Résumé
Biological control of the snail hosts of schistosomes has been considered in the last few decades as an alternative to molluscicides. Several groups of organisms have been proposed to control snail hosts, but very few have proven their efficacy in the field. Competitor snails can be considered as the most efficient biological control agents and numerous promising laboratory studies and field experiments have been carried out, mainly in the Caribbean. Two species of competitor caenogastropod snails belonging to the Ampullariidae (Marisa cornuarietis) and Thiaridae (Melanoides tuberculata) families have succeeded in eliminating or reducing populations of schistosome-transmitting snails, especially Biomphalaria glabrata in several different habitats in St Lucia, Martinique, and Guadeloupe. However, their efficiency is context-dependent. Caenogastropods are good competitors in relatively stable habitats only when long-term resource exploitation rather than colonization is the limiting factor. At the same time, unassisted invasions by these species and by other freshwater snails, including numerous pulmonates, were detected in the 1950s, followed by rapid spread in the following decades to most Neotropical areas. These invasions were largely responsible for the general decline of B. glabrata in islands, such as Martinique and Guadeloupe, replicating at a larger scale the results of biological control programs. No extinction of local snail species occurred following the invasion by exotic snails, except for B. glabrata in Martinique. Thus, biological invasions could qualify as efficient “unintentional biological control” agents. However, the downside of biological invasions is that snail hosts can be invasive and establish new sites of parasite transmission in formerly parasite-free areas. Moreover, the apparent lack of extinctions may mask the ongoing declines of local species leading to future extinctions, especially if new invaders continue to appear on a yearly basis.