Changes in the Populations of Two Lymnaeidae and Their Infection by Fasciola hepatica and/or Calicophoron daubneyi over the Past 30 Years in Central France

SIMPLE SUMMARY: Two parasitic diseases affecting humans and ruminants have a particular life cycle, because freshwater molluscs intervene in their transmission by ensuring the development of parasite larval forms. As the climate is changing, researchers have begun to investigate the effects of global warming on these snails and the larval forms of parasites that they harbour. Several authors have already conducted analyses of these diseases, but no field studies have been carried out so far. Therefore, in the present study, snail counts were conducted on 39 farms with acidic soils between 1976 and 1997, in 2013–2014, and in 2020–2021. The results showed that the number of snail populations decreased over time and that many populations have fewer and fewer individuals. This decline has also been faster in recent years. The infection of snails by one of the parasites has decreased over time. Conversely, snails are increasingly infected with the other parasite. These changes are due to the generalized use of a drug used to treat one of the diseases in ruminants and probably also due to the heatwave episodes that occurred for several years. As these larval forms are again infecting animals after their departure from the snails, practitioners must take these changes into account when treating ruminants for these diseases. ABSTRACT: Field investigations were carried out during three periods (from 1976 to 1997, in 2013–2014, and in 2020–2021) on 39 cattle-raising farms on acidic soils to track changes in the populations of two Lymnaeidae (Galba truncatula and Omphiscola glabra) and their infection with Fasciola hepatica and/or Calicophoron daubneyi. Compared to the survey between 1976 and 1997 on these farms, there was a significant decrease in the number of the two lymnaeid populations and the size of the G. truncatula populations in both 2013–2014 and 2020–2021. This decline was significantly faster in the last nine years than it was before 2013. The area of habitats colonized by G. truncatula showed no significant variation over the years, while that of habitats with O. glabra significantly decreased in the period covered by the three surveys. The prevalence of F. hepatica infection in snails significantly decreased over the years, while C. daubneyi infection increased over time in both lymnaeid species. These changes are due to the use of triclabendazole to treat fasciolosis in ruminants since the 1990s, and are probably a consequence of the successive heatwaves that have occurred since 2018 in the region.