Summary and Conclusions

Female albino mice were re-exposed to cercariae of Schistosoma mansoni at intervals of one hour, five hours, one day, two days, one week, one month and two months after an original exposure, together with previously unexposed control mice. Exposures were to 50 cercariae per mouse by tail for one hour. For all except the longest interexposure interval the parasites of the initial and challenging infections were tagged by their sex, and the mice were initially exposed at the age of about six weeks. When the interval between exposures was two months, each exposure was to both male and female cercariae, and the initial infection was presented to the mice when they were one week old. In all studies autopsies were performed six weeks after the second exposure.

The following conclusions have been drawn:

• 1.  The percentage of schistosomes maturing from the challenging exposure in the superinfected mice was significantly reduced below that in the control mice when the interexposure interval was one hour, five hours, one day and two months.
• 2.  On the other hand, the number of adult schistosomes maturing in the superinfected mice from challenging exposures presented two days, one week and one month after the initial exposure was essentially equal to that from specific control exposures.
• 3.  This reduction was the result of a local inhibition of penetration by the cercariae of the challenging exposure when the second exposure followed the first by an interval of one hour, five hours and one day. Each exposure was to cercariae of one sex only.
• 4.  When the interexposure period was longer than one day, no significant inhibition of penetration was noted. No clue was found to the mechanism of reduction of the worm burden from the challenging exposure when it was presented two months after the original exposure.