Short report. Schistosoma mansoni miracidia are killed by the defense system of an Argentine strain of Biomphalaria straminea

Biomphalaria straminea snails from Argentina fail to shed cercariae even if exposed to high doses of Schistosoma mansoni EC miracidia. Alternative exp lanations for this failure are that miracidia are unable to penetrate the s nail's epithelium or that the miracidia are killed by the snail's defense s ystem. To discriminate between these 2 possibilities, B. straminea snails w ere individually exposed to increasing doses of miracidia. Susceptible B. g labrata were used as controls. Exposed snails were fixed 12 hr after exposu re, and histological sections of the whole specimens were examined. Miracid ia were seen to penetrate the epithelium of B. straminea and B. glabrata at similar rates (14.7%), independent of the exposure level. Regardless of th e miracidial close, 94% of the penetrating miracidia appeared encapsulated by the B. straminea defense system, whereas in B. glabrata, only 42% of the miracidia underwent encapsulation. These results show that resistance of B . straminea to S. mansoni EC strain is due to an efficient defense system t hat destroys miracidia once they have penetrated.