Death of schistosome cercariae during penetration of the skin

Abstract

A large percentage of the cercariae of Schistosoma mansoni die while penetrating the abdominal skin of rats or mice but relatively few cercariae die in the skin of hamsters. Comparison of hamsters and mice infected from the same pool of cercariae showed that the higher percentage of cercariae recovered as adult worms from hamsters than from mice was due to the lower percentage of cercariae which died during penetration of hamster skin. In similar comparisons of rats with mice the lower percentage of adult worms recovered from rats was not entirely accounted for by the higher percentage of cercariae which died in rat skin. Losses of schistosomula at later stages of development are apparently higher in rats than in mice. Cercariae die within 10 min after beginning to penetrate the skin of mice. At this stage the cercariae, which are now termed schistosomula, are attempting to penetrate the Malpighian layer of the epidermis. Five minutes later many schistosomula have entered the dermis. The cause of death of cercariae has not been determined. It is not related to the development of sensitivity to water which accompanies the transition from cercaria to schistosomulum. Sensitivity to water appears much sooner after penetration in some cercariae than in others; even after 30 min in the skin some cercariae can tolerate a return to water. In rats, hyper-immunized by a series of infections with cercariae through the abdominal skin, the percentage of challenging cercariae which die in the abdominal skin shows a slight increase over controls. No further mortality of schistosomula occurs during the next 24 h. Age resistance to infection with S. mansoni could not be demonstrated in comparisons of young mice (1 month) with old mice (1 year) of the Parkes and CBA strain. We should like to thank Dr F. Hawking for performing the skin biopsies on the dogs and Mrs P. Clark and Miss J. Marsh for their excellent technical assistance.