Vibrio aestuarianus subsp. francensis has been identified as a pathogenic agent responsible for recurring mortality in adult Pacific cupped oysters (Magallana gigas) in several countries. However, studies attribute these mortalities to the interaction of environmental factors and pathogens. Little information available in the literature highlights the need to clarify the mechanisms of action of V. aestuarianus in M. gigas . The objective of this study was to apply an in situ hybridization (ISH) system (RNAscope®) to detect the presence of Vibrio spp. in M. gigas.
Twenty−eight oysters (M. gigas) were manually collected during a mortality event (October 2016−March 2017) from San Teodoro lagoon , one of Sardinia’ s leading oyster culture facilities, with a water temperature of 14°C. The collected samples were investigated by molecular, histopathological, and in situ hybridization (ISH) techniques.
In 29% of the oysters , the mantle exhibited a moderate to severe, nodular to multifocal hemocytic inflammatory infiltrate associated with a concentration of V. aestuarianus greater than 104 copies/µL in qPCR. ISH demonstrated the presence of Vibrio spp. in 78% of the subjects associated with an inflammatory process in the mantle and gills, with a stronger and diffuse signal in oysters displaying moderate to severe inflammation (r=0.66, p<0.05) ( Figure 1). Our results suggest that V . aestuarianus is a major contributor to oysters mortality.