2.11 Cestodes

The Eucestoda (real tapeworms) live in the intestine of vertebrates, their definitive hosts (DHs), and are unique in propagating simultaneously by sexual and vegetative processes. As adults, they are highly host specific. The scolex supports attachment organs (long grooves: bothria, leaf-like structures: bothridia, suckers: acetabula) and ends in a germinating zone, which produces the strobila by budding proglottids (fig. 2.73, page 156). These parasites obtain their food by resorbtion via the body wall, a syncytic tegument. The hermaphroditic proglottids exhibit proterandry and, when gravid, are filled with eggs.

The hexacanth first larva (in the case of aquatic intermediate hosts (IHs) the coracidium; in the case of terrestrial hosts the oncosphera), develops in arthropods or vertebrates as far as the second larva ("metacestode or metacestoid"), which is a waiting stage. This reaches the DH either directly or via a second IH.

The second larva is primarily acystic and long-oval with sucking grooves (procercoid) or suckers (cercoscolex) on the anterior end of a cercomer showing posteriorly the 3 pairs of hooks of the oncosphera. Other forms are cystic with suckers, which may be evaginated but also redrawn (cysticercoid) or invaginated (cysticercus). Both types possess a cercomer. The plerocercus is cystic with fully developed suckers but without a cercomer. In cases in which the body is elongate (acystic) and solid with two sucking grooves, they are designated as plerocercoid; in cases with four evaginated suckers and a short strobila, they are termed strobilocercus (fig. 2.75 above, page 157).

Other more complex larval forms come into being by vegetative propagation of the second larvae from the cysticerci. Tetrathyridium: acystic with four invaginated suckers and vegetative splitting of the scolex. Coenurus: cysticercus with numerous invaginated scolices. Hydatid: cysticercus with inner and outer budding of secondary germ bladders, inside of which are invaginated free scolices. Polycercus: likewise but with evaginated free scolices. (fig. 2.75 below, page 157). The unlimited, even if slow, growth of the second larvae makes them pathogenic for the IHs, as they affect the fitness of the IH, conditioning them to become prey for their predators.

