Studies on Infection Route of Schistosoma japonicum to Intermediate Host, Oncomelnia nosophora

It has already been experimentally confirmed by many investigators that schistosoma miracidia hatched out in the water and invaded into the oncomelania snails through their body surface and developed up to cercaria in the snail tissues within about eleven weeks. Oncomelania snail is, however, ordinarily a kind of land-inhabiting snails which spend great part of their life time on the ground. Therefore, attack of schistosoma miracidia to the snail may be limited in an exclusive oppotunity of being submerged in the water by raining or irrigation. Miyagawa (1916) and Suzuki (1913) described an evidence that hatching process of schistosoma egg was inhibited in fecal substances of the host. There is no any reason to deny a possibility that the snail contact with unhatched schistosoma eggs on the ground in natural world. The author attempt an experiment to give orally Schistosoma japonicum eggs to the snail, Oncomelania nosophora, Yamanashi strain, and to follow up its migration and development in the snail tissues. First of all in this study, viability and hatchability of the eggs were tested under various conditions of media in which the eggs were incubated. As the results, it was demonstrated that distilled water of pH 7.6 was the most ideal medium for hatching of the eggs, but saline and succarose solution were not suitable for maintaining of the viability. On the other hand, the author pointed out an important and interesting fact that S. japonicum eggs placed on an agar plate which was kept in high moisture and constant temperature of 27℃ to 29℃ did not hatch throughout the period of ten days investigation and most of them could suvive for long time at least three days. The snails and eggs containing mature miracidia were placed together on the agar plate mentioned above for 15 minutes to 2 hours to bring them into contact each other. After this procedure the snails were isolated on filter paper disk and fed with rice powder. Number of the eggs remaining on the agar plate were counted. The snails were killed and histologically examined at designated intervals after the infection. The eggs were found to reduce in number which varied from seven to eighteen eggs per snail. In the histological examination of the snail, the eggs were first detected in the esophagus and stomach shortly after ingestion and then in the intestine and rectum within 4 hours. 12 to 48 hours the empty egg shell and hatched miracidia were found, some of which were going to penetrate into the wall of digestive canal or submucous connective tissues. Other miracidia were found, furthermore, to migrate into the liver, kidney and uterine. These invasion of miracidia produced sometimes infiltration of a small lymphocyte like cell and amoebocyte, and granuloma bilding around degenerated miracidia in the snail tissues. However, the author has never found any other larval forms of further developed stages, such as typical sporocyte and cercaria during the period of observation over 11 weeks.
著者は日本住血吸虫の中間宿主宮入貝O. nosophoraの感染経路に関する研究の一端として,日本住血吸虫卵を宮入貝に経口的に与え貝体内での虫卵の移行,発育を観察した.先ず色々のmedium日虫卵の生存と孵化に及ぼす影響を観察したが,その至適な水はpH7.6～6.8の蒸溜水である.アルカリに傾いた蒸溜水,食塩水,ブドー糖液では孵化が抑制されるが,濃度が高くなると同時に虫卵の生存に障害的に作用し,孵化能力を失う.日虫卵を寒天板に置く事によって虫卵は孵化せず,少くとも3日間生きたまゝ保存する事が出来る.甲府産の宮入貝を寒天上で日虫卵と接触させると好んで捕喰する.虫卵は食道,胃,小腸,直腸と下降し,12時間頃から消化管内で孵化し,48時間頃になると一部は繊毛を失なって消化管壁,中腸腺,腎腔その他の組織に侵入,胚細胞の分裂が見られる.その後77日まで観察したが,スポロチスト,セルカリアに発育したものは検出出来なかった.
熱帯医学 Tropical medicine 12(4). p221-235, 1971