Oncosphere activation

The hatching process has been most thoroughly studied in the Hymenolepididae and has been described for H. diminuta (Fig. 7.14) (331, 439, 441, 906) H. microstoma, H. citelli, and H. nana (204, 796). In the natural host, the factors which stimulate oncosphere activity are not clear. 

Benitez, L., Carate, T., Harrison, L.J., Kirkham, P., Brookes, S.M. and Rarkhouse, R.M. (1996) Cloning and sequencing of the gene encoding the principal 18-kDa secreted antigen of activated oncospheres of Taenia saginata. Molecular and Biochemical Parasitology 78, 265-268. 

Taeniid cestodes have a prey-predator life cycle involving two mammalian hosts. The definitive host is a carnivore or omnivore, which harbours the adult tapeworm parasite in the small intestine. Mature infective eggs are released with the faeces and, when these are ingested by a suitable species of intermediate host, the oncosphere contained within the egg is liberated through the influence of intestinal secretions, particularly bile. The activated parasite penetrates the mucosa of the small intestine... 

Early studies with T. taeniaeformis in mice (Miller and Gardiner, 1932 Campbell, 1938b Leid and Williams, 1974 Musoke et al., 1975 Mitchell et al., 1977, 1980), T. pisiformis in rabbits (Campbell, 1938a), T. saginata in cattle (Lloyd and Soulsby, 1976) and T. ovis in sheep (Blundell et al., 1968 Rickard et al., 1977 Heath et al., 1979 Harrison et al., 1993) demonstrated that antibodies in the sera of infected or immunized hosts were capable of transferring passive immunity to na ive hosts. The presence and activities of these protective antibodies were found to be evident in vitro through the demonstration that activated oncospheres were killed if cultured in the presence of serum from infected or vaccinated... 

Rickard, M.D. and Adolph, A.J. (1977) Vaccination of lambs against infection with Taenia ovis using antigens collected during short-term in vitro incubation of activated T. ovis oncospheres. Parasitology 75, 183-188. 

The egg and reproductive system thus present an elegant model for the study of a number of areas of physiological interest, especially the ultrastructure and cytochemistry of the spermatozoa, ova and embryonic envelopes (see p. 166), as well as the physiological processes involved in egg hatching and the subsequent activation of the released oncosphere. 

Hatching of the cyclophyllidean egg involves two processes (a) the rupture of the egg shell (when present) and the passive digestion and/or disruption of the embryophore, (b) activation of the hexacanth embryo to become motile and rupture its enclosing oncospheral membrane. These processes frequently require different stimuli for their initiation. The hatching process has been reviewed by Lethbridge (442), and Ubelaker (888). 

Rather surprisingly, it was found (16), that the presence of glucose in the maintenance saline had no effect on survival (Figs. 8.19), which suggests that (like some trematode cercariae) the larvae are non-feeding during the hexacanth penetration phase. It is possible that the appropriate systems for active transport or diffusion do not develop until after penetration. Since so little is known regarding the metabolism of the oncosphere (see Chapter 7) it is not profitable to speculate further on this question without more experimental data. 

The following species of Taenia have been grown from oncospheres to active larvae with suckers T. hydatigena, T. ovis, T. pisiformis, T. serialis and E. granulosus (308, 314). The techniques of Heath Smyth (314), or modifications of them, have been used widely by other workers for the production and collection of E/S antigens of oncospheres in the preparation of potential vaccines (see Chapter 11, p. 302). Only limited success has been achieved with the culture of larval T. saginata (310, 471) and almost none with T. solium, and the in vitro culture of these important species remains a challenge. 

Halawa, B. Jakacka, B. (1977a). [Proteolytic activity of Taenia saginata oncospheres examined by the isotope method.] In Polish. Wiadomosci Parazytologiczne, 23 363-7. [HA/48/3531]... 

---