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Diseases of Aquatic Organisms

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DAO 158:81-99 (2024)  -  DOI:

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

Céline Garcia1,*, Maud Charles2, Bruno Chollet1, Aurélie Nadeau1, Delphine Serpin1, Laure Quintric3, Jean-François Pépin4, Maryline Houssin2, Coralie Lupo1

1Ifremer, ASIM Adaptation et Santé des Invertébrés Marins, F-17390 La Tremblade, France
2LABÉO, 14280 Saint-Contest, France
3Ifremer, IRSI, SEBIMER Service Bio-informatique d’Ifremer, 29280 Plouzané, France
4Ifremer, LITTORAL, 17390 La Tremblade, France
*Corresponding author:

ABSTRACT: Since 2014, mass mortalities of 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. This retrospective study was conducted to assess the association between F. halioticida and these mussel mortalities. Mussel batches (n = 45) from the Atlantic coast and English Channel were selected from archived individual samples (n = 863) collected either during or outside of mortality events between 2014 and 2017. All mussels were analysed by real-time PCR assays targeting F. halioticida; 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 were more likely to have been sampled during a mortality event, and positive 16S rRNA metabarcoding batches increased the strength of the association with mortality by 11.6 times. The role of F. halioticida 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 in mussel mortalities.

KEY WORDS: Francisella halioticida · Mussel mortality · Causal inference · Molecular epidemiology · Histology · 16S rRNA metabarcoding · Real-time PCR

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Cite this article as: Garcia C, Charles M, Chollet B, Nadeau A and others (2024) Understanding the role of Francisella halioticida in mussel mortalities in France: an integrative approach. Dis Aquat Org 158:81-99.

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