In situ hybridization detection of initial infective stages of Marteilia refringens (Paramyxea) in its host Mytilus galloprovincialis

Marteilia pararefringens infections are more frequent than revealed by the Norwegian surveillance programme, highlighting the need for its improvement

Norway had historically been considered free of marteiliosis in bivalves since the disease surveillance programme began in 1995. However, in 2016, Marteilia pararefringens, a protistan parasite of mussels Mytilus spp., was described in a heliothermic lagoon-a poll-previously used to produce flat oyster spat. To study whether the parasite was introduced, and possibly spread, via the historical flat oyster networks on the south and west coast, we sampled aquaculture polls that were part of different networks of farmers and wild, natural polls with no aquaculture activity. Additionally, we sampled mussel banks influenced by polls and sheltered bays that could have a similar environment to that of polls. We identified 7 sites with M. pararefringens-infected mussel populations: 5 were polls used in flat oyster production and 2 were in fjord areas with no known connection to any bivalve aquaculture. Prevalence ranged between 2 and 88%. At one site, Trysfjorden, we found M. pararefringens in atypical organs, including the gills, mantle, and intestine. Marteilia-like cells were also observed in the epithelium, lumen, and surrounding connective tissue of metanephridia and in the sinus of the anterior retractor muscle. Our results demonstrate that the parasite is more widespread than previously thought and is neither isolated to polls nor connected directly to aquaculture activity. Lastly, our findings highlight the need for an improved sampling strategy in surveillance programmes to detect marteiliosis in mussels.

Species which may act as vectors or reservoirs of diseases covered by the Animal Health Law: Listed pathogens of molluscs

Vector or reservoir species of five mollusc diseases listed in the Animal Health Law were identified, based on evidence generated through an extensive literature review, to support a possible updating of Regulation (EU) 2018/1882. Mollusc species on or in which Mikrocytos mackini, Perkinsus marinus, Bonamia exitiosa, Bonamia ostreae and Marteilia refringens were detected, in the field or during experiments, were classified as reservoir species with different levels of certainty depending on the diagnostic tests used. Where experimental evidence indicated transmission of the pathogen from a studied species to another known susceptible species, this studied species was classified as a vector species. Although the quantification of the risk of spread of the pathogens by the vectors or reservoir species was not part of the terms of reference, such risks do exist for the vector species, since transmission from infected vector species to susceptible species was proven. Where evidence for transmission from infected molluscs was not found, these were defined as reservoir. Nonetheless, the risk of the spread of the pathogens from infected reservoir species cannot be excluded. Evidence identifying conditions that may prevent transmission by vectors or reservoir mollusc species during transport was collected from scientific literature. It was concluded that it is very likely to almost certain (90-100%) that M. mackini, P. marinus, B. exitiosa B. ostreae and M. refringens will remain infective at any possible transport condition. Therefore, vector or reservoir species that may have been exposed to these pathogens in an affected area in the wild or at aquaculture establishments or through contaminated water supply can possibly transmit these pathogens. For transmission of M. refringens, the presence of an intermediate host, a copepod, is necessary.

Transcriptional Alteration of Gene Biomarkers in Hemocytes of Wild Ostrea edulis with Molecular Evidence of Infections with Bonamia spp. and/or Marteilia refringens Parasites

The European flat Ostrea edulis is highly susceptible to intracellular parasitic infections, particularly bonamiosis and marteiliosis. The defensive response of oyster to both bonamiosis and marteiliosis is typically mediated by hemocytes, which play a pivotal role in immune system homeostasis. In the present study, we first used a DNA-based tool in order to rapidly and specifically detect the presence of parasites in oysters from natural banks in the middle Adriatic Sea. In a second step, we used qRT-PCR to analyze the mRNA levels of a set of genes (i.e., superoxide dismutase (SOD), glutathione S-transferase (GST), metallothionein (MT), heat shock protein (HSP) 70 and 90, inhibitor of apoptosis (IAP), fas ligand (FAS), galectin (GAL) and extracellular superoxide dismutase (Ec-SOD)) expressed by hemocytes of flat oysters infected by the parasites, present singularly or in combination, compared to hemocytes from non-infected specimens. The results indicate that the presence of parasite DNA may be associated to a general upregulation of host genes related to apoptosis, detoxification and oxidative stress protection, with the exception of Ec-SOD, whose trend to a downregulation might reflect a mechanism for parasite escape before internalization.

