BackgroundMassive mortality outbreaks affecting Pacific oyster (Crassostrea gigas) spat in various countries have been associated with the detection of a herpesvirus called ostreid herpesvirus type 1 (OsHV-1). However, few studies have been performed to understand and follow viral gene expression, as it has been done in vertebrate herpesviruses. In this work, experimental infection trials of C. gigas spat with OsHV-1 were conducted in order to test the susceptibility of several bi-parental oyster families to this virus and to analyze host-pathogen interactions using in vivo transcriptomic approaches.ResultsThe divergent response of these oyster families in terms of mortality confirmed that susceptibility to OsHV-1 infection has a significant genetic component. Two families with contrasted survival rates were selected. A total of 39 viral genes and five host genes were monitored by real-time PCR. Initial results provided information on (i) the virus cycle of OsHV-1 based on the kinetics of viral DNA replication and transcription and (ii) host defense mechanisms against the virus.ConclusionsIn the two selected families, the detected amounts of viral DNA and RNA were significantly different. This result suggests that Pacific oysters are genetically diverse in terms of their susceptibility to OsHV-1 infection. This contrasted susceptibility was associated with dissimilar host gene expression profiles. Moreover, the present study showed a positive correlation between viral DNA amounts and the level of expression of selected oyster genes.

Marine mussel production is of substantial economic interest in numerous coastal areas worldwide, making crucial the study of pathologies that affect them. Disseminated neoplasia (DN) has recently been suggested to be linked to blue mussel, Mytilus edulis, mortality outbreaks observed in France since 2014, although the evidence remains indirect. In order to improve DN detection and monitoring, we compared the sensitivity of four diagnostic tools, namely haemocytology, histology, flow cytometry, and genetics. Haemocytological examination gave the best results in sensitivity and had the advantage of being non-invasive, allowing disease progression to be followed in affected mussels. Using this approach, we showed that DN progression is usually slow, and we provide evidence of remission events. We observed a high diversity of forms and mitotic features of neoplastic cells located in the vesicular connective tissue but rarely in the haemolymph. Circulating cells occur as four main types but are homogenous in morphology and DNA content within a single individual. Polyploidy proved very high, from 8N to 18N. Genetic analysis of haemolymph DNA showed that a Mytilus trossulus genetic signal was associated with almost all the DN cases here diagnosed by haemocytological examination, regardless of the DN type. This result corroborates DN is a transmissible cancer that first originated in a M. trossulus host and subsequently crossed into M. edulis. No pre-neoplastic conditions were detectable. The prevalence of the disease was quite low, 1 which, together with the low morbidity observed in the lab, suggest DN is unlikely to be the direct cause of mortality outbreaks in France.

A multi-parametric approach was adopted to identify mortality causes in mussels.  A 'mortality window' during the spring season was identified.  Brest, Lannion and St. Brieuc (France) have different cumulative mussel mortality rates.  Haemocytic infiltrations and Marteilia sp. are parameters linked to mortality.  There was no specific significant involvement of disseminated neoplasia or Vibrio.

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This note describes the first detection of the bacteria Francisella halioticida in mussels Mytilus spp. from locations in Normandy and northern Brittany (France) experiencing high mussel mortalities, while it was not detected in the Bay of St Brieuc (northern Brittany), an area which was not affected by abnormal mussel mortality. The distribution of the bacteria in mussels seems to be restricted to inflammatory granulomas as observed in Yesso scallops Mizuhopecten yessoensis from Canada and Japan. F. halioticida has been identified as being involved in mass (>80%) mortality of abalones Haliotis gigantea in Japan and high (up to 40%) mortality of Yesso scallops Mizuhopecten yessoensis in Canada as well as in lesions reducing marketability of Yesso scallops in Japan. The impact of this bacterium on the health of mussels needs to be investigated in future research, especially since the cause of high mussel mortalities that have been occurring in France for the past few years is still undetermined.