Oyster disease in a changing environment: Decrypting the link between pathogen, microbiome and environment.

Shifting environmental conditions are known to be important triggers of oyster diseases. The mechanism(s) behind these synergistic effects (interplay between host, environment and pathogen/s) are often not clear, although there is evidence that shifts in environmental conditions can affect oyster immunity, and pathogen growth and virulence. However, the impact of shifting environmental parameters on the oyster microbiome and how this affects oyster health and susceptibility to infectious pathogens remains understudied. In this review, we summarise the major diseases afflicting oysters with a focus on the role of environmental factors that can catalyse or amplify disease outbreaks. We also consider the potential role of the oyster microbiome in buffering or augmenting oyster disease outbreaks and suggest that a deeper understanding of the oyster microbiome, its links to the environment and its effect on oyster health and disease susceptibility, is required to develop new frameworks for the prevention and management of oyster diseases.

PCR test to specifically detect the apicomplexan 'X' (APX) parasite found in flat oysters Ostrea chilensis in New Zealand.

Described here is a polymerase chain reaction (PCR) test to detect the apicomplexan-X (APX) parasite of a flat oyster species, Ostrea chilensis, endemic to New Zealand. The test primers target sequences in the in situ hybridisation probes identified to bind specifically to APX 18S rRNA and amplify a 723 bp DNA product. The test did not amplify 18S rRNA gene sequences of other apicomplexan species, including Toxoplasma gondii, Neospora caninum, Selenidium spp., Cephaloidophorida spp., Lecudina spp. and Thiriotia sp. Of 73 flat oysters identified by histology to be infected with APX at different severities, 69 (95%) tested PCR-positive. Failure to amplify an internal control indicated the presence of PCR inhibitors in the 4 PCR-negative samples. The high analytical sensitivity, specificity and speed of the PCR test should make it a useful tool for detecting APX.

Experimental infection of European flat oyster Ostrea edulis with ostreid herpesvirus 1 microvar (OsHV-1µvar): Mortality, viral load and detection of viral transcripts by in situ hybridization.

Ostreid herpesvirus 1 (OsHV-1) infections have been reported in several bivalve species. Mortality of Pacific oyster Crassostrea gigas spat has increased considerably in Europe since 2008 linked to the spread of a variant of OsHV-1 called µvar. In the present study we demonstrated that O. edulis juveniles can be infected by OsHV-1µvar when administered as an intramuscular injection. Mortality in the oysters injected with OsHV-1µvar was first detected 4 days after injection and reached 25% mortality at day 10. Moreover, the high viral load observed and the detection of viral transcripts by in situ hybridization in several tissues of dying oysters suggested that OsHV-1µvar was the cause of mortality in the O. edulis juveniles. This is therefore the first study to provide evidence about the pathogenicity of OsHV-1µvar in a species that does not belong to the Crassostrea genus. Additionally, we present a novel method to detect OsHV-1 transcripts in infected individuals' using in situ hybridization.

Herpesvirus infection in European flat oysters Ostrea edulis obtained from brood stocks of various geographic origins and grown in Galicia (NW Spain).

We evaluated differences in productive traits and disease susceptibility among Ostrea edulis stocks. We produced 4 to 5 families from each of 4 oyster populations (Irish, Greek and 2 Galician) in a hatchery. Spat corresponding to 19 different families were transferred to a raft in the Ría de Arousa (Galicia, Spain) for grow-out. Samples of each family were histologically processed every month for 2 yr. One of the pathological conditions disclosed by histological examination was characterised by the occurrence of numerous abnormal cells throughout the connective tissue of various organs, showing hypertrophied nuclei with marginated chromatin and a characteristic large intranuclear acidophilic inclusion. Ultrastructural examination showed that the abnormal cells contained herpesvirus-like particles. In situ hybridisation assay using a DNA probe specific for Ostreid herpesvirus 1 (OsHV-1) confirmed that the abnormal cells were infected by OsHV-1 or a closely related herpesvirus. All cases of this pathological condition, except one, were detected during the first year of grow-out; thus it was mostly restricted to juvenile stages. The disease was detected in oysters of each origin but it was not found in all families of each origin, thus suggesting significant parental influence in the susceptibility to this disease or significant influence of the infective status of the parents on the infection of the progeny (vertical transmission). This pathological condition was likely responsible for oyster mortality to some extent during the first year of grow-out.