Detection and characterization of Bonamia ostreae in Ostrea edulis imported to China

Robinson A. Co-designing and building an expert-elicited non-parametric Bayesian network model: demonstrating a methodology using a Bonamia Ostreae spread risk case study

The development and use of probabilistic models, particularly Bayesian networks (BN), to support risk-based decision making is well established. Striking an efficient balance between satisfying model complexity and ease of development requires continuous compromise. Codesign, wherein the structural content of the model is developed hand-in-hand with the experts who will be accountable for the parameter estimates, shows promise, as do so-called nonparametric Bayesian networks (NPBNs), which provide a light-touch approach to capturing complex relationships among nodes. We describe and demonstrate the process of codesigning, building, quantifying, and validating an NPBN model for emerging risks and the consequences of potential management decisions using structured expert judgment (SEJ). We develop a case study of the local spread of a marine pathogen, namely, Bonamia ostreae. The BN was developed through a series of semistructured workshops that incorporated extensive feedback from many experts. The model was then quantified with a combination of field and expert-elicited data. The IDEA protocol for SEJ was used in its hybrid (remote and face-to-face) form to elicit information about more than 100 parameters. This article focuses on the modeling and quantification process, the methodological challenges, and the way these were addressed.

What do the terms resistance, tolerance, and resilience mean in the case of Ostrea edulis infected by the haplosporidian parasite Bonamia ostreae

The decline of the European flat oyster Ostrea edulis represents a loss to European coastal economies both in terms of food security and by affecting the Good Environmental Status of the marine environment as set out by the European Council's Marine Strategy Framework Directive (2008/56/EC). Restoration of O. edulis habitat is being widely discussed across Europe, addressing key challenges such as the devastating impact of the haplosporidian parasite Bonamia ostreae. The use of resistant, tolerant, or resilient oysters as restoration broodstock has been proposed by restoration practitioners, but the definitions and implications of these superficially familiar terms have yet to be defined and agreed by all stakeholders. This opinion piece considers the challenges of differentiating Bonamia resistance, tolerance, and resilience; challenges which impede the adoption of robust definitions. We argue that, disease-resistance is reduced susceptibility to infection by the parasite, or active suppression of the parasites ability to multiply and proliferate. Disease-tolerance is the retention of fitness and an ability to neutralise the virulence of the parasite. Disease-resilience is the ability to recover from illness and, at population level, tolerance could be interpreted as resilience. We concede that further work is required to resolve practical uncertainty in applying these definitions, and argue for a collaboration of experts to achieve consensus. Failure to act now might result in the future dispersal of this disease into new locations and populations, because robust definitions are important components of regulatory mechanisms that underpin marine management.

Haplosporidian host:parasite interactions

The host:parasite interactions of the 3 serious haplosporidian pathogens of oysters, on which most information exists, are reviewed. They are Bonamia ostreae in Ostrea spp. and Crassostrea gigas; Bonamia exitiosa in Ostrea spp.; and Haplosporidium nelsoni in Crassostrea spp. Understanding the haemocytic response to pathogens is constrained by lack of information on haematopoiesis, haemocyte identity and development. Basal haplospridians in spot prawns are probably facultative parasites. H. nelsoni and a species infecting Haliotis iris in New Zealand (NZAP), which have large extracellular plasmodia that eject haplosporosomes or their contents, lyse surrounding cells and are essentially extracellular parasites. Bonamia spp. have small plasmodia that are phagocytosed, haplosporosomes are not ejected and they are intracellular obligate parasites. Phagocytosis by haemocytes is followed by formation of a parasitophorous vacuole, blocking of haemocyte lysosomal enzymes and the endolysosomal pathway. Reactive oxygen species (ROS) are blocked by antioxidants, and host cell apoptosis may occur. Unlike susceptible O. edulis, the destruction of B. ostreae by C. gigas may be due to higher haemolymph proteins, higher rates of granulocyte binding and phagocytosis, production of ROS, the presence of plasma β-glucosidase, antimicrobial peptides and higher levels of haemolymph and haemocyte enzymes. In B.exitiosa infection of Ostrea chilensis, cytoplasmic lipid bodies (LBs) containing lysosomal enzymes accumulate in host granulocytes and in B. exitiosa following phagocytosis. Their genesis and role in innate immunity and inflammation appears to be the same as in vertebrate granulocytes and macrophages, and other invertebrates. If so, they are probably the site of eicosanoid synthesis from arachidonic acid, and elevated numbers of LBs are probably indicative of haemocyte activation. It is probable that the molecular interaction, and role of LBs in the synthesis and storage of eicosanoids from arachidonic acid, is conserved in innate immunity in vertebrates and invertebrates. However, it seems likely that haplosporidians are more diverse than realized, and that there are many variations in host parasite interactions and life cycles.

