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Ostreid herpesvirus 1 (OsHV-1) is an infectious viral disease of bivalve mollusks with a wide geographic distribution across five continents. Since its initial identification in 1991, the disease has been identified as the cause of mortality in more than ten species of bivalve mollusks. The transmission of OsHV-1 is thought to be facilitated by water. In 2019 and 2021, mass mortalities of blood cockle (Anadara kagoshimensis) hatchery broodstock associated with OsHV-1 infection were reported. The presence of OsHV-1 DNA was confirmed in all hatchery samples with levels exceeding 106 copies per milligram of DNA. OsHV-1 was not detected in the wild populations from which the broodstock was collected, indicating that the virus was introduced into the hatchery with seawater from the open ocean. The results of the histopathological examination indicated that tissue lesions and associated hemocyte infiltration were most prevalent in the connective tissues of the digestive tract, mantle, gills and foot of the diseased A. kagoshimensis. In situ hybridization (ISH) signals of OsHV-1 DNA were frequently observed in infiltrated hemocytes, putative fibroblasts, and on occasion, in muscle cells with weak staining. Transmission electron microscopy analysis demonstrated the presence of herpes-like viral particles within the nucleus of infected cells. The genomes of the OsHV-1 variants from cases 2019 and 2021 were successfully resolved through direct high-throughput sequencing of infected tissues. A phylogenetic analysis of the complete genome sequences of 32 OsHV-1 variants revealed that the segregation of the virus was primarily structured by host species and geographic origin, with some exceptions. We propose that the sympatric distribution of different host species and the potential occurrence of cross-species transmission of OsHV-1 are responsible for the inconsistency between the phylogenetic segregation and their host species. This is the first report on the pathogenic and genomic characterization of OsHV-1 infecting A. kagoshimensis. The epidemiological and phylogenetic analyses offer new insights into the evolution of the virus and provide valuable information for the prevention and control of the disease. [Display omitted] • OsHV-1 infection results in breeding failure of Anadara kagoshimensis due to mass mortalities of broodstock. • The virus was introduced into the hatcheries with seawater from the open sea. • Our findings indicate the possibility of potential cross-host transmission of OsHV-1 among sympatric bivalve species. [ABSTRACT FROM AUTHOR]

Genotype-by-environment (GxE) interaction in aquaculture is usually estimated for continuous traits and based on data from a limited number of 2–3 rearing sites. Here we report the results of a GxE study for resistance to Pacific oyster mortality syndrome (POMS), a multi-factorial disease that severely impacts Pacific oyster production worldwide. The syndrome is largely associated with Ostreid Herpes Virus 1 (OsHV-1). Resistance to OsHV-1 in Crassostrea gigas has been shown to be heritable, meaning that selective breeding is a suitable strategy for reducing mortalities. However, limited information was available about GxE interaction or the possible need to consider it in selective breeding. Survival of two cohorts (C1 and C2), consisting of a total of 104 full-sib families, was evaluated during the summer of 2013 in 7 sites along the French Atlantic, Channel and Mediterranean coasts. Mean survivals in autumn 2013 were 12.6% ± 10.9 and 4.6% ± 6.4 for C1 and C2, respectively. Genetic parameters were computed by MCMC, which is suitable for binary data like survival. Heritability estimates ranged from 0.16 to 0.42 depending on site and cohort, with a mean of 0.24 [0.20; 0.27] when including all data. GxE interactions were estimated by the genetic correlations between pairs of sites. Genetic correlations were high (ρ > 0.80) for C1 between most tidal Atlantic and Channel sites, and intermediate between tidal sites and a Mediterranean lagoon site, while they were lower and more variable for C2 (0.21–0.77). Expected genetic gains were maximal when production site was the same as selection site. They were closed to this expected maximum when the selection site was different from the production site along the Atlantic or Channel coast. Limited GxE interaction along the French Atlantic coast is favorable to wide dissemination of genetically improved oysters along this coast. Limited potential improvement was shown in the Mediterranean site if selection was carried out elsewhere, confirming the specificity of this environment. Consequently, a specific strategy such as dedicated breeding should be used to achieve genetic progress for this site. • Intermediate heritability for survival to Pacific oyster mortality syndrom (POMS) was estimated. • Limited GxE interactions along Atlantic and Channel areas were favorable to dissemination of progress for survival to POMS. • Strong GxE interaction between Atlantic or Channel sites (intertidal sites) and the Mediterranean site (subtidal site). • The results suggest the development of breeding programs specific to subtidal and intertidal environments. [ABSTRACT FROM AUTHOR]

