A comparative study of the life-histories of mammalian schistosomes

Available data in the literature pertaining to the life-history characteristics of all known species of mammalian schistosomes have been gathered, and correlations between such variables as length of pre-patent period, adult worm size, rate of progeny production and progeny size have been explored. Accommodation of the schistosome life-cycle to the constraints imposed by certain host characteristics such as life-expectancy and size is discussed. Of the 23 known species of mammalian schistosomes, 20 species apparently rely to a major extent on relatively large-bodied and long-lived mammals such as primates, ungulates and proboscideans for their transmission. Only 1 species, Schistosomatium douthitti, is exclusively dependent on rodents for its transmission. S. douthitti attains maturity within its definitive host faster than any other mammalian schistosome, and is the only species known to be capable of producing viable eggs by facultative parthenogenesis. For all species of mammalian schistosomes, adult worm size, as estimated by female length, is positively correlated with the number of uterine eggs contained within the female (r = 0.682). For the 7 species for which data exist, rate of egg production/worm pair/day is positively correlated with uterine egg counts (r = 0.873) and inversely correlated with egg length (r = -0.787) and miracidium length (r = -0.953). Length of the pre-patent period is positively correlated with egg length (r = 0.503). With respect to the molluscan host, the number of cercariae produced by snails is positively correlated with the shell size of the snail (r = 0.657). For the 5 species for which data exist, the rate of egg production is inversely correlated with shell size of the intermediate host (r = -0.955) and the common logarithm of the number of cercariae produced (r = -0.893). Comparisons between species suggest that exceptionally low rates of cercariae production in the intermediate host may be compensated for by rapid rates of egg production in the definitive host, implying a degree of integration in the schistosome life-cycle not previously appreciated. Most species of mammalian schistosomes have long-lived definitive hosts, and snail hosts capable of producing many cercariae; compensatory relationships are therefore less obvious in such species. Additional quantitative data on all aspects of schistosome life-histories, particularly rate and duration of egg production, are needed to confirm or refute the relationships discussed above.