Schistosoma mansoni: Resistance to reinfection in the rat

Abstract

Resistance to reinfection by Schistosoma mansoni cercariae in Sprague-Dawley rats was analyzed, using the liver perfusion assay. The effects of varying experimental parameters, e.g. dose of cercariae in primary or challenge infections, time interval between infections, and time of worm recovery by liver perfusion, on the level of resistance were determined. Primary infections with doses ranging from 50 to 1000 cercariae/rat induce a significant resistance to reinfection, from 40% to 80%. A primary infection with 10 cercariae/rat was without effect. Resistance is established at a time interval of 4 weeks between primary and challenge infections, and its persists for at least 12 weeks. Increasing the challenge-infection dose to 5000 cercariae/rat abolishes resistance to reinfection. It is concluded that resistance is not due to a nutritional limitation on worm survival (“athrepsia”). Therefore, once-infected rats produce toxic factors which limit challenge worm survival. Worm burdens in the liver were measured during a 2-month period in once- and twice-infected rats. We demonstrate that the time course of appearance of challenge worms in the liver is not significantly altered by the presence of primary-infection worms. Similarly, arrival of challenge worms does not influence the expulsion of primary-infection worms. Thus, resistance is the result of an absolute decrease of challenge-infection worms in twice-infected rats.