The cestodes (tape worms) of the genus Echinococcus (family Taenidae) may give rise to infection -echinococcosis or hydatidosis in the human host. Echinococcal disease was apparently known to Hippocrates (460-370 BC), who described water-filled cysts in the liver of infected patients. The aetiology of the disease was, however, not identified until the seventeenth century. It was first in 1808 that Rudolphi used the term echinococcus to name the parasite. The word 'echinococcus' is derived from the Greek and means hedgehog berry. In the beginning of this century Deve described the asexual cycle of the worm and furthermore demonstrated that the scolex plays a major role both in cyst formation and parasitic life cycle.

The life cycle involves two mammalian hosts. Definitive hosts of the Northern Biotype are always carnivores, most often members of the dog (Canidae) or cat (Felidae) families. The animals harbour the adult tapeworm in the small intestine and become infected by ingesting the larval (metacestode) form of the parasite with intermediate host tissue.

The reindeer and tile elk (Alces alces) are the principal intermediate hosts in northern regions. In Eurasia, the range of the elk extends southward to about latitude 50°N. Intermediate hosts are mainly herbivorous, but also omnivorous mammals that become infected by ingesting eggs passed in the faeces of infected definite hosts, which may contaminate grass or other vegetation or drinking water. Humans are incidental intermediate hosts of the larval form (Figure 18.1).

Four species of Echinococcus are recognized, three of which cause distinctive forms of disease: Echinococcus granulosus (CHD), E. multilocularis (AHD), and E. vogeli (polycystic hydatid disease, PHD). Echinococcus oligarthrus, has so far been recognised only in South America where only a couple of cases of human infection have been reported to date (Lopera et al. 1989). The adult cestodes are small (2-10mm) and they consist of a scolex with four suckers and a double row of rostellar hooks and two to five proglottids. Taxonomically significant morphologic differences between adult Echinococcus species include the form of the strobila, the position of the genital pore in mature and gravid proglottids, the size of rostellar hooks, the number and distribution of testes and the form of the gravid uterus. The eggs are secreted into the faecal mass and pass into the environment via host faeces. Despite the fact that the definitive host may harbour many thousands of these small worms, infections are rarely symptomatic in this host (Schantz 1989).

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