Cestoda (Cestoidea) is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebrates as adults, and often in the bodies of various animals as juveniles. Over a thousand species have been described, and all vertebrate species can be parasitised by at least one species of tapeworm. Several species parasitise humans after being consumed in underprepared meat such as pork (Taenia solium), beef (T. saginata), and fish (Diphyllobothrium spp.), or in food prepared in conditions of poor hygiene (Hymenolepis spp. or Echinococcus spp.).
T. saginata, the beef tapeworm, can grow up to 12 m (40 ft); the largest species, the whale tapeworm Polygonoporus giganticus, can grow to over 30 m (100 ft).
The worm's scolex ("head") attaches to the intestine of the definitive host. In some species, the scolex is dominated by bothria (tentacles), which are sometimes called "sucking grooves", and function like suction cups. Other species have hooks and suckers that aid in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex.
While the scolex is often the most distinctive part of an adult tapeworm, it is often unnoticed in a clinical setting as it is inside the patient. Thus, identifying eggs and proglottids in feces is important.
Body systemsThe main nerve centre of a cestode is a cerebral ganglion in its scolex. Motor and sensory innervation depends on the number and complexity of the scolex. Smaller nerves emanate from the commissures to supply the general body muscular and sensory ending. The cirrus and vagina are innervated, and sensory endings around the genital pore are more plentiful than other areas. Sensory function includes both tactoreception and chemoreception. Some nerves are only temporary.
ProglottidsThe body is composed of successive segments (proglottids). The sum of the proglottids is called a strobila, which is thin, and resembles a strip of tape. From this is derived the common name "tapeworm". Like some other flatworms, cestodes use flame cells (protonephridia), located in the proglottids, for excretion. Mature proglottids are released from the tapeworm's posterior end and leave the host in feces.
Because each proglottid contains the male and female reproductive structures, they can reproduce independently. Some biologists have suggested that each should be considered a single organism, and that the tapeworm is actually a colony of proglottids.
The layout of proglottids comes in two forms, craspedote, meaning proglottids are overlapped by the previous proglottid, and acraspedote which indicates a non-overlapping conjoined proglottid.
Once anchored to the host's intestinal wall, the tapeworm absorbs nutrients through its skin as the food being digested by the host flows past it and it begins to grow a long tail, with each segment containing an independent digestive system and reproductive tract. Older segments are pushed toward the tip of the tail as new segments are produced by the neckpiece. By the time a segment has reached the end of the tail, only the reproductive tract is left. It then drops off, carrying the tapeworm eggs to the next host, since, by that point, the proglottid is, in essence, a sac of eggs.
REPRODUCTION AND LIFE CYCLE OF THE TAPEWORMS
True tapeworms are exclusively hermaphrodites; they have both male and female reproductive systems in their bodies. The reproductive system includes one or many testes, cirrus, vas deferens and seminal vesicle as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs. There is a common external opening for both male and female reproductive systems, known as genital pore, which is situated at the surface opening of the cup-shaped atrium. Even though they are sexually hermaphroditic, self-fertilization is a rare phenomenon. In order to permit hybridization, cross-fertilization between two individuals is often practiced for reproduction. During copulation, the cirrus of one individual connects with that of the other through the genital pore, and then exchange their spermatozoa.
The life cycle of tapeworms is simple in the sense that there are no asexual phases as in other flatworms, but complicated in that at least one intermediate host is required as well as the definitive host. This life cycle pattern has been a crucial criterion for assessing evolution among Platyhelminthes. Many tapeworms have a two-phase life cycle with two types of host. The adult Taenia saginata lives in the gut of a primate such as a human. Proglottids leave the body through the anus and fall onto the ground, where they may be eaten with grass by animals such as cows. This is known as the intermediate host. The juvenile form migrates and establishes as a cyst in the intermediate hosts body tissues such as muscles, rather than the gut. They cause more damage to this host than it does to its definitive host. The parasite completes its life cycle when the intermediate host passes on the parasite to the definitive host, this is usually done by the definitive host eating an infective intermediate host, such as possibly a human with a preference for raw meat?in whose gut the adult Taenia establishes itself.
Tapeworm infestation is the infection of the digestive tract by adult
parasitic flatworms called cestodes or tapeworms. Live tapeworm larvae
(coenuri) are sometimes ingested by consuming undercooked food. Once
inside the digestive tract, a larva can grow into a very large adult
tapeworm. Additionally, many tapeworm larvae cause symptoms in an
intermediate host. For example, cysticercosis is a disease of humans
involving larval tapeworms in the human body.
Among the most common tapeworms in humans are the pork tapeworm (T.
solium), the beef tapeworm (T. saginata), the fish tapeworm
(Diphyllobothrium spp.), and the dwarf tapeworm (Hymenolepis spp.).
Infections involving the pork and beef tapeworms are also called
taeniasis. Tapeworms of the genus Echinococcus infect and cause the
most harm to intermediate hosts such as sheep and cattle. Infection
with this type of tapeworm is referred to as Echinococcosis or hydatid
disease. Symptoms vary widely, as do treatment options, and these
issues are discussed in detail in the individual articles on each worm.
With a few notable exceptions like the fish tapeworm, most cestodes
that infect humans and livestock are cyclophyllids, and can be
identified as such by the presence of four suckers on their scolex or
Most occurrences are found in areas which lack adequate sanitation and
include Southeast Asia, West Africa, and East Africa.
Although tapeworms in the intestine usually cause no symptoms, some
people experience upper abdominal discomfort, diarrhea, and loss of
appetite. Anemia may develop in people with the fish tapeworm.
Infection is generally recognized when the infected person passes
segments of proglottids in the stool (looks like white worms),
especially if a segment is moving.
Rarely, worms may cause obstruction of the intestine. And very rarely,
T. solium larvae can migrate to the brain causing severe headaches,
seizures and other neurological problems. This condition is called
neurocysticercosis. It can take years of development before the patient
has those symptoms of the brain.
Tapeworms are treated with medications taken by mouth, usually in a
single dose. The drug of choice for tapeworm infections is niclosamide.
Praziquantel and albendazole can also be used.
INGESTION OF EGGS
Tapeworm eggs are generally ingested through food, water or soil
contaminated with human or animal (host) feces. For example, if a pig
is infected with a tapeworm, it may pass eggs or segments (proglottids)
of the adult tapeworm through its feces into soil. Each segment
contains thousands of microscopic tapeworm eggs. These eggs can be
ingested via food contaminated with the feces. Once the eggs have been
ingested, they develop into larvae, which can migrate out of the
intestines and form cysts in other tissues such as the lungs or liver.
This type of infection is not common with beef or fish tapeworms, but
can occur with the pork tapeworm called cysticercosis and can also
occur with dog and sheep tapeworms called echinococcosis.
INGESTION OF LARVAE CYSTS IN MEAT OR MUSCLE TISSUE
Tapeworm infection can also be caused by eating raw or undercooked meat
from an animal or a fish that has the larval form of the tapeworm cysts
in its muscle tissue. Once ingested, the larvae then develop into adult
tapeworms in the intestines. Adult tapeworms can measure up to 50 feet
(15 m) long and can survive as long as 20 years. Some tapeworms attach
themselves to the walls of the intestine, where they cause irritation
or mild inflammation, while others may pass through to the stool and
exit the body. Unlike other tapeworms, the dwarf tapeworm can complete
its entire life cycle egg to larva to adult tapeworm in one host.
This is the most common tapeworm infection in the world and can be
transmitted between humans. Even while being treated for certain
tapeworm infections, reinfection can result from ingesting tapeworm
eggs shed by the adult worm into the stool, as a result of insufficient
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