Strains cestodes

Mitchell, G.F., Goding, J.W. and Rickard, M.D. (1977) Studies on immune responses to larval cestodes in mice. Increased susceptibility of certain mouse strains and hypothymic mice to Taenia taeniaeformis and analysis of passive transfer of resistance with serum. Australian Journal of Experimental Biology and Medical Science 55, 165-1 86. 

Mitchell, G.F., Rajasekariah, G.R. and Rickard, M.D. (1 980) A mechanism to account for mouse strain variation in resistance to the larval cestode, Taenia taeniaeformis. Immunology 39, 481 t89. 

Another well-recognised complication in the study of cestode metabolism is the fact that a number of species (e.g. Echinococcus granulosus, Hymenolepis diminuta, Taenia crassiceps) have now been shown to exist as complexes of different strains, which may, often quite considerably, differ in their biochemistry. This important aspect is considered, in depth, in Chapters 5, 6 and 10. Furthermore, there is evidence that parasites from different host species or different strains of host show differences in metabolism, and the sex and circadian rhythm of the host can also influence the biochemistry of the parasite under study (59). 

Data on the chemical analysis of cestodes are of limited value unless the nutritional status of the host is known, as significant fluctuations in individual parasite components can occur. Furthermore, the chemical composition may vary with the strain of both parasite and host, the host species, the age of the cestode and its degree of maturation. This is partly illustrated in Table 4.1, which shows the variation in biochemical composition of Echinococcus granulosus obtained from different hosts. Some of the available data must also be accepted with caution on technical grounds, because a number of the older analytical methods have been shown, by more modern workers, to be unreliable. 

An important point which should be re-emphasised in relation to in vitro studies is the now substantial evidence that certain cestodes exist as a complex of intraspecific variants or strains (pp. 97-98). These strains may exhibit considerable quantitative and qualitative differences in carbohydrate metabolism, thereby complicating the interpretation of results. This particular aspect is elaborated on later. Furthermore, there is the additional problem of differing protocols used by independent research workers which can often make in vitro data comparisons difficult. 

Cestodes produce a range of end-products as a result of their respiratory metabolism (Table 5.4). Bryant Flockhart (104) have usefully divided the patterns of respiratory metabolism among parasitic helminths into three types. The metabolism of larval and adult cestodes fits broadly into the first two categories of this biochemical classification and these are illustrated in Fig. 5.4. Type 1 contains the homolactate fermenters in which carbohydrate is degraded, via glycolysis, to lactate and excreted. The ANU (Australian) strain of H. diminuta tends towards this type of metabolism (see below). 

The phenomenon of biochemical strain variation in helminths is proving of increasing interest to parasite biochemists and the area has been comprehensively reviewed by Bryant Flockhart (104). Probably the most remarkable examples of strain variation in cestodes occur in the hydatid organisms, Echinococcus spp. Accounts of the biochemistry and physiology (492) and the extent and significance of strain variation (501, 865) are available for this group. 

A major factor - and one which often introduces uncertainty into experimental work - is the influence of the species or strain of host or the cestode isolate used. This question clearly overlaps the whole question of host-specificity which may have a morphological, physiological or immunological basis. 

There are many examples of cestode species which, although capable of infecting different hosts, may either fail to remain established or develop at a different rate. The best-known example is probably H. diminuta, which becomes mature in rats in 12-19 days and normally survives for the life span of the host, at least in infections of 10 or fewer worms per rat (353). In contrast, in mice, H. diminuta is expelled within 7-14 days without reaching maturity. In most mice strains, growth ceases abruptly about day 10 and worms are expelled. This phenomenon has been much investigated (19). 

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