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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - VIVALDI (Preventing and mitigating farmed bivalve diseases)

Teaser

Summary

The European shellfish industry is a major contributor to global production of marine bivalves and has a significant social impact. Since 2008, massive mortality outbreaks in Pacific oyster spat have been reported, attributed to the oyster herpesvirus OsHV-1. More recently, increased mortality has been reported in adult Pacific oysters associated with Vibrio aestuarianus and among blue mussels with the detection of V. splendidus. Similarly, the Brown Ring Disease caused by V. tapetis and Perkinsosis have a significant impact on the production of clams. In Spain, the parasite Marteilia cochillia has contributed to the collapse of cockle fishery since 2012. Disease management methods rely, among others, on preventive measures, which are regulated at EC level. VIVALDI project can contribute to identify possible specific adaptations to marine molluscs.
In this context, VIVALDI is bringing new knowledge on the interactions between shellfish, environment and pathogens and is developing practical tools and approaches aimed at better preventing and controlling diseases in the main European farmed shellfish species. These species include oysters (Crassostrea gigas and Ostrea edulis), mussels (Mytilus edulis and M. galloprovincialis), clams (Ruditapes philippinarum) and scallops (Pecten maximus). The project addresses the most harmful pathogens affecting these species: the virus Ostreid herpesvirus 1 (OsHV-1), Vibrio species including V. aestuarianus, V. splendidus and V. tapetis, as well as micro- eukaryotes such as the parasites Perkinsus olseni and Bonamia ostreae.

Work performed

Pathogen diversity and lifecycles:
- Investigations on oyster pathogensâ€™ genomes reveal that the virus OsHV-1 is evolving via animal transfers and when it switches from one host species to another;
- Differences in the virulence of the bacteria Vibrio aestuarianus have been revealed;
- New bivalve pathogens have been characterised;
- Rapid identification of bacteria will become possible thanks to the creation of a MALDI-TOF MS database;
- -Rapid quantification of the parasite Perkinsus olseni in clam is now possible by Real-Time PCR
- Several diagnostic approaches allow to successfully detect aquatic pathogens in sea water, after filtration or using plastic membranes as passive sensors. Magnetic Beads could be used prior to PCR to improve the detection OsHV-1 in aquaculture facilities.
Host and pathogen response:
- Important pathways involved in the bivalvesâ€™ response against pathogens were identified: e.g. autophagy (mechanism contributing to cellular components degradation and recycling), seems involved in the oyster defence against the virus OsHV-1;
- We have discovered that mussels show an extremely high expression of antimicrobial peptides that might be very effective against pathogens;
- Tissular distribution and pathogenesis of oyster pathogens OsHV-1 and Vibrio aestuarianus and the bacteria Vibrio tapetis in clams have been successfully explored;
- Improved survival of oysters challenged with UV inactivated OsHV-1 has been revealed.
Development of selective breeding programmes:
- We have produced and identified oyster tolerant families (able to support high viral load without dying) and resistant families (able to limit the infection).
- A massive genotyping approach based on 20 000 SNPs (Single Nucleotide Polymorphisms) combined with whole genome sequencing will allow us to identify genomic regions associated with the resistance/tolerance to the viral infection.
- The impact of the diet on the resistance to OsHV-1 has also been observed.
- Parentage assignation tools based on SNP panels were developed and tested in oysters and clams.
- Simulation exercises have shown that it is possible to select for survival and growth without increasing inbreeding.
Microbiota:
- Microbial communities are investigated in bivalves under different environmental conditions and infections.
- Experimental studies have shown that oyster depuration does not remove Vibriofrom bivalve tissues and exposure to nanoparticles such as n- TiO2 modifies the composition of the bacteriome
- These first results suggest thatmicrobiota could be used as a health marker.
Environmental parameters:
-Environmental parameters influence disease development: OsHV-1 infection is drastically reduced at 29Â°C or increased in the presence of green algae.
-Laboratory and field experiments have highlighted interactions between plankton and Vibrio : e.g the microalgal species Alexandrium minutum seems to boost Vibrio tasmaniensisâ€™ virulence, whereas Isochrysis galbana inhibits the growth of V. aestuarianus.
- Modelling the spread of pathogens by water currents was achieved within key sites monitored in VIVALDI. Such models are of interest to simulate outbreak and test the efficiency of management measures.
Disease management measures:
- Strategies to avoid OsHV-1 in hatcheries/nurseries and reduce mortalities were reviewed.
- Field experiments confirm that husbandry practices need to be locally adapted.
- Experimental studies have demonstrated that UV-based technology inactivates pathogens in the water in entrance of hatcheries/nurseries but also oyster gametes and larvae from the wastewater.
- A model allows to categorise shellfish farms depending on the risk of introduction and spread of oyster pathogens. This tool might help the competent authorities in Member States to better implement risk-based surveillance.
Stakeholder analysis and mapping was carried out in several countries in order to better understand the key stakeholders in the shellfish

Final results

- Characterizing the diversity of oyster pathogens: these results are of interest for future detection tools;
- Passive sensors, magnetic beads and electrochemical biosensors to improve the detection aquatic pathogens: such tools will be useful for pathogen surveillance in the environment and development of early warning systems;
- Better understanding of the life-cycles of some pathogens is required to develop effective disease management plans;
- Characterizing bivalve microbiome under different scenarios could help to identify bivalve health markers;
- Identifying key mechanisms involved in the response of bivalves during an infection may help to identify markers related with resistance against diseases;
- Panels of SNP markers are now available for oysters and clams. Combined with oyster and clam phenotyping, these panels will allow us to identify markers associated with resistance/tolerance to some pathogens;
- Marker panels for parentage assignment available for oysters and clams. These tools will optimise breeding programs;
- Existing strategies to avoid OsHV-1 in hatchery/nursery were identified and completed by field studies, so as to identify the best husbandry practices to reduce mortality;
- UV treatment showed efficacy to inactivate pathogens and remove oyster gametes and larvae from the wastewater;