Distribution Pattern of the Snail, Austropeplea ollula, in Relation to Cercarial Dermatitis

Study was made on the distribution pattern of Austropeplea ollula, a snail, as intermediate host of dermatitis-producing cercaria of avian schistosome. Distribution was analyzed for 3 populations of the host snail, i.e., 1) overwintering population found in rice stubbles in dried rice field, 2) large snail population which grew after overwintering in the rice stubbles, and 3) small snail population newly produced by the large snails in early summer. All the 3 populations were distributed not randomly but unevenly following the negative binomial distribution model. The degree of unevenness was large in the order of newly born > overwintering > large snail populations. Judging from the above distribution pattern and degree of unevenness, the over-wintering population seemed to leave the rice stubbles in the following spring, and to spread out to the periphery of the rice field after maturing and laying eggs around the central part of the paddy. Small snails born in autumn were thought to survive winter in dried rice fields by crawling into underground rice stubbles at watery places. Surface soil including more fine sand was proven to be moister than that with less fine sand.