Resistance of mice to secondary infection with Schistosoma mansoni. II. Evidence for a correlation between egg deposition and work elimination.

Mice reinfected with Schistosoma mansoni 6--8 weeks after a primary infection largely or completely eliminated the second infection prior to the 7-week adult worm stage. In contrast, challenge worm counts were not lower than controls at the 6-day lung schistosomulum stage. At reinfection intervals of 12 or more weeks, worm counts were reduced at both stages. The reduction in lung schistosomulum count was proportional to the number of schistosome eggs present in the lungs, with no significant reduction being detected at any challenge time in mice free of lung eggs. Isolated schistosome eggs injected intravenously into the lungs of normal mice induced moderate to high levels of resistance to infection, while eggs injected subcutaneously or imtraperitoneally did not. It is concluded that the deposition of schistosome eggs in sites encountered by migrating schistosomula may be essential for mice to become resistant to reinfection with S. mansoni.