| Summary: | This study was undertaken to investigate viral and bacterial infections in Pacific oysters in Ireland. In recent years, ostreid herpes virus-1 (OsHV-1) and variants, in particular OsHV-1 microVar (OsHV-1 µVar), and bacteria belonging to the genus Vibrio, specifically V. splendidus and V. aestuarianus, have become a major problem for the Pacific oyster industry. Both pathogen groups have been associated with significant mortalities, yet little is known about how OsHV-1 µVar and Vibrio spp. are sustained in the environment outside this host species and what impact these pathogens may be having on the wider ecosystem health. Therefore, the overall focus of this study was to investigate and gain a more comprehensive understanding of how these viruses and bacteria might be establishing themselves in the marine environment and to make proposals on the ecological influences they may have outside of oysters. In contrast to previous studies where detection was restricted to the summer, OsHV-1 µVar detection occurred year round in C. gigas gill tissue, environmental (water, biofilm and sediment) and notably other macroinvertebrates. Vibrio splendidus was the only Vibrio spp. detected; it was also persistent throughout the year in oysters, invertebrate species and biofilm. This study represents the first record of OsHV-1 µVar in Mytilus edulis. The present study identified for the first time, the transmission of this virus from both Mytilus edulis and Mytilus spp. to naïve C. gigas, which occurred in the laboratory under an increased and increasing temperature regime. This study confirms that these pathogens are capable of maintaining themselves year round in a marine environment and that other macroinvertebrate and a vertebrate species in close proximity to oyster culture sites are susceptible to OsHV-1 µVar infection, although their role as potential carriers or alternative hosts is still not fully understood. The seasonal samples from different sources (macroinvertebrate species and environmental media samples) were ...
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