SCHISTOSOMA-JAPONICUM, ZOOPHILIC STRAIN, IN ONCOMELANIA-HUPENSIS-CHIUI AND ONCOMELANIA-HUPENSIS-FORMOSANA - MIRACIDIAL PENETRATION AND COMPARATIVE HISTOLOGY

Miracidial penetration and intramolluscan development of a zoophilic s train of Schistosoma japonicum were studied in the highly susceptible Oncomelania hupensis chiui and a highly resistant population of Oncome lania hupensis formosana. When exposed to 20 miracidia, an average of 3.6 miracidia penetrated O. h. chiui and 3.4 penetrated O. h. formosan a. The distribution of sporocysts in the 2 snail subspecies was simila r: 70.4% migrated to the cephalopedal sinus, 14.2% to the anterior vis cera, and 3.1-4.9% to the posterior viscera, tentacle, mantle collar, and foot. Tn O. h. chiui only 1 out of 118 sporocysts degenerated and there was no host cellular response. In O. h. formasana, all the sporo cysts survived for 12 hr, but by 5 days postinfection (DPI) all were d ead 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. japo nicum All of the O. h. formasana snails of 10 and 15 DPI showed pronou nced 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 i n response to degradation products released by the dead parasites.