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Diseases of Aquatic Organisms

    DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03782

    Understanding the role of Francisella halioticida in mussel mortalities in France: an integrative approach

    Céline Garcia*, Maud Charles, Bruno Chollet, Aurélie Nadeau, Delphine Serpin, Laure Quintric, Jean-François Pépin, Maryline Houssin, Coralie Lupo

    *Corresponding author:

    ABSTRACT: Since 2014, mass mortalities in mussels Mytilus spp have occurred in production areas on the Atlantic coast of France. The aetiology of these outbreaks remained unknown until the bacterium Francisella halioticida was detected in some mussel mortality cases. Hence, a retrospective study was conducted to assess the association between this bacterium and mussel mortalities. Mussel batches (n=45) from the Atlantic coast and English Channel were selected from archived individual samples (n=863) collected either during mortality events or outside of them between 2014 and 2017. All mussels were analysed by real-time PCR assays targeting F. halioticida and in addition, 185 were analysed using histological analysis and 178 by 16S rRNA metabarcoding. F. halioticida DNA was detected by real-time PCR and 16S rRNA metabarcoding in 282 and 34 mussels, respectively. Among these individuals, 82% (real-time PCR analysis) and 76% (16S rRNA metabarcoding analysis) were sampled during a mortality event. Histological analyses showed that moribund individuals had lesions mainly characterized by necrosis, haemocyte infiltration and granulomas. Risk factor analysis showed that mussel batches with more than 20% of PCR positive individuals had a higher risk of being sampled during a mortality event and positive 16S rRNA metabarcoding batches increased the strength of the association with mortality by 11.6 times. F. halioticida's role in mussel mortalities was determined by reviewing the available evidence. To this end, a causation criteria grid, tailored to marine diseases and molecular pathogen detection tools, allowed more evidence to be gathered on the causal role of this bacterium on mussel mortalities.