Low internal transcribed spacer rDNA variation in New Zealand Bonamia ostreae: evidence for a recent arrival

Bonamia ostreae is a haplosporidian parasite of oysters that was first reported to occur in the Southern Hemisphere in 2015 in the New Zealand flat oyster Ostrea chilensis. Until that report, B. ostreae had been restricted to populations of O. edulis within the Northern Hemisphere. This large range extension raised questions regarding B. ostreae dispersal, including whether B. ostreae is a recent introduction and from where it originated. The whole 18S rRNA gene of New Zealand B. ostreae revealed 99.9-100% sequence homology to other published B. ostreae 18S rDNA sequences. Internal transcribed spacer (ITS) rDNA sequences (n = 29) were generated from New Zealand B. ostreae and compared to published B. ostreae sequences from 3 Northern Hemisphere sites: California, USA (n = 18), Maine, USA (n = 7), and the Netherlands (n = 6) to investigate intraspecific variation. Low ITS rDNA variation was observed from New Zealand B. ostreae isolates, and high levels of variation were observed from Northern Hemisphere B. ostreae sequences. We hypothesise that the low ITS rDNA diversity found in New Zealand B. ostreae is the result of a founder effect resulting from a single introduction from a limited number of propagules. The high level of ITS rDNA variation from the Northern Hemisphere prevented inferences of dispersal origins. New Zealand B. ostreae were genetically differentiated from all sites, and additional genetic data are required to better determine the origin of B. ostreae in New Zealand.

Infection dynamics of Bonamia exitiosa on intertidal Ostrea angasi farms

Bonamia spp. cause epizootics in oysters worldwide. In southern Australia, Bonamia exitiosa Hine, Cochennac and Berthe, 2001 threatens aquaculture of Ostrea angasi Sowerby, 1871. Bonamia spp. infections can display strong seasonality, but seasonal dynamics of B. exitiosa-O. angasi are unknown. Ostrea angasi naïve to B. exitiosa infection were stocked onto farms in three growing regions, and B. exitiosa was monitored seasonally for one year. Environmental parameters we measured did not correlate with B. exitiosa prevalence or infection intensities. Extreme temperatures suggest O. angasi culture systems need development. Bonamia exitiosa prevalence increased over time. After three months, O. angasi had B. exitiosa prevalence of 0.08-0.4, and after one year, the prevalence was 0.57-0.88. At some sites, O. angasi had >0.5 B. exitiosa prevalence in >6 months, but at other sites, >9 months passed before prevalence was >0.5. Bonamia exitiosa infection intensities were low with no seasonal pattern but were affected by the interaction of site, season and oyster meat:shell ratio. Understanding infection and initiating a breeding programme for resistance would provide benefits for O. angasi industry expansion.

Microbial Diseases of Bivalve Mollusks: Infections, Immunology and Antimicrobial Defense

A variety of bivalve mollusks (phylum Mollusca, class Bivalvia) constitute a prominent commodity in fisheries and aquacultures, but are also crucial in order to preserve our ecosystem’s complexity and function. Bivalve mollusks, such as clams, mussels, oysters and scallops, are relevant bred species, and their global farming maintains a high incremental annual growth rate, representing a considerable proportion of the overall fishery activities. Bivalve mollusks are filter feeders; therefore by filtering a great quantity of water, they may bioaccumulate in their tissues a high number of microorganisms that can be considered infectious for humans and higher vertebrates. Moreover, since some pathogens are also able to infect bivalve mollusks, they are a threat for the entire mollusk farming industry. In consideration of the leading role in aquaculture and the growing financial importance of bivalve farming, much interest has been recently devoted to investigate the pathogenesis of infectious diseases of these mollusks in order to be prepared for public health emergencies and to avoid dreadful income losses. Several bacterial and viral pathogens will be described herein. Despite the minor complexity of the organization of the immune system of bivalves, compared to mammalian immune systems, a precise description of the different mechanisms that induce its activation and functioning is still missing. In the present review, a substantial consideration will be devoted in outlining the immune responses of bivalves and their repertoire of immune cells. Finally, we will focus on the description of antimicrobial peptides that have been identified and characterized in bivalve mollusks. Their structural and antimicrobial features are also of great interest for the biotechnology sector as antimicrobial templates to combat the increasing antibiotic-resistance of different pathogenic bacteria that plague the human population all over the world.

Legislative and regulatory aspects of molluscan health management

The world population is growing quickly and there is a need to make sustainable protein available through an integrated approach that includes marine aquaculture. Seafood is already a highly traded commodity but the production from capture fisheries is rarely sustainable, which makes mollusc culture more important. However, an important constraint to its continued expansion is the potential for trade movements to disseminate pathogens that can cause disease problems and loss of production. Therefore, this review considers legislative and regulatory aspects of molluscan health management that have historically attempted to control the spread of mollusc pathogens. It is argued that the legislation has been slow to react to emerging diseases and the appearance of exotic pathogens in new areas. In addition, illegal trade movements are taken into account and possible future developments related to improvements in areas such as data collection and diagnostic techniques, as well as epidemiology, traceability and risk analysis, are outlined.