[One more biological characteristic of coccidia in the genus Cryptosporidium (Sporozoa: Apicocomplexa)].

Suckling rats were used as a model host for our previous EM studies of the endogenous development of Cryptosporidium parvum isolated from spontaneously infected calves (Beĭer e. a., 1990; Beĭer, Sidorenko, 1990). In the course of repeated infections it was noticed that the oocysts discharged by the recipient host (rat) were obviously larger than those in the donor's (calf's) isolates. Keeping in mind the presumable taxonomic significance of coccidian oocysts as a constant and rather reliable tool for species discrimination we made a comparative quantitative and statistical analysis of the mean values of oocysts of C. parvum, originating from several sources (table 2): a random human isolate (N 1), several random isolates from spontaneously infected calves (N 2-5), isolates from calves (N 6-8) used as the infecting material for suckling rats, and fecal samples from the experimentally infected rats (N 9-15). The results obtained have shown that the oocysts discharged by rats (N 9-15) were larger that those of calf origin (N 6-8), with the differences being statistically significant with 95% confidence. Besides, within the same host (rat) at least two oocyst groups were distinguished (N 9-11 and 12-15, resp.) whose differences in mean values also appeared statistically significant. The larger oocysts displayed differences in morphology bearing distinct walls which were never observed either in the donor isolates or in the smaller population of the recipient oocysts. The established differences in oocyst dimensions lay presumably within the frames of the normal reaction of C. parvum and other Cryptosporidium species, due to biological peculiarities of unusual life cycles of these unique coccidia: their homoxenous (i.e. confined to one host body only) development is combined with polyxeny (i.e. a wide host specificity that involves the number of host species representing different environmental conditions). The parasite's ability to change its functional morphology and size may appear some kind of preadaption to the number of varying conditions met by these polyxenous coccidia. This and other relevant assumptions are discussed in the paper.