Description Of The Organism Taxonomy

Since these early times, the taxonomic designation of the causative agents of echinococcosis has remained controversial, with numerous revisions (Rausch, 1967; Krotov, 1979; Kumaratilake and Thompson, 1982; Thompson and Lymbery, 1988; Thompson et al., 1994, 1995). Echinococcus belongs to the order Cyclophyllidea, which characteristically includes tapeworms that have four muscular suckers and hooks on their scolex for attachment to the mucosa of the definitive (final) host (Figure 22.1). Echinococcus is a member of the family Taeniidae, whose members all have indirect life-cycles, with two mammalian hosts and larval (metacestode) stages of the fluid-filled, 'bladder-worm' type. However, the hydatid metacestode of Echinococcus contains numerous scoleces as a result of asexual multiplication within the cyst. The degree of definitive host specificity is much greater than with the intermediate host, and is restricted to canids, vulpines and felids. In contrast, intermediate hosts include numerous species of herbivorous or omnivorous animals. Although a total of 16 species and 13 subspecies were originally described in the genus Echinococcus, subsequent taxonomic revisions recognised only four valid species, Echinococcus granulosus, E. multilocularis, E. oligarthrus and E. vogeli. Their characteristics are summarised in Table 22.1.

Taxonomic uncertainty has been largely due to the limitations of morphological descriptions and lack of evidence for geographical or ecological segregation (reviewed in Kumaratilake and Thompson, 1982; Thompson and Lymbery,

1988; Thompson and Allsopp, 1988; Eckert and Thompson, 1988; Thompson et al., 1995).

The situation has been exacerbated by a lack of appreciation of the extent and significance of variability in Echinococcus. As a result, certain features that characterized a particular population were overlooked because of uncertainty regarding its taxonomic status. It is now clear that many of the populations previously given taxonomic status do exhibit strongly defined and distinct characteristics. Because of the epidemio-logical significance of the variation exhibited between populations of Echinococcus, particularly E. granulosus, such variant populations were designated informally as being different strains (Table 22.2) and there are clear morphological, behavioural and genetic characteristics by which most of them can be distinguished (Thompson et al., 1995). The situation with E. multilocularis is not as clear-cut and, although there is increasing evidence of variability in a range of behavioural and other phenotypic characteristics between geographically separated populations, compared to E. granulosus, there is little evidence of genetic distinctness between populations of E. multilocularis (Thompson and Lymbery, 1988; Thompson et al., 1994; Haag et al., 1997). However, both mitochondrial and rDNA sequencing of isolates of E. multilocularis from Europe, North America and Japan have confirmed the genetic distinctness of Eurasian and North American 'strains' of E. multilocularis (Bowles et al., 1992; Rinder et al., 1997).

Recent, comprehensive molecular genetic and phylogenetic analyses of Echinococcus populations based on sequence data from the mitochondrial cytochrome c oxidase subunit I (COI) and NADH dehydrogenase 1 (ND1) genes, and the nuclear rDNA internal transcribed spacer 1 (ITS1) (Bowles et al., 1992, 1995; Bowles and McManus, 1993a,b; Lymbery, 1995; Thompson et al., 1995; Lymbery and Thompson, 1996) has revealed that many of these strains most likely represent distinct species, and the reinstatement of their formal taxonomic status has been proposed (Thompson et al., 1995; Lymbery and Thompson, 1996). As for any infectious disease, an evolutionarily sound species-level classification for the genus Echinococcus is essential for the control of hydatid disease. Molecular genetic studies have therefore laid the foundation for a

Definitive host

Sexually mature adult worm between villi with scolex attached in crypt of Lieberkulm

Evaginated pmloscoiex actively descends between intestinal villi

Detached gravid proglottid containing eggs voided in faeces

Invaginated protcscolex ingested by definitive host

Resistant egg containing one hos pile re ingested by, intermediate host

Intermediate host

Egg hatches in stomach and released onchospherc is activated in small intestine

Oncosphere penetrates mucosa and enters circulation

Metacestode (hydatid cyst)

develops in viscera; protoscoleces produced by asexual multiplication

Fig. 22.1 Life-cycle and development of Echinococcus. Redrawn and designed by Russ Hobbs after Thompson, 1995

Table 22.1 Characteristics of the four species currently recognised within the genus Echinococcus

Geographical distribution

Host range

Definitive hosts

Intermediate hosts

Metacestode Nature of cyst

Location of cysts

Adult

Mean number of segments (range)

Total length of strobila (mm)

E. granulosus

Cosmopolitan

Primarily canids

Primarily ungulates, also marsupials and primates, including humans

Unilocular, endogenous proliferation, no infiltration or metastasis

Visceral, primarily liver and lungs

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