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

Massive mortality outbreaks in oysters have been globally reported with heavy economic and social impact on Pacific oyster production.
Several studies highlighted the multifactorial triggers of mortality including environmental and physiological factors (i.e water temperature and oyster genetics) and pathogens as Ostreid Herpesvirus-1 (OsHV-1) and Vibrio species.
OsHV-1 has been associated with significant mortality in Pacific oyster and was often co-detected with V. aestuarianus. Moreover, the potential role of V. aestuarianus in mass mortality events has been only marginally evaluated. In Sardinia, oyster farming represents a developing business and recently mortality episodes caused important economic losses.
The aim of this study was to evaluate the occurrence and related lesions in target organs of OsHV-1, V. aestuarianus and V. splendidus in Pacific 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 specimens from San Teodoro, 59 from Tortolì, 10 from Marceddì, and 13 from Calich), during 2016, 2017 and 2018 years. In particular, oysters affected by V. aestuarianus 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%), mostly with a multifocal distribution, in connective tissue and hemolympahic vessels.
OsHV-1 was not detected in oyster from San Teodoro but only in Tortolì lagoon (47%). However, no histopathological lesions were associated with OsHV-1.
V. splendidus was detected mostly in oysters from Marceddì lagoon (90%) without tissue lesions.

Overall, a relationship between V. aestuarianus bacterial load, degree of hemocytes infiltration in the mantle tissue and mortality episodes in oysters was demonstrated.
To the best of our knowledge, differently from what have been reported by reference studies, that do not support a primary effect of V. aestuarianus in oyster mortality events, we suggest a potential role of V. aestuarianus in oysters mortality in one of the most devoted lagoon to extensive aquaculture in Sardinia.