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The ostreid herpesvirus (OsHV-1) has the potential to devastate Crassostrea gigas culture in Australia as it has done in many other countries, highlighting the need for a better understanding of disease expression and transmission. The aim of this study was to assess the spatial distribution of OsHV-1 associated mortalities in one of only two infected areas in Australia, Woolooware Bay (Botany Bay New South Wales). In October 2011, healthy sentinel Pacific oysters were placed in three different locations at three different tidal levels and OsHV-1 associated mortalities were closely monitored over 7 months. The outbreak started in November 2011 and the disease remained active until April 2012. Three major mortality events were detected. Rather than being a propagating epizootic, it appeared that most oysters were infected from the same environmental source. The distribution of OsHV-1 associated mortalities was spatially clustered, highly variable and clearly dependent on the age of oysters and their position in the water column. Non-random distribution of mortalities at macro scale (sites several km apart) and micro scale (within rearing trays), and vertical clustering patterns in the water column are discussed in relation to mechanisms of transmission in water. We hypothesise that OsHV-1 may be carried through water by particles, possibly plankton. Key words: Crassostrea gigas, Ostreid herpesvirus 1, summer mortalities, spatial distribution, plankton, disease transmission Funded by the Fisheries Research and Development Corporation, the University of Sydney and the Sydney Metropolitan Catchment Management Authority

International audience; Since 2008, mass mortalities of 1-yr-old Crassostrea gigas associated with the ostreid herpesvirus OsHV-1 μVar have occurred along all the coasts of France when seawater temperature reaches 16 to 17°C. The present study aimed to characterize the effect of temperature on oyster survival in combination with OsHV-1 DNA quantification by standard real-time PCR and total vibrio population levels in oyster tissues. To examine the effect of seawater temperature on disease transmission and related mortality of oysters, cohabitation experiments were conducted between healthy naïve oysters and oysters previously exposed to field conditions in areas where mortalities were occurring. Oysters initially maintained in controlled conditions (free of mortality and negative for OsHV-1), and then transferred to an area where high mortalities were occurring among farmed stocks, became infected with OsHV-1 and exhibited high loads of vibrios followed by significant mortalities. When previously exposed oysters were maintained indoors at 13.0°C for 40 d and then at 20.6°C, they exhibited no mortality, were negative for OsHV-1 detection, and did not transmit the disease to healthy oysters. Survival of previously exposed oysters maintained indoors at 8 temperatures ranging from 13.4 to 29.0°C varied from 25 to 48% and was negatively correlated with holding temperature. Concomitantly, survival of naïve cohabiting animals (62 to 98%) decreased with increasing seawater temperature until a plateau was reached between 16.2 and 21.9°C, and increased at higher temperatures. Therefore, the optimal temperature range for disease transmission from field-exposed to naïve animals was between 16.2 and 21.9°C. Our results suggest that a long-term period (40 d) at low temperature (13°C) may offer a method of mitigating mortalities in oysters that have been exposed to an infective environment.

Although there are a number of ostreid herpesvirus 1 (OsHV-1) variants, it is expected that the true diversity of this virus will be known only after the analysis of significantly more data. To this end, we analyzed 72 OsHV-1 “specimens” collected mainly in France over an 18-year period, from 1993 to 2010. Additional samples were also collected in Ireland, the United States, China, Japan, and New Zealand. Three virus genome regions (open reading frame 4 [ORF4], ORF35, -36, -37, and -38, and ORF42 and -43) were selected for PCR analysis and sequencing. Although ORF4 appeared to be the most polymorphic genome area, distinguishing several genogroups, ORF35, -36, -37, and -38 and ORF42 and -43 also showed variations useful in grouping subpopulations of this virus.

