What are tapeworms? |

Definition

Tapeworms are flatworm members of the class Cestoda, with seven species known to infect humans with intestinal worms. Other species can create diseases in humans by forming cysts in body tissues.

Natural Habitat and Features

Tapeworms are parasitic flatworms (also known as cestodes),
with more than one thousand known species in worldwide distribution. The worms are
located in the intestinal tract of the final host, although a larval-stage disease
is also encountered in other body tissues. Tapeworms have a complicated life cycle
of intermediate and final hosts. In intermediate hosts the
cestode travels through the circulation system and forms a larval cyst within body
tissues of the host. When the final host eats the intermediate host, the cestode
develops into the mature worm in the intestine, where it absorbs nutrients through
its skin as it passes through the host’s intestine. Depending on the species, the
worms can range in size from less than one inch to more than 50 feet long.

The head of a tapeworm is called a scolex, and it usually has suckers that enable the worm to attach to the intestine; some species also have hooks. The body of the worm is referred to as the strobila, which is made up of segments called proglottids. Proglottids can produce thousands of eggs, which are released to pass from the host in the feces. Most tapeworms are hermaphroditic, and a single tapeworm can produce many thousands of viable eggs. Eventually, proglottids break off from the body of the tapeworm and also pass in the feces. Some tapeworms, such as the Taenia
species, may live for as long as twenty years, while Hymenolepsis nana (dwarf tapeworm) lives for one year only.

Tapeworms causing intestinal infestation in humans are usually known by the
name of the intermediate host. The worms that infest humans are T.
saginata (beef tapeworm), T. solium (pork tapeworm),
T. asiatica (earlier confused with T.
saginata), Diphyllobothrium latum (fish tapeworm),
Dipylidium caninum (dog tapeworm), H. diminuta (rat tapeworm), and H.
nana (dwarf tapeworm). The dwarf tapeworm is so named not only because
of its small size but also because the worm was thought to lack an intermediate
host.

Pathogenicity and Clinical Significance

Most tapeworm infestations in humans involve mature worms in the intestines and involve few symptoms, if any. When symptoms are noticed, they usually manifest as intestinal distress, such as nausea, loss of appetite, abdominal pain, weight loss, or diarrhea. Because of a lack of symptoms, the host can carry the tapeworm for many years without being aware he or she is doing so. Although most tapeworm infestation does not cause a visible problem for the host, a worm that grows very large may block the bile duct, the pancreatic duct, or the intestine.

Tapeworms are generally acquired from intermediate hosts by eating poorly cooked or raw beef, pork, or fish containing encysted larvae. A second route of transmission also involves intermediate hosts, whereby a person accidentally ingests the host through eating insects in dried foods such as grains and cereals (H. diminuta). Another route is more direct: children accidentally ingesting fleas or lice from dogs (D. caninum). Children also are most susceptible to acquiring H. nana.

A larval-stage infestation, which can be a much more serious disease, comes from directly ingesting tapeworm eggs, either from contaminated food and water or from self-infection with T. solium eggs by hosts who already carry pork tapeworm. In addition to T. solium, such larval-stage disease is caused by Spirometra species, T. multiceps, and Echinoccocus granulosus, E. multilocularis, and E. vogeli.

When eggs from these species are swallowed, they form larvae that move through
the blood to body tissues and then form cysts, using the human as an intermediate
host. This form of infestation has various names, depending on the species and
type of cyst involved. When the larvae come from T. solium, the
disease is known as cysticercosis. Such larval-stage
diseases can have serious health consequences, particularly if the cysts lodge in
the heart, eye, or central nervous system, including the brain. If the cysts
migrate to other organs within the body, they can disrupt the functioning of the
organ; in the eye they may lead to blindness. When cysts form in the central
nervous system, including the brain (neurocysticercosis), they can result in
headaches, seizures, meningitis, hydrocephalus, dementia, and death.

Drug Susceptibility

Most intestinal infestations are treated with praziquantel in a single oral dose. With treatment, there is a
good chance of ridding the body of the parasite altogether. After treatment, it is
possible that only part of the worm will be seen in the feces, as the host may
begin to break the worm down by digestion before it is excreted. Other possible
anthelmintic drugs include albendazole and niclosamide.

Cysticercosis and other larval-stage infestations are not necessarily treated
unless they involve the brain. When they are treated, praziquantel may also be
used, though in persons with neurocysticercosis, side effects may require
administration of corticosteroids and praziquantel. In some cases, surgery is
necessary to remove the cysts of larval disease.

Bibliography

Ash, Lawrence R., and Thomas C. Orihel. Atlas of Human Parasitology. 3d ed. Chicago: American Society of Clinical Pathologists, 1990. Short articles with very good photographs of both adult worms and eggs of all common cestode diseases.

Grove, David I. A History of Human Helminthology. Wallingford, England: CAB International, 1990. Contains chapters on a variety of helminthic diseases, with detailed chapters on the most common cestode diseases.

Icon Health. Tapeworms: A Medical Dictionary, Bibliography, and Annotated Research Guide to Internet References. San Diego, Calif.: Author, 2004. A helpful reference guide to tapeworms and tapeworm infection.

Roberts, Larry S., and John Janovy, Jr. Gerald D. Schmidt and Larry S. Roberts’ Foundations of Parasitology. 8th ed. Boston: McGraw-Hill, 2009. A graduate-level textbook covering all aspects of parasitology. The text is highly technical and intended for the informed reader. The book is well illustrated.

“Tapeworm.” In Johns Hopkins Family Health Book, edited by Michael Klag. New York: HarperCollins, 1999. Describes six types of tapeworm that infest humans as adult worms. Includes advice to patients on what to look for and when to call a doctor.

“Tapeworm Infestation.” In The American Medical Association Encyclopedia of Medicine, edited by Charles B. Clayman. New York: Random House, 1994. A discussion of the basic life cycle and clinical effects of pork, beef, fish, and dwarf tapeworms.