Hélène BOURAS

Study of the host-pathogen relationship between the blue mussel Mytilus sp. and Francisella halioticida

PhD thesis title: Emergence of pathogens in bivalve species: towards a better understanding of the diseases and towards a set-up of management methods

Shellfish farming is an important sector of French coast areas and is in difficulty since several years due to excessive and repetitive mortality, most particularly on pacific oysters (Crassostrea gigas) and blue mussels (Mytilus edulis). The abnormally high mortality rate of blue mussels was first reported in Charente before broadening to the North with reported mortalities in Pays de la Loire, Britany and lately in Normandy and Hauts-de-France. No causality is known as of today but the recent primary identification in France of the bacterium Francisella halioticida is a hypothesis worth of further studies. This bacterium has been correlated with mortality in the giant abalone Haliotis gigantea in Japan and the Japanese scallop Pectinopecten yessoensis in Japan and Canada. In addition, other bivalve species suffered from abnormally high mortality rate in France such as the clam (Ruditapes philippinarum) in the autumn 2016 and the scallop (Pecten Maximus) in 2018.

The management of mollusc diseases is complicated due to the absence of acquired immunity in these organisms and due to their direct contact with their surrounding environment. In the absence of prevention and management of these diseases, the future of the shellfish farming industry in Normandy is concerning. This is why it is crucial to understand, explain and attempt to fight against these outbreaks by developing reliable and adapted diagnostics tools, as well as optimised management methods to assure the biosecurity of bivalve mollusc species.

To answer these requirements, the present thesis relies on two approaches:

• To increase the scientific knowledge of bivalve species diseases. It is given a priority to study the pathogenicity of F. halioticida towards the blue mussels M. edulis through a study in a monitored environment. The host-pathogen relationship is going to be studied (via transcriptomic, proteomic and peptidomic approaches) to allow a better understanding of the infection pattern and the host immune reaction following the infection. In addition, a retrospective study of the diagnostics tools performed in case of mortality events will be performed in order to propose a line of optimisation of these methods. 
• To set-up a prevention, a management and potentially a control plan for these diseases. It is considered to use the knowledge obtained in the first approach to come up with rapid and reliable diagnostics methods. It is also considered to set-up a monitoring network for shellfish farming diseases and emerging pathogens in Normandy if possible. 

Supervised by Céline Zatylny-Gaudin (BOREA, Caen) and Maryline Houssin (Labéo, St-contest ; BOREA, Caen)

Funding: Université de Caen – Fonds départementaux 50 et 14 - EPC