Although viruses have been associated with mortality outbreaks in different mollusc species, little information is available on their exact affiliation and taxonomic position. Herpes viruses have attracted particular attention because of their economic and ecological impact on both farmed and wild marine molluscs over the past 20 years. Investigations based on molecular approaches have been carried out on the herpes virus infecting Pacific oysters in France. Molecular and ultra-structural data were used to classify the virus under the name ostreid herpesvirus 1 (OsHV-1), as the first known species in the family Malacoherpesviridae. Since 2008, massive mortality outbreaks were reported in several Crassostrea gigas oyster farms in France, Ireland, and the United Kingdom. The outbreaks were attributed to adverse environmental conditions associated with a particular variant of OsHV-1, called OsHV-1 μVar., Bien que des épisodes de mortalité associés à la détection de virus aient été rapportés chez différentes espèces de mollusques, peu d’informations sont aujourd’hui disponibles concernant l’affiliation exacte de ces virus et leur position taxinomique. Parmi les virus infectant les coquillages, les herpèsvirus ont fait l’objet d’une attention particulière du fait de leurs impacts économiques et écologiques sur les animaux en élevage et sauvages. Des recherches par des approches de biologie moléculaire ont été menées en France sur le virus infectant les huîtres creuses, Crassostrea gigas. Sur la base des données moléculaires et ultra-structurales, ce virus a été classé, sous le nom d’ostreid herpesvirus 1 (OsHV-1), comme première espèce virale de la famille Malacoherpesviridae. Depuis 2008, des épisodes de surmortalité ont été rapportés dans différentes régions de production des huîtres creuses C. gigas, en France, en Irlande et au Royaume Uni. Ils ont été attribués à des conditions environnementales défavorables associées à la découverte d’un variant génotypique viral particulier, appelé OsHV-1 μVar., Renault Tristan. Les virus infectant les mollusques marins: un exemple d'actualité, les herpèsvirus. In: Bulletin de l'Académie Vétérinaire de France tome 164 n°4, 2011. pp. 359-364.

Since its molecular characterisation, Ostreid herpesvirus 1 (OsHV-1) has been regularly detected in Crassostrea gigas in France. Although its pathogenicity was demonstrated on larval stages, its involvement during mortality outbreaks at the juvenile stage was highly suspected but not evidenced. To investigate mortality outbreaks, the French National Network for Surveillance and Monitoring of Mollusc Health (REPAMO) carried out two surveys in juvenile C. gigas. The first survey lasted from 1998 to 2006 and was an epidemiological inquiry occurring when oyster farmers reported mortality outbreaks. The second survey, a longitudinal one, was set up in 1998 to complete the network observations on OsHV-1. Data analysis showed a specific pattern of mortality outbreaks associated with OsHV-1 detection. Ostreid herpesvirus 1 detection mainly appeared during the summer, suggesting the influence of the seawater temperature on its occurrence. It mostly presented a patchy distribution in the field in contrast to the nursery. Significant relationship between OsHV-1 detection and spat mortality was found, preferentially in sheltered and closed environments. The longitudinal survey confirmed most of the network observations. Although subsequent works particularly epidemiological surveys would be useful to confirm the causal link between the detection of OsHV-1 and the mortality outbreaks in juvenile C. gigas, the role of OsHV-1 in oyster mortality is progressing.

Herpes- and herpes-like viruses are known to infect a wide range of bivalve mollusc species throughout the world. Abnormal summer mortalities associated to the detection of ostreid herpesvirus 1 (OsHV-1) have been currently reported in France among larvae and spat of the Pacific cupped oyster Crassostrea gigas. In the present work, we have developed an experimental protocol of horizontal transmission based on the cohabitation between healthy and experimentally infected oysters. Through a cohabitation trial, the kinetics of OsHV-1 detection in different oyster organs and seawater samples were investigated and characterized for the first time using real time quantitative PCR.

Herpes and herpes-like virus infections have been reported in various marine mollusc species associated with high mortality rates. Following the characterisation and genome sequencing of ostreid herpesvirus 1 (OsHV-1), specific diagnostic tools have been developed based on conventional PCR techniques or in situ hybridisation. We have now developed a real-time PCR assay for rapid, sensitive and quantitative detection of OsHV-1, and compared it with a conventional PCR technique described previously. The new assay utilised SYBR((R)) Green chemistry with specific primers C(9)/C(10) targeting the C region. The melt curve analysis of OsHV-1 DNA or DNA extracted from infected material showed only one melting temperature peak (75.75+/-0.1 degrees C). The assay had a detection limit of 4 copies/microL of viral genomic DNA and a dynamic range of 5 logs. Using infected oyster samples as template, the assay was about 100-fold more sensitive than single PCR method using C(2)/C(6) primers. The assay was applied successfully for rapid diagnosis (100 min) and quantitation of OsHV-1 in different developmental stages of Crassostrea gigas. Although it already exists a competitive PCR method to quantify OsHV-1 DNA, quantitative data that will emerge in future using the new sensitive and reliable assay will illuminate aspects of pathogenesis, in particular the viral loads in asymptomatic oysters and the kinetics of infection in specific target tissues.

