In Echinococcus granulosus, the fully developed metacestode is typically unilocular, subspherical in shape, fluid-filled and has the least complex structure of the four species (Figure 22.2). The rate of cyst development is slow and variable and dependent on a number of factors, including the strain of parasite, the species and strain of host and the intensity of infection. Cysts increase in diameter by 1-5 cm/year (Heath, 1973), whereas brood capsule formation may vary from a few months to years, and in humans and other 'abnormal' hosts it may not occur at all. The production of brood capsules and protoscoleces is not a factor of cyst size and appears to be dependent upon the nature of the host-parasite relationship. A cyst in which brood capsules and protoscoleces have developed is referred to as being 'fertile', whereas in a 'sterile' cyst they are absent. The life-span of hydatid cysts of E. granulosus can be as long as 16 years in horses (Roneus et al., 1982) and 53 years in humans (Spruance, 1974).
Histologically, a unilocular hydatid cyst of E. granulosus consists of an inner germinal or nucleated layer, supported externally by a tough, elastic, acellular laminated layer of variable thickness, surrounded by a host-produced fibrous or adventitial layer (Figures 22.2, 22.3). Typically, cyst growth is expansive by concentric enlargement, and asexual proliferation of the germinal layer from which brood capsule formation takes place entirely endogenously. Pouching of the cyst walls may give rise to secondary chambers, communicating with the central cavity (Vanek, 1980), and sometimes the central cavity may be partly separated from the secondary chambers by incomplete septa. Occasionally, cysts may abut and coalesce, forming groups or clusters of small cysts of variable size. In some hosts, particularly humans, where abnormally large cysts often develop, daughter cysts may form within the primary cyst (Figure 22.2).
The germinal layer is similar in structure to the metabolically active cellular covering (the tegument) of the adult worm. Undifferentiated cells in the germinal layer proliferate and form brood capsules, which originate as small buds that proliferate towards the cystic cavity (Figure 22.2). Brood capsules enlarge, vacuolate and become stalked. Within their lumen, a repetition of the asexual budding process takes place, leading to the production of numerous proto-scoleces. The formation of protoscoleces is asynchronous and a number of different developmental stages are usually present in a brood capsule at the same time. Fully developed protoscoleces are characterised by the possession of hooks on the invaginated rostellum.
The thin germinal layer is supported externally by the laminated layer. All species of Echinococcus are characterised by the possession of a laminated layer which, because it is periodic acid-Schiff (PAS)-positive (Figure 22.3; Kilejian et al., 1961), provides a useful diagnostic marker. It is a polysaccharide protein complex secreted by the germinal layer. The laminated layer assists in supporting the cyst and allows an often considerable intracystic tension to develop (Cameron
Fig. 22.3 Sections through cysts in human liver stained with haematoxylin and eosin (A) and periodic acid-Schiff (PAS; B, D). Sections (A) and (B) are both Echinococcus granulosus, and C and D both E. multilocularis. Note the preferential staining of the laminated layer, L, by PAS in (B) and (C), which makes it much easier to detect scattered exogenous vesicles of E. multilocularis (C). The germinal layer (arrow) gives rise to brood capsules, b, containing protoscoleces, P, in E. granulosus, and overlies the germinal layer, which in turn overlies the laminated layer, L. In E. granulosus, there is a characteristic fibrous advential layer, Ad separating the laminated layer from host tissue, whereas with E. multilocularis the proliferating parasite lesions are scattered within a dense mass of connective tissue. Figure produced by Russ Hobbs
Fig. 22.3 Sections through cysts in human liver stained with haematoxylin and eosin (A) and periodic acid-Schiff (PAS; B, D). Sections (A) and (B) are both Echinococcus granulosus, and C and D both E. multilocularis. Note the preferential staining of the laminated layer, L, by PAS in (B) and (C), which makes it much easier to detect scattered exogenous vesicles of E. multilocularis (C). The germinal layer (arrow) gives rise to brood capsules, b, containing protoscoleces, P, in E. granulosus, and overlies the germinal layer, which in turn overlies the laminated layer, L. In E. granulosus, there is a characteristic fibrous advential layer, Ad separating the laminated layer from host tissue, whereas with E. multilocularis the proliferating parasite lesions are scattered within a dense mass of connective tissue. Figure produced by Russ Hobbs and Webster, 1969; Slais, 1973). It may also protect the cyst from immunological attack by offering an immunologically inert barrier that can deny access to host defence cells (Coltorti and Varela-Diaz, 1974; Rogan and Richards, 1989; Leducq and Gabrion, 1992). Immunoglobulin, however, can pass through the laminated layer and the capacity to regulate penetration of macro-molecules into the cyst appears to be a function of the germinal rather than the laminated layer (Coltorti and Varela-Diaz, 1974).
The host fibrous capsule (adventitial layer), which typically surrounds fully developed, viable cysts of E. granulosus, is the product of a three-layered host cellular inflammatory reaction initiated in the early stages of post-oncospheral development (Figures 22.2, 22.3; Cameron and Webster, 1969; Smyth and Heath, 1970; Slais and Vanek, 1980). The initial intensity of this reaction varies between hosts and governs the fate of the developing metacestode. If too intense, it will cause the degeneration and eventual death of the parasite, whereas in suitable intermediate hosts the initial reaction resolves, leaving a fibrous capsule (Thompson and Lymbery, 1990). The latter situation is common where a stable hostparasite relationship has evolved, as appears to be the case, for example, with strains (species) adapted to sheep, cattle and horses.
lymph or blood is responsible for the distant metastatic foci that characterise the disease in humans (Ali-Khan et al., 1983; Eckert et al., 1983; Mehlhorn et al., 1983).
E. multilocularis develops rapidly in its natural intermediate rodent host, producing proto-scoleces in a few months, after which there is little if any further increase in size (Rausch, 1975; Rausch and Wilson, 1973). In humans, growth is very different and proliferation continues indefinitely, although there may be few if any protoscoleces produced (Rausch and Wilson, 1973). The larval mass proliferates peripherally and at the same time regressive changes occur centrally. Thus, a progressively enlarging mass of necrotic tissue with a relatively thin zone of viable proliferating parasite is produced. The term 'alveolar hydatid' is used to describe this form of growth, which is not a feature of the development in natural intermediate host species.
The metacestode of E. multilocularis has been found on a number of occasions in extraintestinal sites in dogs and cats (Geisel et al., 1990; Deplazes et al., 1997a; Losson and Coignoul, 1997). It is not known whether such infections resulted directly from the ingestion of eggs or indirectly by autoinfection as a result of a previously acquired worm burden, but they illustrate the unusual developmental potential of E. multilocularis.
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