Abstract
From May to October 2019, multiple mass mortality events (MMEs) of Pinna nobilis were observed along Croatian coastline starting from the south-east and rapidly progressing in north-western direction. Time dynamics of the MMEs closely followed general speed and direction patterns of surface sea-currents, advancing approximately 350 km in less than 3 months. Surveillance, clinical evaluation, and sample collection were performed on multiple sites with various degrees of mortality rates. Moribund P. nobilis individuals were collected and subjected to pathological, molecular, and microscopical investigation. Affected animals were positive for Mycobacterium in 70% of the individuals, and Haplosporidium pinnae was present in 58% of the cases. Observed pathological lesions were most severe where concurrent presence of both pathogens was confirmed (in 45.8% of moribund individuals). Moderate to strong lesions were observed in animals positive for Mycobacterium only (25% of cases), and lesions were absent or minor to moderate when only H. pinnae was confirmed (16% of cases). Considering the rapid and severe spread of the MMEs, the areas less exposed to major sea currents appeared to be at lower risk of pathogen transmission. Surveillance activities along the Croatian coastline identified several P. nobilis populations in such "lower risk" areas without apparent mortality or clinical symptoms. Such areas are of particular interest as source of potentially healthy individuals to support active recovery actions.