INVASION OF THE RIVERS OF THE LITTORAL CENTRAL REGION OF VENEZUELA BYTHIARA-GRANIFERA AND MELANOIDES-TUBERCULATA (MOLLUSCA, PROSOBRANCHIA,THIARIDAE) AND THE ABSENCE OF BIOMPHALARIA-GLABRATA, SNAIL HOST OF SCHISTOSOMA-MANSONI

Prior to 1975, some streams of the littoral central region of Venezuel a were active transmission sites for intestinal schistosomiasis. The s nail intermediate host, Biomphalaria glabrata, was controlled but not eliminated by repeated molluscicide applications. Two thiarid snails, Thiara granifera and Melanoides tuberculata, began to invade these str eams around 1975. The rapid expansion of these two species was confirm ed during more recent surveys. Although other factors may be involved, the existence of stable populations of the two thiarid species is pro bably linked to the absence of recolonization by B. glabrata; the latt er species has not been observed in the study region during the last 1 5 years.