Schistosoma mansoni, NIH-SM-PR-2 strain, in susceptible and nonsusceptible stocks of Biomphalaria glabrata: comparative histology.

Of 10 inbred stocks of Biomphalaria glabrata examined, eight were susceptible to the NIH-Sm-PR-1 strain (PR-1) of Schistosoma mansoni, whereas only three were susceptible to the NIH-Sm-PR-2 strain (PR-2). By employing a histological ranking system based on parasite development and host tissue response, we have observed a range of nonsusceptibility among stocks nonsusceptible to PR-2. In three nonsusceptible stocks, sporocysts were rapidly surrounded by host amebocytes and were destroyed within a few days. However, in four other nonsusceptible stocks, sporocysts were surrounded by only a minimal host reaction and some sporocysts persisted for at least 3 weeks. Nondevelopment of sporocysts in these four stocks may result from biochemical unsuitability or a weak from of resistance in the host. Parasite development and host tissue response also varied among three stocks of snails that were susceptible to PR-2. In two susceptible stocks, most PR-2 sporocysts developed normally, and the small proportion that underwent degeneration elicited no host cellular response within the first week of infection. However, in a third susceptible stock, most sporocysts were surrounded by a minimal host reaction and very few appeared to develop normally.