Background Ostreid herpesvirus-1 (OsHV-1) is the major bivalve pathogen associated with severe mortality events in a wide host range. In the early summer of 2012 and 2013, mass mortalities of blood clam (Scapharca broughtonii) broodstocks associated with a newly described variant of OsHV-1 (OsHV-1-SB) were reported. Methods In this study, the complete genome sequence of the newly described variant was determined through the primer walking approach, and compared with those of the other two OsHV-1 variants. Results OsHV-1-SB genome was found to contain 199, 354 bp nucleotides with 38.5 % G/C content, which is highly similar to those of acute viral necrosis virus (AVNV) and OsHV-1 reference type. A total of 123 open reading frames (ORFs) putatively encoding functional proteins were identified; eight of which were duplicated in the major repeat elements of the genome. The genomic organization of OsHV-1-SB could be represented as TRL-UL-IRL-IRS-US-TRS, which is different from that of OsHV-1 reference type and AVNV due to the deletion of a unique region (X, 1.5Kb) between IRL and IRS. The DNA sequence of OsHV-1-SB is 95.2 % and 97.3 % identical to that of OsHV-1 reference type and AVNV respectively. On the basis of nucleotide sequences of 32 ORFs in OsHV-1-SB and the other nine OsHV-1 variants, results from phylogenetic analysis also demonstrated that OsHV-1-SB is most closely related to AVNV. Conclusions The determination of the genome of OsHV-1 with distinguished epidemiological features will aid in our better understanding of OsHV-1 diversity, and facilitate further research on the origin, evolution, and epidemiology of the virus. Electronic supplementary material The online version of this article (doi:10.1186/s12985-015-0334-0) contains supplementary material, which is available to authorized users.

Diseases pose an ongoing threat to aquaculture, fisheries and conservation of marine species, and determination of risk factors of disease is crucial for management. Our objective was to decipher the effects of host, pathogen and environmental factors on disease-induced mortality of Pacific oysters (Crassostrea gigas) across a latitudinal gradient. We deployed young and adult oysters at 13 sites in France and we monitored survival, pathogens and environmental parameters. The young oysters came from either the wild collection or the hatchery while the adults were from the wild only. We then used Cox regression models to investigate the effect of latitude, site, environmental factors and origin on mortality risk and to extrapolate this mortality risk to the distribution limits of the species in Europe. We found that seawater temperature, food level, sea level atmospheric pressure, rainfall and wind speed were associated with mortality risk. Their effect on hatchery oysters was generally higher than on wild animals, probably reflecting that hatchery oysters were free of Ostreid herpesvirus 1 (OsHV-1) whereas those from the wild were asymptomatic carriers. The risk factors involved in young and adult oyster mortalities were different, reflecting distinct diseases. Mortality risk increases from 0 to 90% with decreasing latitude for young hatchery oysters, but not for young wild oysters or adults. Mortality risk was higher in wild oysters than in hatchery ones at latitude > 47.6°N while this was the opposite at lower latitude. Therefore, latitudinal gradient alters disease-induced mortality risk but interacts with the initial health status of the host and the pathogen involved. Practically, we suggest that mortality can be mitigated by using hatchery oysters in north and wild collected oysters in the south.

A herpes-type virus infection, the first to be found in an invertebrate animal, is reported in the oyster Crassostrea virginica. Intranuclear herpes-type viral inclusions were more prevalent in the oyster at elevated water temperatures of 28 degrees to 30 degrees C than at normal ambient temperatures of 18 degrees to 20 degrees C. The inclusions were associated with a lethal disease at the elevated temperatures.

Background: Massive 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. Results: The 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. Conclusions: In 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.