Schistosoma japonicum, zoophilic strain, in Oncomelania hupensis chiui and O. H. formosana: miracidial penetration and comparative histology.

Miracidial penetration and intramolluscan development of a zoophilic strain of Schistosoma japonicum were studied in the highly susceptible Oncomelania hupensis chiui and a highly resistant population of Oncomelania hupensis formosana. When exposed to 20 miracidia, an average of 3.6 miracidia penetrated O. h. chiui and 3.4 penetrated O. h. formosana. The distribution of sporocysts in the 2 snail subspecies was similar: 70.4% migrated to the cephalopedal sinus, 14.2% to the anterior viscera, and 3.1-4.9% to the posterior viscera, tentacle, mantle collar, and foot. In O. h. chiui only 1 out of 118 sporocysts degenerated and there was no host cellular response. In O. h. formosana, all the sporocysts survived for 12 hr, but by 5 days postinfection (DPI) all were dead and by 10 DPI they had disintegrated completely; the encapsulation was encountered in only 3 out of 78 sporocysts or 3 out of 23 snails, suggesting that it played a minor role in the defense against S. japonicum. All of the O. h. formosana snails of 10 and 15 DPI showed pronounced leukocytosis and elevated phagocytosis in the heart and increased cell debris in tissue spaces, which coincided with the disappearance of the parasites. It is suggested that these phenomena were provoked in response to degradation products released by the dead parasites.