Pathogens evaluation in imported molluscs: evidence from case studies in the northern Adriatic Sea

In recent years, the Italian mollusc farming sector has faced significant challenges, including climate-related stressors, disease outbreaks and the rapid spread of invasive species such as the blue crab (Callinectes sapidus). These factors have caused a marked decline in local production, particularly of Manila clams, leading to increased movement and importation of molluscs from other countries. While supporting production, these practices raise concerns about the introduction and dissemination of non-native species and pathogens. This study investigates pathogen occurrence in imported Mytilus galloprovincialis in the northern Adriatic Sea within the framework of the MUSSEL-AID project. Mussels were sampled biweekly from offshore farms in Veneto (Italy), including sites with exclusively local stocks and one farm primarily relying on imported mussels. Health assessments were conducted through histological, cytological and molecular analyses. Preliminary findings from the farm importing mussels from the Atlantic coast of Spain revealed multiple parasitic infections. Marteilia pararefringens was frequently detected, sometimes in co-infection with a haplosporidian parasite. In addition, Perkinsus olsenii was identified in a subset of specimens. Notably, these parasites were not detected in mussels of Adriatic origin. Overall, the results highlight the risks associated with mollusc translocation, particularly the potential introduction of emerging or non-endemic pathogens into local aquaculture systems. Strengthened biosecurity measures, systematic health monitoring of imported stocks and early detection strategies are essential to mitigate pathogen spread in vulnerable marine farming areas.