Distribution of the snail Biomphalaria glabrata, intermediate host of Schistosoma mansoni, within a St Lucian field habitat.

A total of 6360 mud samples were obtained, in 62 collections made with an exhaustive sampling device, from banana drains on the West Indian island of St Lucia during fortnightly samplings over a 2(1/2)-year period. Analysis of counts of the snail Biomphalaria glabrata from these samples showed that this species had a contagious distribution. This finding is consistent with other evidence that banana drains form a rigorous habitat for B. glabrata. Its distribution was more contagious than that of Oncomelania quadrasi in certain Philippine habitats and several species of aquatic snail in various African irrigation canals. The exact transformation for normalizing the snail counts for standard statistical techniques was z = x(0.287) but the more convenient cube root transformation is probably adequate. However, if too few snails are collected (15 or fewer per 100 samples) or if the frequency distribution of snail counts is discontinuous, with too many widely separated high frequency counts, neither transformation will be entirely satisfactory.