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Human tapeworms, Taenia spp.

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Taenia spp. (tapeworms) taxonomy and origin

Taenia solium lineage:
cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Acoelomata - Platyhelminthes - Cestoda - Eucestoda - Cyclophyllidea - Taeniidae - Taenia -
Human tapeworms
Taenia solium - pork tapeworm
Taenia saginata - beef tapeworm
Taenia asiatica - Asian tapeworm

Zoonotic tapeworms
Taenia crassiceps
Taenia ovis
Taenia taeniaeformis
Taenia hydatigena
Taenia multiceps
Taenia serialis
Taenia brauni

Taenid tapeworms are helminth endoparasites belonging to order Platyhelminthes (flatworms).

The Taenids are unique in having mammals as both definitive (harboring adult parasites) and intermediate (harboring larval parasites) hosts.
Their transmission and life cycles are linked to specific predator–prey assemblages: a carnivorous or omnivorous definitive host gets infected by eating its prey, an intermediate host. Switching among carnivorous definitive hosts (e.g., among canids, felids, hyaenids and humans) that have historically exploited common prey resources played a crucial role in Taenid speciation (formation of new biological species).

To answer the question how species of human-specific taenids arose, it was proposed initially that in early times of animal husbandry (not more than 10,000 years ago), tapeworms circulating between dogs and domesticated ruminants were the source of first Taenia that colonized humans. More recent phylogenetic studies based on molecular data had estimated age for divergence of human-specific T. saginata and T. asiatica at 0.78–1.71 million years ago which suggest that the parasites adapted to Homo sapiens much earlier than domestication started. Shift of human diet from herbivory to omnivory and, later, to facultative carnivory 2.0–2.5 million ago was necessary for the adaptation of the tapeworms to human host.

Maintenance of T. solium within populations of Homo may have further been facilitated by cannibalism in a unique "human–human cycle".

T. asiatica as a separate species was discovered relatively recently. It had been mistaken for Taenia saginata Goeze (1782) for more than 200 years and later was classified as subspecies of T. saginata. However, extensive epidemiological, molecular, comparative morphology and phylogenetic analyses have indicated the independent and specific status of T. asiatica (Eom & Rim 1993).

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Brief facts

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Developmental stages of Taenia spp. (Taenia solium, Taenia saginata, Taenia asiatica) and associated diseases.

Life cycles of all taenids are very similar. T. solium is distinguished by its ability to use humans not only as definitive hosts but also as intermediate hosts. Because of this, the species is most important within this group of parasites. There is currently no evidence that cysticerci of either T. saginata or T. asiatica can develop in humans.

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Life cycle diagram of human tapeworms

Taenia spp. (taenosis/cysticercosis) life cycle

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Pathways of animal cystisercosis

Human hosts may release several segments daily, each containing thousands of eggs, and a single infected agricultural worker can be responsible for epizootic outbreaks in cattle or swine. Notably, eggs may remain infective in the environment for several months under suitable conditions.

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Eradication and control

Although all tapeworms represent a considerable animal and human health burden, T. solium remains as a primary target for eradication world-wide because of its ability to cause cysticercosis in humans. Autoinfection (when individual parasitized by adult worm ingests eggs expelled from its own body and becomes ill from cysticercosis) is considered a common occurence. The following characteristics of human-specific tapeworms make them vulnerable to eradication:

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Desinfection

Note: Undercooked, smoked or pickled meat can be infective.

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Brief comparison of human tapeworms

Definitive host human human human
Characteristics T. solium T. asiatica T. saginata
Intermidiate host pig, human pig cattle
Larvae location muscle, fat, brain viscera, liver, spleen, lung and other internal organs muscles including heart (one of easier diagnosed site), tongue
Adult worm scolex double-crown of hooklets present, rostellum present hooklets absent, rostellum with 4 suckers present hooklets absent, rostellum absent, 4 suckers with small apical depression
Geographic distribution world-wide with "hot spots" in Mexico and several countries in Central America, South Asia, and sub--Saharan Africa Southeast Asia - Korea, Taiwan, Indonesia, Vietnam, China world-wide including Europe, Africa, Middle East, and Asia; most highly endemic Taenia in United States

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Treatment

Human cysticercosis

Because extraneural cyctocercosis is usually benign, in majority of cases no treatment is necessary. However, because a proportion of people with cysticercosis may also be T. solium tapeworm carriers it may be unavoidable. Neurocysticercosis is associated with substantial morbidity and mortality and often requires treatment in order to alleviate symptoms and improve quality of life of infected people. The management of NCC may involve:

Human taenasis

Both niclosamide (2 g orally in a single dose) and praziquantel (5 mg/kg orally in a single dose) are effective against the adult tapeworm. Praziquantel may reach serum levels high enough to affect brain cysts if the tapeworm carrier also has NCC, causing seizures or severe headaches. An elctrolyte-polyethylenglycol salt (EPS) purge is usually indicated short after the chemotherapy to expel the worm and avoid the potential risk of having proglottids or eggs returned to the stomach and digested, leading to internal autoinfection. It also greatly improves the recovery of tapeworm scolices and gravid proglottids which indicates cure as well as provides parasite material for species identification of the infecting worm.

Porcine cysticercosis

A veterinary benzimidazole and oxfendazole, is more than 95% effective in killing the cysts in the pig when given in a single dose of 30 mg/kg. Infested meat in oxfendazole-treated pigs needs at least eight weeks for all the cysts to degenerate and up to 12 weeks to achieve a clear, acceptable appearance of the pork for human consumption.

Bovine cysticercosis

Oxfendazole was developed for use as an anthelmintic for ruminants and is administered to cattle at an oral dose of 4.5 mg/kg.

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Other tapeworms that can affect humans

In humans domestic cats and dogs are potential sources of zoonotic cystocerosis and coenurosis infections and undercooked game meat can be source of zoonotic taenosis.

Species Primary intermediate hosts/Alternative Primary definitive host/Alternative Type of larval infection Larval tissue distribution Geographic distribution
T. crassiceps wild rodents / cats, humans foxes/canids (wolves, coyotes, dogs) cysticercus (can multiply by asexual budding) subcutaneous tissues, peritonealor pleural cavities Canada, northern United States
T. ovis dogs, wild carnivores / rare in cats sheep, goat cysticercus muscles world-wide
T. taeniaeformis rodents / very rare in humans felids and canids cysticercus liver, spleen world-wide
T. hydatigena sheep, goats, cattle, pigs, reindeer / rare in rabbits, rodents, humans dogs, wolves, coyotes, lynxes / rare in cats cysticercus liver, abdominal cavity attached to omentum and mesentery world-wide
T. multiceps sheep, goats, cattle / human dogs, wolves, foxes, jackals coenurus (2 - 6 cm in diameter) muscles, brain, spinal cord, eye some parts of Americas, Europe and Africa
T. serialis hares, rabbits / squirrels, other rodents, cats, human canids including dogs, coyotes, etc. coenurus (up to 5 cm in diameter) subcutaneous tissues, muscles, retroperitoneally North America, Europe, Africa
T. brauni rodents / human canids coenurus subcutaneous tissues, eye Africa

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References

Websites

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