Cestoda

Scolex of Taenia solium with hooks and bothria present.

The scolex, which attaches to the intestine of the definitive host, is often minute in comparison with the proglottids. It is typically a four-sided knob, armed with suckers or hooks or both.[2] In some species, the scolex is dominated by bothria, or "sucking grooves" that function like suction cups. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolices.[10] Other species have ruffled or leaflike scolices, and there may be other structures to aid attachment.[2]

In the larval stage the scolex is similarly shaped and is known as the protoscoleces.[11]

Circular and longitudinal muscles lie under the neodermis, beneath which further longitudinal, dorso-ventral and transverse muscles surround the central parenchyma. Protonephridial cells drain into the parenchyma. There are four longitudinal collection canals, two dorso-lateral and two ventro-lateral, running along the length of the worm, with a transverse canal linking the ventral ones at the posterior of each segment. When the proglottids begin to detach, these canals open to the exterior through the terminal segment.[2]

The main nerve centre of a cestode is a cerebral ganglion in its scolex. Nerves emanate from the ganglion to supply the general body muscular and sensory endings, with two lateral nerve cords running the length of the strobila.[2] The cirrus and vagina are innervated, and sensory endings around the genital pore are more plentiful than in other areas. Sensory function includes both tactoreception (touch) and chemoreception (smell or taste).[8]

Two proglottids of Taenia solium. This species has 7 to 13 branches of the uterus on each side (above and below in this micrograph).

Once anchored to the host's intestinal wall, tapeworms absorb nutrients through their surface as their food flows past them.[12] Cestodes are unable to synthesise lipids, which they use for reproduction, and are therefore entirely dependent on their hosts.[13]

The tapeworm body is composed of a series of segments called proglottids. These are produced from the neck by mitotic growth, which is followed by transverse constriction. The segments become larger and more mature as they are displaced backwards by newer segments.[2] Each proglottid contains an independent reproductive tract, and like some other flatworms, cestodes excrete waste through flame cells (protonephridia) located in the 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". Proglottids are continually being produced by the neck region of the scolex, as long as the scolex is attached and alive.[14]

Mature proglottids are essentially bags of eggs, each of which is infective to the proper intermediate host. They are released and leave the host in feces, or migrate outwards as independent motile proglottids.[14] The number of proglottids forming the tapeworm ranges from three to four thousand. Their layout comes in two forms: craspedote, meaning any given proglottid is overlapped by the previous proglottid, or acraspedote, indicating the proglottids do not overlap.[15]

Parasite fossils are rare, but recognizable clusters of cestode eggs, some with an operculum (lid) indicating that they had not erupted, one with a developing larva, have been discovered in fossil shark coprolites dating to the Permian, some 270 million years ago.[1][27]

The fossil Rugosusivitta, which was found in China at base of the Cambrian deposits in Yunnan [28] just above the Ediacaran-Cambrian border, has great similarities to present day Cestodians. If correct, this would be the earliest example of a Platyzoan and also one of the earliest bilaterian body-fossils and might thus provide an insight to the living mode of Cestodians before they became specialized parasites.

The position of the Cestoda within the Platyhelminthes and other Spiralian phyla based on genomic analysis is shown in the phylogenetic tree. The non-parasitic flatworms, traditionally grouped as the "Turbellaria", are paraphyletic, as the parasitic Neodermata including the Cestoda arose within that grouping. The approximate times when major groups first appeared is shown in millions of years ago.[29][30]

Gyrocotylidea: body flatwormlike, not divided into proglottids

Amphilinidea: body wormlike, not divided into proglottids

"Tetraphyllidea": elaborate four-leaved scolex

The evolutionary history of the Cestoda has been studied using ribosomal RNA, mitochondrial and other DNA, and morphological analysis and continues to be revised. "Tetraphyllidea" is seen to be paraphyletic; "Pseudophyllidea" has been broken up into two orders, Bothriocephalidea and Diphyllobothriidea.[31][32][33] Hosts, whose phylogeny often mirrors that of the parasites (Fahrenholz's rule), are indicated in italics and parentheses, the life-cycle sequence (where known) shown by arrows as (intermediate host₁ [→ intermediate host₂] → definitive host). Alternatives, generally for different species within an order, are shown in square brackets.[31][32][33]

The Taeniidae, including species such as the pork tapeworm and the beef tapeworm that often infect humans, may be the most basal of the 12 orders of the Cyclophyllidea.[34]

Like other species of mammal, humans can become infected with tapeworms. There may be few or no symptoms, and the first indication of the infection may be the presence of one or more proglottids in the stools. The proglottids appear as flat, rectangular, whitish objects about the size of a grain of rice, which may change size or move about. Bodily symptoms which are sometimes present include abdominal pain, nausea, diarrhea, increased appetite and weight loss.[36]

There are several classes of anthelminthic drugs, some effective against many kinds of parasite, others more specific; these can be used both preventatively[37] and to treat infections.[38] For example, praziquantel is an effective treatment for tapeworm infection, and is preferred over the older niclosamide.[39] While accidental tapeworm infections in developed countries are quite rare, such infections are more likely to occur in countries with poor sanitation facilities or where food hygiene standards are low.[36]

In ancient Greece, the comic playwright Aristophanes and philosopher Aristotle described the lumps that form during cysticercosis as "hailstones".[40] In Medieval times, in The Canon of Medicine, completed in 1025, the Persian physician Avicenna recorded parasites including tapeworms.[40] In the Early Modern period, Francesco Redi described and illustrated many parasites, and was the first to identify the cysts of Echinococcus granulosus seen in dogs and sheep as parasitic in origin; a century later, in 1760, Peter Simon Pallas correctly suggested that these were the larvae of tapeworms.[40]

Tapeworms have occasionally appeared in fiction. Peter Marren and Richard Mabey in Bugs Britannica write that Irvine Welsh's sociopathic policeman in his 1998 novel Filth owns a talking tapeworm, which they call "the most attractive character in the novel"; it becomes the policeman's alter ego and better self.[35] Mira Grant's 2013 novel Parasite envisages a world where people's immune systems are maintained by genetically engineered tapeworms.[41] Tapeworms are prominently mentioned in the System of a Down song "Needles": their inclusion within the song result in a lyrical dispute among band members.[42]

There are unproven claims that, around 1900, tapeworm eggs were marketed to the public as slimming tablets.[43] A full-page coloured image, purportedly from a women's magazine of that period, reads "Fat: the enemy ... that is banished! How? With sanitized tape worms. Jar packed. No ill effects!"[35] When television presenter Michael Mosley deliberately infected himself with tapeworms he gained weight due to increased appetite.[44] Dieters still sometimes risk intentional infection, evidenced by a 2013 warning on American television.[45]