In order to determine the infected or susceptible areas from Clonorchis sinensis, the author has, during the past one and a half years, investigated the geographical distribution of Bythiniae, covering 266 different towns and villages out of 398 in the whole prefecture, and come to the following conclusions:- 1. The area in which Bythiniae live is the south-western part of the prefecture, i. e., along the down-streams of the three largest rivers; the area is oblong, its diameter being about 60 k. m. from east to west and about 16 k. m. from south to north. Across this area runs the Sanyo Railway Line. The north, east and south boundaries are surrounded by a doubtful zone of about 4 k. m. in width, while the west boundary reaches to a slightly affected area of the neighbouring prefecture, Hiroshima.
The severely infected region, located nearly centrally, is a lowland of clayey soil along the shore of the Kojima Bay, with many canals in which the water flows very slowly or is rather stagnant. In these canals, there are many kinds of water plants growing abundantly, and there can be seen numberless fresh-water fish such as Pseudorasbora parva living. The most severely affected area lies within about 3 k. m. from the shore. In this district, it is not difficult, during the summer time, to catch several hundreds of Bythiniae by a few scoops of a net having a diameter of two-thirds of one meter. 2. The density of Bythiniae is nearly in inverse proportion to the distance from the severest center. On the contrary, it is nearly in direct proportion to the number of Clonorchis cysts in a second intermediate host, and to the number of cases of the fluke carriers, and furthermore to the extent of clayey soil, and accordingly to the activity of rush cultivation, the cultivation being made practically in clayey soil. The original paper contains the outline map, the explanation of which will be: Dotted………………doubtful zone
Lined………………slight or moderate zone
Black triangle……name of county
◎……………………city of Okayama