Abnormal shell mortalities have been observed around the world for about fifteen years now.
In 2008, a particular genotype of the OsHV-1 virus thus appeared; it seems to be at the origin of episodes of massive mortality affecting young cupped oysters in Europe and particularly in France.
From 2012, these are the adult cupped oysters that present mortalities associated with the bacteria Vibrio aestuarianus in French oyster basins.
Monitoring these diseases, understanding them better, and even anticipating their emergence is a necessity not only to maintain sustainable production but also because these animals are sentinels that reveal the imbalances in coastal ecosystems.
Flat oysters Ostrea edulis create, for example, underwater mini-reefs that serve as support and shelter for many other species. Abundant in coastal aquatic ecosystems, shellfish play an essential role in their structure and operation.
We must also obviously mention the importance of shellfish farming, which achieves more than 20% of world aquaculture production. In France, oyster production (oyster farming) represented, in 2013, 72% of the turnover of the shellfish industry. The Hexagon is the first producer of oysters in Europe.
A set of imbalances
The presence of a pathogenic organism is not always synonymous with disease and mortality in shellfish. In fact, pathogenic organisms tend to develop and induce mortalities when there is an imbalance in the interactions between shellfish, pathogenic organisms, the environment and cultural practices; this imbalance favors the emergence of diseases.
Due to their mode of production – most often in an open environment – and the lack of antibody production, disease control in shellfish cannot be based on the use of treatments or vaccines. The implementation of surveillance programs and the restriction of animal transfers are the only measures currently available to prevent the introduction of diseases into free areas.
Once installed, it is always possible to minimize their impact on shellfish populations, by proposing appropriate stock management measures and / or by developing disease-resistant animal selection programs.
In 2016, a vast European scientific program (Vivaldi) was launched in 10 countries to improve our knowledge of shellfish diseases and develop tools to better control their effects.
Identify the “culprits”
The identification and distinction of species of pathogenic microorganisms is necessary to adjust the methods of detection and control of associated diseases.
For a long time, this identification relied on microscopy. For the past twenty years, the use of molecular tools, such as PCR and DNA sequencing, has made it possible to characterize new species and reveal a greater diversity of pathogenic microorganisms in shellfish.
The OsHV-1 virus mentioned above is a member of the herpesvirus family which has so far only been detected in bivalves – cupped oysters, flat oysters, clams and scallops. The complete genome of several OsHV-1 viruses found in cupped oysters from different regions of the world has been sequenced.
This study (which has not yet been published) shows differences depending on the geographical origin of the samples. She confirms that there was not just one OsHV-1 virus but a “virus constellation” within an infected oyster.
Another problem: the “reservoirs”, that is to say the “compartments” where pathogenic organisms can be present apart from the shellfish. It can be water, sediment or other marine organisms. Other studies carried out in the harbor of Brest have, for example, shown that the parasite Marteilia refringens, which affects the flat oyster, is present in sediment and water.
Until now, the diagnostic approach has been based on the search for pathogenic organisms in shellfish. Then systems making it possible to detect their presence in water, in particular before the onset of infection, were developed. The fact of being able to detect the OsHV-1 virus in water before the episodes of oyster mortality gives us the possibility of using such tools for the early detection of the virus.
More resilient individuals
Like mammals, shellfish defend themselves to prevent the development of disease. The work completed on the cupped oyster have in particular made it possible to show the involvement of autophagy, a mechanism for degradation and recycling of intracellular components, in the oyster’s response to the OsHV-1 virus.
Within the same population, some individuals are particularly sensitive to pathogenic organisms and others prove to be more resistant. Scientists have studied in the oyster dig the genes that can explain this difference.
Identifying key genes could help understand how oyster populations cope with disease and thus select more resistant animals. However, this selection must not be made to the detriment of genetic diversity or other interesting characteristics (size of the oysters, taste qualities, etc.).
Numerical simulations have made it possible to define good practices to be implemented in hatcheries, in order to limit the loss of genetic diversity.
Contrary to popular belief, mollusks have a sort of immune memory. This is in the form of patterns in proteins, able to recognize pathogenic organisms with which the animal has already been confronted and to trigger defense mechanisms. Works have made it possible to study the means of stimulating this response.
Oysters seem to defend themselves better against the OsHV-1 virus when they have been previously exposed to a molecule resembling the virus. This phenomenon, called “priming”, could pave the way for forms of immunostimulation. Better yet: this ability could be transferable. Initial results seem to show that oyster offspring brought into contact with these “stimulating” molecules survive a viral infection better, even if they have never been confronted with it.
The determining role of the environment
The environment plays an essential role in the emergence of diseases in marine molluscs; and the effects of temperature, salinity, acidity, nutrients and cohabitation with other species were studied.
It was thus shown than above 29 ° C, the OsHV-1 virus no longer causes mortality in cupped oysters. In contrast, the pH of seawater does not seem to have an impact on the virus’s ability to induce infection.
In addition, cohabitation with competing species, such as mussels or ascidians, seems to be beneficial for the cupped oyster. Several mechanisms can explain this phenomenon, including competition for food. The oyster has fewer nutrients available, which reduces its development, and may decrease the multiplication of the virus. Additional work is underway to better understand these results.
The microbiota – that is to say all the naturally present microorganisms – of shellfish has also been explored for several years using new sequencing tools. The structure of microbial communities seems to vary depending on the species, their habitat and also the season. The microbiota is unique to each individual and even to each organ. Interestingly, an imbalance in the oyster microbiota has been observed during mortality events: a decrease in microbial diversity is then reported.
All of these observations motivate scientists to continue their research in order to identify profiles indicative of good shellfish health or, on the contrary, indicative of dysfunction.