Abstract Due to massive mortality of Crassostrea gigas spat in France since 2008, a "plan de sauvegarde" was set up from 2011 to 2014 (hereafter referred to as PS1 to PS4), in order to reduce the shortage of spat. This plan involved the participation of commercial hatcheries, the French Research Institute for Exploitation of the Sea (Ifremer), and the Direction des Pêches Maritimes et de l'Aquaculture (DPMA) of the French Ministry of Agriculture. It was based on selecting diploid lines of C. gigas for their higher resistance to the oyster herpesvirus OsHV-1 (2nR group), and one of these lines was subsequently tetraploidized (4nR group). Both the 2nR and 4nR groups were produced by Ifremer, and then transferred to commercial hatcheries that produced the selected triploids (3nR groups). We report here the mortality rates of the 3nR group for each of the four "plan de sauvegarde" campaigns and compare these with the mortalities of the classic production of commercial hatcheries (both 2n and 3n), benchmarks of selected (2nR group) and unselected (2n-control group) oysters produced by Ifremer, and wild-caught spat, representing a total of 104 diploid and triploids batches. For PS1, the 3nR group had a mean mortality of 67% and did not show any advantage over the 2n- and 3n-commercial groups, suggesting a lack of genetic progress in the 2nR and 4nR groups. For PS2, OsHV-1 resistance was increased in both the 2nR and 4nR groups and, consequently, the 3nR group exhibited a mean mortality of 52%, which was significantly lower than the mortality of the 2n- (87%) and 3n-(76%) commercial groups in 2012. Unfortunately, the mortality of the 3nR group reached 62% and 71% in PS3 and PS4, respectively, although it was expected to be lower than that in PS2. OsHV-1 DNA was quantified in the live oysters at deployment (1356 oysters) and at the endpoint (1171 oysters), as well as in moribund oysters sampled during peak mortality (539 oysters). The results strongly supported the involvement of this pathogen during the main mortality outbreak in May/June. Meanwhile, Vibrio aestuarianus was also suspected to cause unexpected mortality of PS3 oysters in August and September, and it was detected in moribund PS4 oysters during both the mortality events, in May and July. Despite genetic improvement for OsHV-1 resistance, this translated into variable commercial genetic gain. This could be explained by the limited genetic backgrounds of the 2nR and 4nR groups, the reemergence of V. aestuarianus in France since 2012, the changing levels of genetic improvement in both the 3nR group and the commercial groups, as well as the limited broodstock genetic variation where small numbers of males were used. Results on growth and yield are discussed. Highlights • Four "plan de sauvegarde" campains (PS) were set up to reduce the spat shortage in France from 2011 to 2014. • Selection criterion was only based on the resistance to the infection by OsHV-1. • The mortality of 5 groups were compared for each PS representing a total of 104 diploid and triploids batches. • The 3nR group only had a significant decrease of mortality over the 2n and 3n commercial groups in 2012. • The failure of the "plan de sauvegarde" in 2013 and in 2014 could be explained by the role of V. aestuarianus. [ABSTRACT FROM AUTHOR]

