Studies on the relationship between the survival of Schistosoma mansoni larvae in mice and the degree of resistance produced.

These studies were aimed at giving a clearer idea of the source of the antigens responsible for the induction of resistance by larval schistosomes. Firstly, the resistance induced by radiation-attenuated (20 krad.) infections was compared with that induced by the larval phase of unirradiated infections. The superiority of the irradiated larvae was demonstrated by infecting mice with 500 female cercariae which were either unirradiated or irradiated, and challenging them 3 weeks later with male cercariae. At perfusion, when primary and challenge worms could be distinguished by sex, significantly greater resistance was demonstrated in the groups vaccinated with irradiated larvae (56 cf. 31% and 48 cf. 26% in two experiments). Secondly, in order to establish how long immunizing larvae need to survive to stimulate optimum resistance, mice were either treated with schistosomulicidal drugs or had the site of infection excised at various intervals post-vaccination. Treatment or excision at up to 24 h did not result in significant resistance to challenge (6-16%). Similar treatments at 1 week did not prevent significant resistance developing (26-37%) but the levels of resistance were significantly lower than those manifest by vaccinated, untreated mice (51-71%). It is concluded that the immunizing larvae need to persist for between 1 and 2 weeks in order to stimulate optimum protection and that antigens released during protracted survival and/or death of the larvae in either the skin or the lungs are probably key factors in the induction of resistance.