Discussion

Mortality events in cultured bivalves, related to disease can be significant. Viruses, bacteria and protistans have been associated with various diseases, many of which cause significant losses not just in Europe but globally. The origins of many of these pathogens and the associated diseases they cause have not been determined. As the host species are economically significant, the focus of much of the research has been on reducing economic losses. However, methods to screen and detect parasites have become more sensitive in recent years allowing more opportunities to understand the infection paths and the potential routes for dissemination of parasites. The life cycles of many parasites have multiple stages and multiple hosts indicating that transmission within aquatic systems is ongoing and dynamic. Screening for parasites within aquatic systems indicates that other species within the ecosystem may act as reservoirs, carriers or alternate, intermediate or final hosts. Examples from some of the study systems which are currently being focussed on, suggest that spread of parasites to other hosts and other trophic levels may be possible via a range of routes, and so may ultimately have wider ecosystem impacts, some of which we are only beginning to understand.