Fig. 1. Diagrammatic representation of scolices in different orders of tapeworm.
Class of Platyhelminthes
All members of the class Cestoda live as parasites, are extremely dorsoventrally flattened, may reach a length of several meters in some species and thus are called tapeworms. In general, the adults inhabit the intestines of their hosts, being anchored to the intestinal wall by means of type-specific holdfast organs (Fig. 1). Their principal body organization corresponds to that of trematodes with the exception of the lack of an intestine; thus all nutrients have to be taken up through the syncytial tegument (Fig. 1, Fig. 3, Fig. 4). The ontogenesis of the cestoda proceeds in most species as metamorphosis employing different larval stages (Fig. 2). In relatively rare cases (e.g., Echinococcus spp.; Fig. 2), an alternation of different generations is involved in the life cycle; however, all tapeworm need an alternation of hosts (see Eucestoda/Table 1). The classification of the class Cestoda is far from being solved (cf. Classification); however, most systems accept two subclasses which are differentiated with respect to the number of larval hooks. The medically unimportant Cestodaria form 10 larval hooks and are thus described as decacanth, whereas the larvae of the Eucestoda have only six hooks (hexacanth).
Class: Cestoda
Subclass: Cestodaria
Order: Amphilinidea
Order: Gyrocotylidea
Subclass: Eucestoda
Order: Caryophyllidea
Order: Trypanorhyncha
Order: Spathebothriidea
Order: Pseudophyllidea
Family: Diphyllobothridae
Family: Schistocephalidae
Order: Lecanicephalidea
Order: Aporidea
Order: Tetraphyllidea
Order: Diphyllidea
Order: Litobothriidea
Order: Proteocephalata
Order: Nippotaeniidea
Order: Cyclophyllidea
Family: Dioecocestidae
Family: Hymenolepidae
Family: Taeniidae
Family: Mesocestoididae
Family: Dilepidiidae
Farnily: Davaineidae
Family: Anoplocephalidae
Family: Dipylidae
In cestodes a gut is lacking. Therefore, nutritional uptake has to be mediated completely by the tegument surface. Moreover, the tegument has to resist the attack of digestive enzymes and protect against the immune responses of the host. Tapeworms may be affected by the immune response of their host. Initially, destrobilized worms are not permanently damaged, but will recover if they are transplanted into nonimmune hosts. It has been assumed that inactivation of proteases and lipase occurs on the surface of the tegument.
The external tegumentary membrane covering the microtriches has a surface coat which is a highly dynamic structure. Using [3H]-galactose as a label it was shown that the surface coat had a turnover rate of about 6–8 h in Hymenolepis diminuta. Autoradiographic investigations revealed that the constituents of the surface coat are synthesized by the endoplasmic reticulum and Golgi complexes of the perikarya and that they are transported to the apical part of the tegument. Histochemical methods demonstrated the presence of carbohydrates and negatively charged compounds in the surface coat. Biochemical investigations revealed a variety of glycoconjugates in tegumental extracts of Hymenolepis diminuta and Spirometra mansonoides which are assumed to take part in the replenishment of the surface coat.
Extracted surface material and in vivo products of larvae of Taenia taeniaeformis have been shown to contain a sulfated glycoconjugate. Its oligosaccharide chains contain glucosamine, galactose, and glucose at proportions of 4:4:1. This glycoconjugate is secreted and able to activate the complement cascade in vitro. It is assumed that it lowers the complement level in the surroundings of the parasite and thus prevents activation of complement at the external tegumentary membrane.
Gold-labeled lectins have been used to localize, by light and electron microscope, terminal sugar residues of the surface coat of Hymenolepis nana, Hymenolepis microstoma and Hymenolepis diminuta (Fig. 3, Fig. 4). Light microscopic sections of Hymenolepis microstoma were labeled with lectin-gold conjugates. It was shown that the tegument binds wheatgerm agglutinin (WGA) and soybean agglutinin (SBA) strongly, but peanut agglutinin (PNA) and Concanavalin A less intensely. Electron microscopic investigations of Hymenolepis microstoma and H. nana demonstrated that WGA, succinylated WGA, SBA, Abrus precatorius agglutinin (APA), PNA and, to a lesser extent, Concanavalin A were preferentially bound to the spines of the microtriches, which indicated that the surface of these species had exposed N-acetylglucosamine, galactose, and perhaps glucose and/or mannose residues. Ulex europaeus (UEA) 1 and Dolichos biflorus agglutinin (DBA) were not adsorbed, which means that terminal L-fucose and N-acetylgalactosamine residues seem to be absent. Specificity was controlled by competition with the respective sugars or sugar derivatives. Lectin-gold particles adsorbed mainly to the surface of the electron-dense spines. Only a few particles were found at the proximal part of the microtriches. This binding pattern does not appear to be the result of capping, since similar results were obtained with worms which had been fixed with glutaraldehyde prior to incubation. The carbohydrate constituents of the surface coat seem to be of parasite origin and do not represent adherent intestinal mucus from the host. The latter readily adsorbed UEA 1- and DBA-gold particles which were not bound by the tapeworm surface.
H. nana inhabits the posterior part of the ileum of the mouse, whereas Hymenolepis microstoma is attached in the bile duct and extends into the intestine. Nevertheless, no differences were observed in the lectin-binding pattern of both species. However, the rat tapeworm, Hymenolepis diminuta, differs markedly from these two species. It is expelled by normal mice 8–13 days after infection by unknown mechanisms, but it is able to develop in immunodepressed mice. In this species lectins specific for N-acetylglucosamine and galactose are adsorbed only at the scolex and adjacent parts but not on the strobila. The same lectins are adsorbed by the whole tegumental surface of H. nana and Hymenolepis microstoma. The immune system of the mouse above all destroys the strobila of Hymenolepis diminuta and leaves the scolex region intact. Destrobilated worms may survive for at least a few days. Therefore, it is tempting to assume that the glycoconjugates which are present in this region might be responsible for the resistance to immune reactions of the host. A striking difference in the polysaccharide content of two species appears to be in accordance with this assumption. A polysaccharide which is present in larger amounts in a resistant species, Hymenolepis microstoma, appears only in traces in Hymenolepis diminuta which is eliminated by the mouse. No protein or uronic acids were demonstrated. Negative charges are due to acetyl groups.
Befus was able to demonstrate the C3 component of complement on the surface of Hymenolepis diminuta in the mouse, but found it inconsistently and only in small amounts in Hymenolepis microstoma. Contrary to these results, other groups found that only small amounts of C3 were deposited on the surface of Hymenolepis diminuta in vivo, whereas in vitro C3 was bound in large amounts. Mouse complement proved unable to lyse the tegumental membrane of the rat tapeworm. However, that and human sera are able to destroy the surface of the tegument of Hymenolepis diminuta in a few minutes. Therefore, the complement system of the mouse cannot be responsible for the elimination of Hymenolepis diminuta from its gut, and it appears more likely that this is effected by cellular mechanisms.
Taeniasis, Animals, Taeniasis, Man, Cysticercosis, Echinococcosis, Cestode Infections (overview).