Gaita, Vincenzo Emerging infections in the Mediterranean mussel Mytilus galloprovincialis in Campania region (Italy). [Tesi di dottorato]

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Abstract

Emerging Infectious Diseases (EIDs) have been reported affecting several marine organisms in the past years however EIDs of invertebrates have to date been poorly described, due in part to a lack of interest and no expert in the field. Historical data have shown the constant presence of undescribed pathogens belonging to the group of microsporidia and trematode in M. galloprovincialis in Campania Region, although scarce are the information in literature about their emergence and effect on the host. Microsporidia are a spore-forming, obligate, intracellular parasites infecting all major taxa in all environments. Of the 187 genera described, almost half are known to infect aquatic organisms, mainly teleost and crustaceans. In molluscs main descriptions are related to one genus (Steinhausia sp.) reported in bivalves’ and gastropods. The species S. mytilovum and S. ovicola affect bivalves belong to the families Mytilidae and Ostreidae, with a Steinhausia-like parasite being described in the cockle Cerastoderma edule. Major species descriptions include detailed microscopic and ultrastructural images, but no information is present about pathogen phylogeny. In this study we report the constant occurrence of the microsporidian parasite S. mytilovum affecting the population of mussel, M. galloprovincialis, in one natural bed and 13 farms along Campania region coastline (Italy). S. mytilovum affected 9 out of the 13 farms, with a prevalence ranging from 2-30% of the females, depending on the areas and the seasons. The parasite develops at oocyte level within a sporophorous vesicle where it produces a spore in a number of one, three per cell, at cytoplasm and at nuclear level. S. mytilovum elicited an infiltrative (24,8%) or a strong capsular inflammatory response (43,4%) at gonadal follicles and surrounding vesicular connective tissue, in some case accompanied by gonadal atresia (24,8%), eventually associated to loss of gonadal architecture. In other cases (7%) no reaction was observed. Microsporidian systematics currently turn around five taxonomic clades, identified by SSU ribosomal gene sequence data. Neighbor Joining of the 18 S assigned S. mytilovum in a separated branch within the Clade V, defined as the Class Terresporidia, with closest genetic relationship (84% identity) to an undermined invertebrate’s ovarian microsporidian. The second part of the study have identified the systematic presence of metacercariae belonging to the group of helminth trematodes at the foot level of 323 examined mussels. Trematodes can have a variety of target tissues, which can result in serious damage in the mollusc and major economic loss. The metacercaria phase of trematodes infections in bivalves is the most well-documented and affects several organs and tissues, particularly the gonads and gills. Recently, many mussels have shown the presence of inflammatory lesions at the foot level linked to the presence of trematodes metacercariae or unidentified amorphous structures. To date, there is a single report of trematodes with the same tropism, in particular in Atlantic mussels (M. edulis) in the waters of Great Britain and Ireland and reported to belong to the group of Echinostomates digenea, and to the species Echinostephilla patellae. The characterization of the parasites has been carried first with a macroscopic identification, the mussels’ foot was separated and cut in half, revealing occasionally the presence of an encysted parasite. Light microscopy has shown the presence of metacercariae in mussels’ foot in all the sampled areas in 27 out of 323 animals, in a number 1-5 per animal, with a prevalence ranging from 3,3% to 16,67%, highest prevalence was observed in warm months. The presence of the parasite resulted in three different type of lesions: inflammatory reaction associated both to metacercariae and cercariae and nodular lesions. Molecular diagnostic carried out on the 18 S rDNA showed an identity of 94.2% in genebank with digenean trematodes of the genus Opisthomonorcheides spp.

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