Scottish oyster mortality event and association with Vibrio aestuarianus

Pacific oysters, Crassostrea ( Magallana ) gigas, are the most commonly cultured invertebrate species globally. Recent years have seen outbreaks of summer mortality across the globe, mainly caused by viral and bacterial pathogens such as OsHV-1 µvar and Vibrio spp. Despite isolated outbreaks in southern England, the UK has remained largely free from these pathogens. A summer mortality event in Scotland was investigated after Vibrio spp . were implicated. We identified two key Vibrio species ( V. aestuarianus and V. splendidus ) but complete absence of OsHV-1. There was a high prevalence of V. aestuarianus among moribund and dead oysters, along with an increase in V. aestuarianus detected in the water column and sediment. In accordance with strains associated with oyster mortalities elsewhere in Scotland and mainland Europe, V. aestuarianus from this site belongs to Lineage A. Whilst V. splendidus, could be cultured from most infected tissue samples, V. aestuarianus could not be cultured from from frozen tissue , only from unfrozen infected tissue. In vivo infection experiments with V. aestuarianus resulted in significantly higher mortalities than with V. splendidus and were exacerbated by increased temperature. These findings suggest that mortalities are caused by V. aestuarianus , which may have been intensified by handling stress and rising temperatures. • Vibrio aestuarianus appears to be causing repeat mortalities at Scottish oyster farm. • Combinations of bacteria cause different levels of mortality in lab disease challenges. • During mortality events a high prevalence of Vibrio aestuarianus is found among moribund and dead oysters.