International epidemiology of mollusc diseases: Learning the lessons from two recent assessments on susceptible and vector species by the European food safety authority

Describing diseases in populations is the literal aim of epidemiology. From that point of view, molluscs and mollusc diseases put epidemiologists and other related 'ogists' under serious challenges. Two recent assessments conducted by the Animal Health and Welfare (AHAW) Panel of the European Food Safety Authority (EFSA) have highlighted some of these challenges. In an attempt to establish a list of species susceptible to certain diseases, a set of objective criteria pertaining to pathogen replication, viability, host response and pathology, were established. Screening the peer-reviewed literature to document these criteria revealed that studies on diseases of molluscs and techniques applied to case investigation usually provide a great deal of information. However, thorough identification of the pathogen is frequently lacking. In addition, application of the criteria has led to unexpected outcomes of the assessment when species usually regarded as susceptible do not fulfill the criteria. If not susceptibility, the presence of a pathogen in a host leaves very little space for interpretation. From the viewpoint of transfer of pathogens, susceptible species and biological vectors may present equivalent risks. The concept of vector has itself proved to be a subject for controversy and definitions in the international standards for health management have shown to be potentially misleading in assessing the risk of transferring pathogens via transfers of live animals. A second assessment of the AHAW panel to list potential vector species highlighted a range of situations from mechanical carriage to actual infection. Interestingly, some of the conclusions of this risk assessment were later comforted by experimental data. Both the lack of accurate identification of mollusc pathogens and inconsistency in host species categorization result in poor understanding of epidemiology of diseases in molluscs. These issues are illustrated by selected examples.