Mortality associated with ostreid herpesvirus 1 (OsHV-1) has significantly impacted Crassostrea gigas farmers all around the world. Although selective breeding programs to improve OsHV-1 resistance can significantly reduce the mortality rate below 20%, husbandry practices have also been one way to limit or decrease mortality. The main objectives of this study were to describe the influence of two husbandry practices on OsHV-1-induced mortality in C. gigas spat and how different family lines respond to husbandry practices to gain a better understanding of the potential benefits of breeding management as a tool to adapt to improved practices. The first analysis investigated the importance of the size of C. gigas when challenged with OsHV-1 for the first time. For each of the 40 families of C. gigas evaluated, small (S) and large (L) oyster groups were tested for two years in the field. A significant mortality outbreak associated with OsHV-1 occurred during the first spring. At the end of the trial, the mean cumulative mortality of the L group (54.1%) was significantly lower than the mean cumulative mortality of the S group (74.8%), indicating the importance of size in OsHV-1-associated mortality. However, the overall effect of size was not consistent for all families: in 11 of the 40 families tested, there appeared to be no difference in mortality between the S and L groups. Observations at the family level allowed the identification of a large range of susceptibility to OsHV-1 regardless of the size group, with highly resistant families (< 32%) and highly susceptible families (> 80%). The second investigation compared different growing heights. Each of the 40 families was grown at three heights (high Hi, medium Me, and low Lo) corresponding to mean emersion times of 25, 12 and 2%, respectively. After an OsHV-1-related mortality outbreak, the mean mortality among the families was 91.6%, 88.0% and 86.9% for the Lo, Me and Hi conditions, respectively. Growing height had no effect in 31 of the 40 families tested, and these families showed no potential for adaptation to improve growing height practices. Among them, 24 families showed extremely high mortality (> 95%) and 2 families showed the lowest mortality (< 40%). In contrast, nine families showed differences in mortality associated with growing heights. A tendency toward higher mortality was observed for oysters that emerged less, with 85% for Lo compared to 72% for Hi. A protective effect of a high growing height on spat oysters was confirmed. In both experiments, higher relative growth rates were associated with higher mortality by OsHV-1. Finally, we identified oyster families from the two husbandry practices that were resistant or susceptible to OsHV-1 infection regardless of their size, growing height and age. Finally, a moderate heritability of mortality, ranging from 0.44 to 0.49, was determined from the size investigation. Statement of relevance Husbandry practices can decrease OsHV-1 mortality in C gigas, but it depends on the genetic resistance of the oyster stocks used. [ABSTRACT FROM AUTHOR]

The microvariant genotype of Ostreid herpes virus 1 (OsHV-1 μVar) has caused repeated seasonal outbreaks of mass mortality in Pacific oyster ( Crassostrea gigas ) populations in Europe, New Zealand and in New South Wales, Australia. The first occurrence of OsHV-1 related mortality in Tasmania, Australia was in January 2016; mortality was documented and multivariable logistic regression analysis was undertaken to examine risk factors. Overall mortality was 78% but varied from 37%–92% across 6 farms and between 96% (0–20 mm length) and 33% (61–115 mm) across size classes. The odds of mortality were 3.2 to 3.8 times greater for baskets containing oysters with an average shell length ≤ 40 mm compared to those with an average > 61 mm. Generally, the greater the time on farm the lower the mortality, consistent with the increasing age and size of the oysters. The effect of stocking density on mortality changed with increasing time on farm (age). The standard density produced lower mortality than high or low stocking density particularly for spat. Mortality was nearly twice as likely when oysters were handled for routine husbandry in the 7 days prior to the outbreak compared to not handled. Ploidy and hatchery of origin did not alter the disease outcome. The variation in mortality with husbandry requires further investigation to determine if it can be exploited to help maintain productivity. Statement of relevance This descriptive epidemiological study documents the impact of the first occurrence of disease associated with Ostreid herpesvirus - 1 (OsHV-1) in Tasmania, Australia. OsHV-1 is an emerging and economically important pathogen. Describing the impact of OsHV-1 in new environments and identifying the association between farm management and mortality is important to develop effective disease control measures. [ABSTRACT FROM AUTHOR]

Ostreid herpesvirus-1 (OsHV-1) microvariant presents a significant threat to the commercial production of Crassostrea gigas worldwide. Investigating the transmission of the virus in the estuarine environment where oysters are grown has proven to be difficult due to the complexity of the ecosystem. The aims of this trial were to (1) investigate the distribution of OsHV-1 in seawater over time in a recirculation system during an experimental disease outbreak; (2) assess the effects of the biofiltration and ultraviolet light components of the recirculation system on the detection of OsHV-1; and (3) assess the potential for OsHV-1 to associate with particles in the artificial seawater matrix. The source of virus was experimentally infected donor and cohabitating recipient oysters. Both the presence of infected oysters and the biofilter and UV light source significantly affected the detection rate of OsHV-1 in seawater. It was detected more frequently after high dose exposure (n = 6 donors per tank) compared with low dose exposure (n = 2 donors per tank), and when the biofilter and UV were disconnected from the treatment tanks. The odds of detecting OsHV-1 in the treatment tanks, which were periodically disconnected from the biofilter and UV, were 2.56 that of the system sumps which were always connected to the biofilter and UV. It may not be possible to remove OsHV-1 completely from the seawater of a recirculation system during a disease event using biofiltration and UV irradiation alone. The presence of food did not significantly affect the detection rate of OsHV-1 in seawater. The pellet produced by low speed centrifugation of seawater had ~ 2 to 5 fold greater odds of being positive for OsHV-1 by qPCR, in comparison to unprocessed seawater and the supernatant fraction. These results are consistent with the hypothesis that OsHV-1 may be attached to particles, which in this model may include algal feed, oyster faeces, fragments of oyster tissues or microorganisms introduced to the system with the oysters, but confirmation of this is required. Statement of relevance It is important to understand the nature and persistence of OsHV-1 microvariant within seawater in order to undertake aquaculture in hatchery environments, where water reticulation, recirculation, biofiltration, UV and other water treatments may be used. Understanding the distribution and mode of transmission of OsHV-1 within seawater has proven difficult. Experimental studies in controlled environments are important to furthering this understanding. [ABSTRACT FROM AUTHOR]

