Penetration and maturation of Schistosoma mansoni were compared in four strains of mice under controlled conditions, with the C3H as the reference strain. Differences were observed in (1) average numbers of cercariae penetrating, (2) percentage of individual mice penetrated by at least 90% of the available cercariae, (3) worm burdens, (4) maturation of penetrants, and (5) consistency of mice with reference to these characteristics. Criteria 2, 4 and 5 have not been evaluated in this way before.

Best average penetration (47%), greatest consistency, and highest percentage of samples with at least 90% of the cercariae penetrating were in mice of the C3H strain (85%). Of the other strains the Swiss albino strain most resembled strain C3H. Beige and hairless mice were comparatively resistant to penetration since only 60 and 30% respectively of these hosts had at least 90% of the cercariae penetrating. Hairless mice gave the best average worm recovery (23%) and parasite maturation (53%) and beige the poorest (15 and 34%). Mice of the Swiss and beige strains were highly variable in terms of parasite maturation.

The establishment of reproducible differences in infections of hosts at the sub-specific level provides basic information for investigation of factors which influence penetration and maturation and which are thus involved in host specificity and innate resistance in closely related animals.

The authors are grateful to Dr Margaret Deringer, National Cancer Institute, National Institutes of Health, for the hairless mice and to Mildred Walters, Naval Medical Research Institute, for assistance in the production of cercariae.