Population studies on Oncomelania quadrasi, the snail intermediate host of Schistosoma japonicum, in the Philippines. I. Distribution pattern of the snail in the field.

Distribution of Oncomelania quadrasi, the snail intermediate host of Schistosoma japonicum, was studied in Leyte, Philippines for the type of distribution based upon a large number of survey data. Among four distribution models examined, three uneven distribution models, i.e., the double Poisson, Neyman type A and negative binomial fitted well in this order but the Poisson model showed the poorest fitness. This indicates that the snails are distributed unevenly over the field and that the population mostly consists of small clusters of individual snails. Elucidation of the snail distribution pattern is necessary for determination of sample size in snail survey and for data transformation in significance test of the snail density comparison.