The ostreid herpesvirus 1 (OsHV-1) is one of the major pathogens affecting the Pacific oyster Crassostrea gigas , and numerous mortality outbreaks have been observed worldwide. We report the first results of our experimental breeding program using mass selection focused on survival and resistance to OsHV-1 after four generations of selection for two lines. These lines originated from two stocks of adult wild oysters sampled from the Marennes-Oléron Bay in 2008. Each line was spawned in February 2009 to produce the base populations. Both lines were then either protected from OsHV-1 or tested in the field in 2009 where they were exposed to OsHV-1. For each line during 2010 to 2013, one generation per year was produced using either the survivors of the previous generation for the selected group or the oysters protected from OsHV-1 for the control group. After one generation of selection (G 1 ) for both lines, the mean survival of the selected group was 34.5% compared with 12.3% in the control group. For the fourth generation of selection (G 4 ), the survival of the selected group reached 69.0% and the survival of the control group was 7.3%. The gain in survival of the selected C. gigas spat over the control increased by 22.2%, 43.9%, 50.2% and 61.8% for the G 1 , G 2 , G 3 and G 4 generations, respectively. Our study demonstrates that mass selection for survival and OsHV-1 resistance was successful after four generations of selection, thus indicating a significant genetic improvement for the selected trait. A genotype × size interaction was observed with 55.1% of survival in G 4 when selected spat were transferred at 1 g versus 89.9% of survival when they were transferred at 3 g. Our study is the first to provide some estimates of the realized heritability for disease resistance using a mass selection scheme in an oyster species with values ranging from 0.34 to 0.63 depending on the size of the oysters exposed to OsHV-1. Oysters selected for their higher resistance to OsHV-1 infection in G 4 showed higher growth (58.4 mm – 19.4 g) than controls (51.4 mm – 15.2 g), and mass selection had significantly improved the yield for the selected oysters (13.3 kg) over the controls (1.2 kg). Mass selection could be easily implemented by a commercial hatchery that cannot afford family-based selection that requires the production of numerous families for the base population. Statement of relevance Strong impact on breeding program working on OsHV-1 resistance in C. gigas. [ABSTRACT FROM AUTHOR]

Mortalities of oyster seed of Crassostrea gigas associated with ostreid herpes virus OsHV-1 μVar have been observed in many oyster producing countries since 2008. The objective of this study was to further investigate whether low temperature treatments can offer a viable option to mitigate oyster mortalities. An experiment was set-up to further evaluate the effect of low temperature treatments (10 and 13 °C vs. 21 °C) and their duration (6 d to 83 d) on the survival of oysters previously infected with OsHV-1 μVar by means of exposure to field conditions in areas where mortalities were occurring. Average survival of oysters infected with OsHV-1μVar was 71% after 83 d at low temperatures compared to only 23% in controls maintained at 21 °C. During cold-exposure, levels of OsHV-1 DNA in oyster tissues gradually decreased, down to nearly the detection limit after 24 d. However, when cold-acclimated oysters were suddenly exposed at 21 °C in the laboratory, they exhibited high levels of mortality along with an enhancement of OsHV-1 DNA concentration in their tissues. Therefore, OsHV-1 persists in oysters even at low temperature and is reactivated during subsequent thermal elevation to 21 °C. Low temperature treatments did not improve overall survival of oyster seed infected with OsHV-1. These results suggest that moving infected oysters to a cooler area only delays mortality and may increase the risk of infection in neighbouring stocks when rising temperatures become permissive for viral replication. Statement of relevance: This study is relevant for building disease control measures. [ABSTRACT FROM AUTHOR]