Pseudophyllidea: the scolex is long-oval with two elongated shallow slitform bothria. Male and female genital pores are separate, being situated laterally or median-ventrally; vitellaria are well developed. The egg lie within a thin sclerotin capsule, embryonating to coracidium, which hatches in water. IHs: aquatic primitive crustaceans, lacustrine Teleosts; DHs: fish-eating mammals, birds and fish other than elasmobranches (fig. 2.76 above, page 160). Diphyllobothrium latum, the human "broad" tapeworm. The DHs are man and piscivorous mammals. The first IHs are freshwater copepods (Cyclops spec., Diaptomus spec.), whereas the second IHs are fishes (Cyprinidae). In the copepods the eggs develop into procercoids, in the fishes into plerocercoids. Occasionally, a third paratenic IH is used, viz. predatory fishes which collect plerocercoids. If a procercoid is ingested with drinking water by man, it transforms to a sparganum, which is a plerocercoid migrating in the body cavity or muscles causing inflammations and edemas.Cyclophyllidea: rounded scolex with four suckers (acetabula); the rostellum is mostly armed with hooks. Both genital pores are united, mostly laterally. The strobila is proterandric. IHs are mostly terrestrial. The eggs are embryonated and there is normally no free-living stage (fig. 2.76 below, page 160). Hymenolepis diminuta: DHs are rats, other rodents and man; the rostellum has no hooks. IHs are insect larvae (which eat the eggs of the parasite) and their imagines (transfer cysticerci): fleas, Tenebrio and Tribolium. This parasite is used in laboratories throughout the world; it has a complex pattern of migratory behaviour within the intestine of rats during development and is diurnal with respect to feeding. Hymenolepis nana ("dwarf" tapeworm) DH: man and rodents. The rostellum is armed with a crown of hooks. Oncosphera can either develop into a cysticercoid in an insect IH or in the microvilli of the intestinal cells of the DH; the species is thus facultatively monoxenic.Dipylidium caninum: DHs are carnivores such as dogs, cats, foxes and occasionally children; the scolex has 3-4 crowns of hooks and four oval suckers; the genitalia are duplicated in each ellipsoid proglottid. IHs: fleas of dogs, cats and man; the eggs are eaten by the insect larva, whereas the cystcercoid together with imago is eaten by the DHs (fig. 2.77 above, page 161).Taenia saginata ("beef" tapeworm) DH: man; the scolex lacks hooks, the vagina has a sphincter muscle and the gravid proglottis has 14-32 lateral branches of the uterus; single proglottids migrate spontaneously, pressing out eggs. IHs: cattle and reindeer in which the parasite forms a cysticercus. Taenia solium ("pork" tapeworm). DH: man; the scolex has a crown of hooks and there are 7-11 lateral branches of the uterus, the vagina lacks a sphincter muscle; groups of gravid proglottids pass out passively. IHs: pig and wild boar in which the parasite forms a cysticercus. Self-infestation with eggs causes human cysticercosis.Taenia multiceps (syn. Multiceps multiceps). DHs: carnivores such as dogs and foxes, cosmopolitan distribution; the scolex has 2 crowns of hooks. IHs: ungulates, especially sheep, in which coenurus in the brain and spinal cord causes disorder of balance called "staggers" or "gid".Echinococcus granulosus. DHs: dog and other canids, spotted hyena. Prepatency 1-3 months; the scolex has a crown of hooks tightly inserted into the crypts of Lieberkühn in the gut mucosa inside of which they advance permanently (fig. 2.74, page 156); the strobila measures 2-11 mm; there are three proglottids, the last one being gravid with about 500 eggs; strobilae live 3-4 months, colonizing the intestine of the DHs in masses. IHs: ungulates and man; hydatid causes cystic echinococcosis. Further species are E. equinus (DH: dog, IH: horse); E. ortleppi (DH: dog, IH: cattle); E. vogeli (DH: wild canids and man); E. oligarthrus (DHs wild felids and man, IHs unknown).Echinococcus multilocularis. DHs: canids, red and polar fox, prepatency 5-7 weeks, strobila with five proglottids, measures 1-4 mm in length. IHs: rodents Microtus spec., Arvicola spec., muscrat Ondrata zibethica. Hydatid multicystic, with inner germinal layer and outer syncytium provided with microtriches; first protoscolices appear after 6 weeks and causes alveolar (multivesicular) echinococcosis also in man. Foxes contaminate urban areas with eggs. Mesocestoides lineatus. DHs: dog, fox, badger, marten and other carnivores. The scolex is clublike and the proglottids have a paruterine organ. IHs: unknown. Mesocestoides vogae, syn. M. corti: The tetrathyridium splits lengthwise developing new strobilae. Additional strobilae arise by by shoulder budding (fig. 2.75, page 157). These cestodes are only known as laboratory animals.Anoplocehalidae: Moniezia spec., Stilesia spec., Avittelina spec., Thysaniezia spec. DHs: herbivores such as ungulates, cervids, antilopes. Proglottids have a paruterine organ, forming encapsulated eggballs that contain eggs, each in a piriform apparatus. These are eaten by the IHs, oribatides (mites), after which the apparatus is digested and the egg develops in the haemocoel to form a cysticercoid. The mites contaminate pastureland (fig. 2.77 below, page 161). Development of cestodes in vitro is possible with protoscolices of Echinococcus granulosus obtained from hydatides by sterile techniques. They grow in vitro to typical fertile strobilae within 20 days. Hymenolepis diminuta and H. nana develop to form a fertile strobila within 15-29 days. Egg types: Taenia-type from proglottid with small vitellarium: egg with thick outer two-layered and inner membranous embryophore, which dissolves step by step during passage through the stomach and small intestine (fig. 2.76 below, page 160).

Dipylidium-type from proglottid with minute vitellarium: egg inside thin capsule; several eggs are united within an eggball, which is eaten by insects or mites and which dissolves inside their intestines (fig. 2.77 above, page 161).

Stilesia-type: genitalia without vitellarium develop dry-resistant eggs. Several eggs are collected in a paruterine organ as covered eggballs and are ejected by the proglottid (Thysaniezia spec., Stilesia spec.). In Moniezia spec., the eggs develop in a tissue network and are surrounded singly by a piriform apparatus. The eggs are eaten by oribatid mites, which contaminate the pastures of herbivorous DHs (fig. 2.77 below, page 161).
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