Detection and characterisation of haplosporidian parasites of the blue mussel Mytilus edulis, including description of the novel parasite Minchinia mytili n. sp

The edible mussel Mytilus edulis is a major aquaculture commodity in Europe, with 168000 t produced in 2015. A number of abundant, well characterised parasites of the species are known, though none are considered to cause significant mortality. Haplosporida (Rhizaria, Endomyxa) is an order of protistan parasites of aquatic invertebrates, the best studied of which are the oyster pathogens Haplosporidium nelsoni and Bonamia ostreae. While these species are well characterised within their hosts, the diversity, life-cycle and modes of transmission of haplosporidians are very poorly understood. Haplosporidian parasites have previously been reported from Mytilus spp., however the majority of these remain uncharacterised, and no molecular data exist for any species. In this study, we identified 2 novel haplosporidian parasites of M. edulis present in the UK. The first of these, observed by light microscopy and in situ hybridisation infecting the gills, mantle, gonadal tubules and digestive connective tissues of mussels in the Tamar estuary, England, we describe as Minchinia mytili on the basis of 18S sequence data. The second, observed infecting a single archive specimen collected in Loch Spelve, Mull, Scotland, infects the foot muscle, gills and connective tissue of the digestive gland. Sequence data places this parasite in an uncharacterised clade of sequences amplified from tropical bivalve guts and water samples, sister to H. nelsoni. Screening of water and sediment samples collected at the sample site in the Tamar estuary revealed the presence of both sequence types in the water column, suggesting host-free or planktonic life stages.

Hyperspora aquatica n.gn., n.sp. (Microsporidia), hyperparasitic in Marteilia cochillia (Paramyxida), is closely related to crustacean-infecting microspordian taxa

The Paramyxida, closely related to haplosporidians, paradinids, and mikrocytids, is an obscure order of parasitic protists within the class Ascetosporea. All characterized ascetosporeans are parasites of invertebrate hosts, including molluscs, crustaceans and polychaetes. Representatives of the genus Marteilia are the best studied paramyxids, largely due to their impact on cultured oyster stocks, and their listing in international legislative frameworks. Although several examples of microsporidian hyperparasitism of paramyxids have been reported, phylogenetic data for these taxa are lacking. Recently, a microsporidian parasite was described infecting the paramyxid Marteilia cochillia, a serious pathogen of European cockles. In the current study, we investigated the phylogeny of the microsporidian hyperparasite infecting M. cochillia in cockles and, a further hyperparasite, Unikaryon legeri infecting the digenean Meiogymnophallus minutus, also in cockles. We show that rather than representing basally branching taxa in the increasingly replete Cryptomycota/Rozellomycota outgroup (containing taxa such as Mitosporidium and Paramicrosoridium), these hyperparasites instead group with other known microsporidian parasites infecting aquatic crustaceans. In doing so, we erect a new genus and species (Hyperspora aquatica n. gn., n.sp.) to contain the hyperparasite of M. cochillia and clarify the phylogenetic position of U. legeri. We propose that in both cases, hyperparasitism may provide a strategy for the vectoring of microsporidians between hosts of different trophic status (e.g. molluscs to crustaceans) within aquatic systems. In particular, we propose that the paramyxid hyperparasite H. aquatica may eventually be detected as a parasite of marine crustaceans. The potential route of transmission of the microsporidian between the paramyxid (in its host cockle) to crustaceans, and, the 'hitch-hiking' strategy employed by H. aquatica is discussed.

Characterization of the protozoan parasite Marteilia refringens infecting the dwarf oyster Ostrea stentina in Tunisia

Marteilia refringens is a protozoan parasite recognized as a significant pathogen of the European flat oyster Ostrea edulis. The life cycle of this species is still poorly known, although there is evidence of the need for intermediate host(s). In the present study, we have used molecular approaches to identify this parasite in samples of the dwarf oyster Ostrea stentina after reports of massive mortality along the Tunisian coasts. In 2009 we evaluated the status of O. stentina from Monastir and checked if there was an infection with M. refringens, using polymerase chain reaction assays. Of the 103 tested O. stentina, 85 were PCR-positive using a Marteilia genus-specific assay. Additional assays were subsequently carried out on some samples collected in 2010 in Monastir and processed for histology, transmission electron microscopy and complementary molecular analyses. PCR was carried out to amplify the IGS and ITS regions. Histological and transmission electron microscopy analyses allowed us to confirm the presence of this parasite in the digestive gland tissue of O. stentina and to characterize it at the ultrastructural level. This is the first record of the occurrence of M. refringens in the oyster O. stentina along the Tunisian coasts.