Since 2008, mass mortality events of Crassostrea gigas have been occurring along the French coast when seawater temperature exceeds 16 °C. These mortality events are related to a particular genotype of the ostreid herpesvirus named OsHV-1 μVar. The present study aimed to detail various physiological aspects underlying the onset of the disease. In the laboratory, both exposed (infected) and naïve (healthy) oysters were maintained at 13.0 °C and 20.6 °C. These temperatures were respectively lower and higher than the threshold values of 16 °C at which the disease generally occurs. At 20.6 °C, exposed oysters were characterized by a reduction in energetic reserves (carbohydrates and triglycerides) together with a decrease in protein content. Sterol levels were lower in exposed oysters than in naïve individuals, irrespective of temperature. Finally, activities of some key enzymes related to energetics were similar in exposed and naïve oysters and did not change with temperature. This result suggests that although energetic reserves were being diminished in infected oysters, their metabolic activities remained similar to that of healthy animals. [ABSTRACT FROM AUTHOR]

Abstract: French oyster farming has been experiencing severe mortalities since 2008 in Crassostrea gigas spat and juveniles mainly during the summer months. This recent phenomenon can be explained by the involvement of many factors including pathogenic agents such as the Ostreid Herpesvirus 1 (OsHV-1). From July 2008 to September 2010, 424 oyster samples received from French oyster farmers were tested to follow the concentration of OsHV-1 by quantitative real-time PCR. Comparisons of OsHV-1 loads were also made according to oyster age, ploidy, origin (hatchery-bred or wild-collected) and sample status (healthy or dying). The survey revealed that high viral loads detected in spat and juvenile oyster tissues were associated with summer mortality occurrence. No significant differences were observed in pathogen detection in relation to oyster ploidy and origin. Finally, this study allowed us to define a viral load threshold of 8.8×103 of OsHV-1 DNA per mg of tissue, which can be useful to oyster farmers in evaluating mortality risk during the different stages of oyster handling. [Copyright &y& Elsevier]

Ostreid herpesvirus 1 (OsHV-1) and Vibrio aestuarianus are the two main pathogens affecting the production of French oyster (Crassostrea gigas). The absence of genetic correlation between the two diseases is promising for the development of stocks with dual resistance. Using unselected and selected oysters concerning enhanced resistance to OsHV-1 infection, we investigated the first generation of mass selection and the response to selection to increase the resistance to V. aestuarianus for two stocks. For each stock, four groups were produced in June 2013 using either parents unchallenged with the bacteria or counterparts that survived experimental infections by the bacteria. Thus, groups were unselected oysters for both pathogens, selected for either the virus or the bacteria, and dually selected for both pathogens. All groups of each stock were evaluated at the spat and juvenile sizes following experimental infection by V. aestuarianus in May 2014. Regardless of their level of selection for OsHV-1, oysters produced from parents that survived V. aestuarianus showed similar mortalities (47% and 53% for stocks A and B, respectively) during the bacterial challenge than those produced from unchallenged parents (43% and 56%, respectively). Thus, no positive response to selection at the first generation to increase the bacterial resistance was found at the spat and juvenile sizes. At the adult stage and with experimental infection with V. aestuarianus , only stock B showed a positive response to selection for increasing the bacterial resistance with a decrease in mortality of 14% in comparison with unselected oysters. Similar results were observed when oysters were tested for 27 months in the field with the absence of response to selection for stock A, while a 13% decrease in mortality was observed for stock B. For stock B, mortality at endpoint after 27 months in the field reached 89% for the control not selected at all, decreased to 84% for oysters selected for their resistance to the bacteria, was 53% for the oysters selected for their resistance to OsHV-1, and was 32% for oysters selected for dual resistance. Realized heritability estimated at the first generation of mass selection for stock B was higher for oysters selected for dual resistance, ranging from 0.47 to 0.80, than those only selected for V. aestuarianus , ranging from 0.05 to 0.30. Selection for dual resistance in C. gigas could limit the impact of both OsHV-1 and V. aestuarianus on oyster production. • This is the first investigation of mass selection to enhance resistance to Vibrio aestuarianus in Crassostrea gigas. • Only one stock showed a positive response to selection. • Stock B showed a gain of survival of +14% for experimental infection at the adult stage. • Dual selection for OsHV-1 and the bacteria increased the field survival of C. gigas from 11% to 68% for stock B. [ABSTRACT FROM AUTHOR]