Crassostrea gigas mortality in France: the usual suspect, a herpes virus, may not be the killer in this polymicrobial opportunistic disease

Abstract

Successive disease outbreaks in oyster (Crassostrea gigas) beds in France have resulted in dramatic losses in production, and subsequent decline in the oyster-farming industry. Deaths of juvenile oysters have been associated with the presence of a herpes virus (OsHV-1 μvar) and bacterial populations of the genus Vibrio. Although the pathogenicity of OsHV-1 μvar, as well as several strains of Vibrio has been demonstrated by experimental infections, our understanding of the complexity of infections occurring in the natural environment remains limited. In the present study, we use specific-pathogen-free (SPF) oysters infected in an estuarine environment to study the diversity and dynamics of cultured microbial populations during disease expression. We observe that rapid Vibrio colonization followed by viral replication precedes oyster death. No correlation was found between the vibrio concentration and viral load in co-infected animals. We show that the quantity of viral DNA is a predictor of mortality, however, in the absence of bacteria, a high load of herpes virus is not sufficient to induce the full expression of the disease. In addition, we demonstrate that juvenile mortalities can occur in the absence of herpes virus, indicating that the herpes virus appears neither essential nor sufficient to cause juvenile deaths; whereas bacteria are necessary for the disease. Finally, we demonstrate that oysters are a reservoir of putative pathogens, and that the geographic origin, age, and cultivation method of oysters influence disease expression.

Recruitment of the Pacific oyster Crassostrea gigas in a shellfish-exploited Mediterranean lagoon: discovery, driving factors and a favorable environmental window

“…Further technical experiments are necessary at an industrial scale to improve this novel shellfish-farming practice. There may also be other benefits in using native oysters; they may, for instance, have a better resistance to OsHv1-μvar (Petton et al 2015). The use of native oysters may also be considered a more eco-friendly practice for spat supply.…”

Salinity influences disease-induced mortality of the oyster Crassostrea gigas and infectivity of the ostreid herpesvirus 1 (OsHV-1)

“…A more likely hypothesis is that another microorganism killed these animals simultaneously weakened by a salinity shock. In our study, donors were infected in the field where the OsHV-1-induced mortality occurred, and they may have introduced a diversity of Vibrio species or other pathogens to the recipients (Lemire et al 2015, Petton et al 2015b). In addition, acute exposure to low salinity may have altered the defenses of oysters either directly by impairing their immunity (Matozzo & Marin 2011, Carregosa et al 2014, Knowles et al 2014 or indirectly by causing the oysters to allocate energy to regulate intracellular osmotic concentrations (Gilles 1972, Shumway 1977, Shumway et al 1977, Neufeld & Wright 1996, thus compromising the defense response.…”

“…The SI was then (1) directed towards recipient oysters held at 4 salinities with or without prior acclimation or (2) brought to a salinity of 10 or 25 ‰ and directed towards recipient oysters maintained at 25 ‰. Overall, this method, which has been used in many previous studies (Petton et al 2013, 2015a,b, Tamayo et al 2014, Lassudrie et al 2015, Lemire et al 2015, Pernet et al 2015, reproduces the natural infection process and takes into account the fact that OsHV-1 triggers oyster mortality, although other microorganisms can also play a role (Saulnier et al 2010, Lemire et al 2015, Petton et al 2015b.…”

Antimicrobial peptides in marine invertebrate health and disease

“…Remarkably, while marine invertebrates are incredibly diverse in terms of phylogeny and ecological niche, disease patterns are repeatedly found across species. These diseases include temperaturedependent vibrioses [7,8] and polymicrobial diseases [9,10]. Differences are observed in the susceptibility of the animals, from larvae to juveniles and adults, and in the diversity of the pathogens (vibrios, viruses, etc.)…”