

Wandra T, Ito A, Yamasaki H, Suroso T, Margono SS. Taenia solium systicercosis, Irian Jaya, Indonesia. Emerg Infect Dis [serial online] 2003 July. 2004 May 25. Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no7/02-0709.htm 

 

http://www.ars.usda.gov/is/AR/archive/may01/worms0501.htm

 

www.nlm.nih.gov

 

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Webpage constructed by Susanna Tan for Humbio103: Parasites and Pestilence, course taught by Dr. Scott Smith at Stanford University. 5/2004.