Definitive host, development

Tapeworm. A parasitic intestinal cestode worm having a flattened, band-like form. The eggs of tapeworms are ingested by the intermediate host, they produce the larval stage in tissues. When the flesh of intermediate host is eaten, the larvae develop within the alimentary canal of the definitive host into adult tapeworms. 

With the exception of a few species with progenetic plerocercoids (e.g. Schistocephalus) it is only within the definitive host that cestode tissue develops in a strobilar direction, and it is self-evident that strobilar differentiation must be induced by factors present in the intestinal environment. Clearly, any number of known parameters of the intestine (Fig. 3.1, p. 37)... 

In the cyclophyllidean Mesocestoides corti, uniquely, an asexual development pattern occurs in the definitive host intestine. Both the recently ingested larva (a tetrathyridium) and the adult worm are capable of undergoing asexual multiplication. This is illustrated dramatically by an experiment in which 2000 tetrathyridia were fed to a dog and 45 days later some 15 000 worms were recovered on autopsy (739). This unusual development was first shown by Eckert et al. (192) and has since been studied by a number of workers (383, 607, 739, 871, 907). 

Fig. 11.5. Hymenolepis diminuta comparative development in the rat (the natural definitive host) and the mouse (a non-permissive host). In most mice, growth ceases abruptly on day 10+1 and destrobilisation commences, leaving only a scolex and a neck 0.5-2 mm in length. (Modified from Hopkins et at., 1972.)...

Kinetic analyses of the membrane-bound alkaline phosphatase activity of Hymenolepis diminuta (Cestoda Cyclophyllidea) in relation to development of the tapeworm in the definitive host. Journal of Cellular Biochemistry, 25 131-7. 

Development of Hymenolepis diminuta in its definitive host. In Biology of the tapeworm Hymenolepis diminuta, ed. H. P. Arai, pp. 357-423. Academic Press New York. 

In contrast to the protozoa, helminths are multicellular with complex reproductive systems and life cycles involving intermediate hosts for the development of larval stages and a definitive host for the adult form. Adults may be dioecious with separate sexes or hermaphroditic. Helminths are flatworms and flukes such as nematodes and tapeworms. They are common enteric pathogens and can be transferred by directly eating or drinking parasite eggs, which may exist in polluted meat or water. 

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