A DEMOGRAPHIC ANALYSIS OF y-NEOTRICULA APERTA (GASTROPODA: POMATIOPSIDAE) POPULATIONS IN THAILAND AND SOUTHERN LAOS, IN RELATION TO THE TRANSMISSION OF SCHISTOSOMIASIS

The results of field work undertaken along the tropical reaches of the Mekong river in Thailand and Laos are presented. Demographic data concerning the schistosome-transmitting snail y-Neotricula aperta are examined and related to the life-cycle of this species. On the basis of these data, and a review of the literature, the only available, empirically substantiated, model for the life-cycle of y-N. aperta (see Upatham et al., 1980) is shown to be equivocal. In this model snails survive the annual spate in the Mekong river as eggs, which remain dormant below the flood waters, whilst the adults die soon after the waters begin to rise. Here, the above described model of ‘egg survival’ is shown to fail to balance the demographic equation for y-N. aperta. An alternative model of delayed oviposition with post-spate survival of mature snails is proposed and shown to be a better fit to the available data. The implications of this alternative model for the evolutionary biology and ecology of N. aperta are discussed. The possible effects of the Pak-Mul dam project (Northeast Thailand) on local populations of schistosome-transmitting snails are assessed and the hydrodynamic conditions conducive to vector snail proliferation are considered. Attenuation of the normal flood-cycle of the Mekong river anc alteration of the river bed topography will probably lead to increased numbers of N. aperta at Pak-Mul. These changes, together with the influx of casual labour from endemic areas in Laos, may result in an epidemic of human schistosomiasis in Northeast Thailand.