A review of the taxonomy and speciation of the genus Echinococcus Rudolphi 1801.

The taxonomy of Echinococcus is reviewed and consideration given to speciation within the genus. The morphological and other biological characteristics of all species and subspecies so far described are given, and evidence for the existence of other intraspecific variants is presented. A total of 16 species have been described but only four have well-defined characteristics and are generally accepted as being taxonomically valid; E. granulosus, E. multilocularis, E. oligarthrus and E. vogeli. The taxonomic status of intraspecific variants is uncertain. Some authorities consider that the majority of described subspecies are invalid taxonomically. However, concern is expressed that previous taxonomic considerations of Echinococcus have not taken into account the nature of its reproductive mechanism which, like that of most cestodes, defies the application of traditional taxonomic concepts and definitions derived from dioecious cross-fertilising organisms. At the present time intraspecific variants of Echinococcus are referred to as strains. The existence of such strains, particularly of E. granulosus, in different parts of the world is firmly established. Their importance in the epidemiology of hydatidosis has high-lighted the needed to establish criteria for their differentiation and characterisation. The limitations of morphology as a sole differential criterion are emphasised. Attention is given to recent studies on strain variation which have demonstrated the value of applying several different criteria in which the developmental, physiological and biochemical characteristics of different strains are compared in conjunction with morphological investigations.