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Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass being Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cestodaria. All tapeworms are Parasitism of vertebrates, living in the digestive tract or related ducts. Examples are the pork tapeworm ( Taenia solium) with a human definitive host, and pigs as the secondary host, and Moniezia expansa, the definitive hosts of which are .
The scolex, located at the anterior end, is a small (usually less than 1 mm) holdfast organ with specific systems for fastening itself to materials: rostrum, acetabula, suckers, bothria, grooves, and hooks. The small neck region, directly behind the scolex, consists of an undifferentiated tissue region of proglottid proliferation, leading into a zone of increasing and continuous proglottid differentiation. As such, the main and largest section of the body, the strobila, consists of a chain of increasingly mature proglottids. These cytology processes are not well understood at present.
Members of the Eucestoda have no mouth or digestive tract, and instead absorb nutrients through a layer of microtriches over the tegument at the shared body wall surface.Rohde, Klaus. "Eucestoda". AccessScience. McGraw-Hill Ryerson In addition to the body wall, several other systems are common to the whole length of the tapeworm, including excretory canals, nerve fibers, and longitudinal muscles. Encyclopedia of Parasitology The excretory system is responsible for osmoregulation and consists of blind-ending flame bulbs communicating through a duct system. The nervous system, often referred to as a "ladder system," is a system of longitudinal connectives and transverse ring commissures."AccessScience"
Egg formation is a result of copulation. A proglottid can copulate with itself, with other proglottids in the same worm, or with proglottids in other worms, and hypodermic fertilization sometimes occurs. When a gravid proglottid that is distended with an embryo reaches the end of the strobila, it detaches and passes out of the host intact with feces, with or without some tissue degeneration. In the order Pseudophyllidea, the uterus has a pore and the proglottid sheds the shelled embryo, only becoming detached when exhausted.
Some members of the Eucestoda (such as Echinococcus, Sparganosis, Taenia multiceps sp., and Mesocestoides sp.) can reproduce asexually through budding, which initiates a metagenesis of alternating sexually and asexually reproducing generations.
Except for members of the order Taeniidae, the first intermediate host is an arthropod, and except for in the case of Archigetes spp. (which can attain sexual maturity in freshwater oligochaeta), the second host is usually a fish, but can be another invertebrate or vertebrate. After the scolex has differentiated and matured in the larval stage, growth will stop until a vertebrate eats the intermediate host, and then the strobila develops. Adult tapeworms often have a high final host specificity, with some species only found in one host vertebrate.
| Diphyllobothrium latum, Spirometra erinacea euopaei, Taenia solium, Taenia saginata, Taenia asiatica, Mesocestoides leptothylacus (rarely), Rodentolepis (syn. Vampiro-, Hymenolepis) nana, Hymenolepis diminuta |
| Diphyllobothrium latum, Spirometra erinacea euopaei, T. pisiformis, Echinococcus multilocularis, Dipylidium caninum |
| Diphyllobothrium latum, Spirometra erinacea euopaei, T. hydatigena, Taenia ovis, T. pisiformis, T. multiceps, Taenia serialis, E. granulosus, E. multilocaris, Mesocestoides leptothylacus, Dipylidium caninum |
| Caryophyllaeus laticeps, Glaridacris catostomi, Archigetes sieboldi, Triaenophorus sp., Eubothrium sp., Rhinobothrium sp., Phyllobothrium sp., Proteocephalus ambloplites |
| Ligula intestinalis, Schistocephalus solidus |
| Schistocephalus solidus, Rodentolepis nana, Hymenolepis diminuta |
| Anoplocephala magna, Anoplocephala perfoliata |
| Moniezia expans, Avitellina sp., Thysaniezia sp., Stilesia sp. |
| Davainea proglottina, Raillietina tetragona, Amoebotaenia sp., Choanataenia sp., Hymenolepis carioca |
| Taenia ovis, T. multiceps, Taenia serialis, E. granulosus, E. multilocularis, Mesocestoides leptothylacus, Dipylidium caninum |
| Cittotaenia sp. |
At these sites, the parasites lodge and form cysts, a condition called cysticercosis, producing inflammatory reactions and clinical issues when they die, sometimes causing serious or fatal damage. In the eye, the parasites can cause visual loss, and infection of the spine and adjacent leptomeninges can cause paresthesias, pain, or paralysis.Bale, James F. "Cysticercosis". Current Treatment Options in Neurology. 2000. p. 355360
Eggs hatch in the gastrointestinal tract after the consumption of contaminated food, after which the larvae travel to the liver through portal circulation. Here, the larvae are trapped and usually develop into hydatid cysts. While the liver is the first filter for trapping them, the lungs act as the second filter site, trapping most of the larvae that are not trapped by the liver. Some larvae escape from the lungs to cause cysts in other tissues.
When a larva becomes established in tissue, it develops into a "bladderworm" or "hydatid" and can cause various cancer-like cysts that may rupture and interact with nearby organs. Most cases are asymptomatic, and the mortality rate is low, but various complications from these interactions may lead to debilitating illness.
While light infections are usually asymptomatic, autoinfection through eating the eggs of worms in the intestines is possible, and it can lead to hyperinfection. Humans can also become hyperinfected through ingesting grain products contaminated by infected insects. Infections involving more than two thousand worms can cause many different gastrointestinal symptoms and allergic responses. Common symptoms include chronic urticaria, skin eruption, and phlyctenular keratoconjunctivitis.Kim, Bong Jin, et al. "Heavy Hymenolepis nana Infection Possibly Through Organic Food: Report of a Case". The Korean Journal of Parasitology. 2014. pp. 85–87.
Clinical symptoms are due to the large size of the tapeworm, which often reaches a length exceeding . The most common symptom is pernicious anemia, caused by the absorption of vitamin B12 by the worm. Other symptoms include various intestinal issues, slight leukocytosis, and eosinophilia.
The most common symptom is a painful, slowly growing nodule in the subcutaneous tissues, which may migrate. Infection in the eye area can cause pain, irritation, edema, and excess watering. When the orbital tissues become infected, the swelling can cause blindness. An infected bowel may become perforated. Brain infection can cause , hematomas, and abscesses.
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