Infectious agents of Pacific oysters (Crassostrea gigas) farmed in Sardinia: isolation, molecular characterization, tissue localization and inflammatory reaction

Mortality outbreaks in oysters have been globally reported with heavy economic and social impact on Pacific oyster production.The aim of this study was to evaluate the occurrence and related lesions in target organs of OsHV-1, V. aestuarianus andV. splendidusin oysters (Crassostrea gigas) farmed in Sardinia by a microbiological, biomolecular and histopathological approach.Four hundred and forty oyster samples were collected from Sardinian lagoons (358 from San Teodoro, 59 from Tortolì, 10 from Marceddì, and 13 from Calich), during 2016, 2017 and 2018 years. In particular,V. aestuarianuspositive oysters were sampled in the San Teodoro (85%) and in the Tortoli (12%) lagoons. Mantle was the most affected organ and the highest bacterial load was associated with a moderate to severe hemocytes infiltration (46%) in connective tissue and hemolympahic vessels.OsHV-1 was detected only in oysters from Tortolì lagoon (47%) and V. splendidus was detected mostly in oysters from Marceddì lagoon (90%), both without tissue lesions.Overall, a relationship betweenV. aestuarianusbacterial load, degree of hemocytes infiltration in the mantle tissue and mortality episodes was demonstrated.To the best of our knowledge, differently from what have been reported in literature, that do not support a primary effect ofV. aestuarianusin mortality events, we suggest a potential role ofV. aestuarianusin oysters mortality in one of the most devoted lagoon to extensive aquaculture in Sardinia.