Understanding host, pathogen, environment dynamics in the marine environment with a particular focus on the Pacific oyster crassostrea gigas and its pathogens

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O'Reilly, Amy
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University College Cork
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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 used to assess the sensitivity of a range of primer pairs available for detection of OsHV-1 µVar outside of the normal host the oyster. The primer pairs OHVA/OHVB and PCR method were consistently the most sensitive method for the detection of OsHV-1 µVar in all sample types. The response of sudden stimulated varying environmental conditions: temperature, salinity and nutrient levels, on hatchery reared naïve C. gigas spat exposed to OsHV-1 µVar via inoculation were also examined and identified that mortality levels were higher in both treatment groups at higher salinity levels, while reduced salinity conditions resulted in lower mortality and reduced infection, even at elevated seawater temperatures. This study confirms that environmental parameters such as salinity and increased temperature are associated with viral proliferation and that interspecies transmission of OsHV-1 µVar is occurring. This study supports previous findings that the primer pair sets OHVA/OHVB and OHVC/OHVD and PCR are very effective tools for disease diagnosis in C. gigas for which they were designed. However, this study also established their effectiveness to detect OsHV-1 µVar in a wide range of other macroinvertebrate and a vertebrate species in close proximity to the oyster culture sites and also in environmental media samples in situ (seawater-plankton, sediment and biofilm) which has not been previously demonstrated in the literature. Overall, the study demonstrated that the virus in particular is demonstrating plasticity on introduction to various habitats, being able to establish itself in a range of other hosts and carriers. The wider ecological implications of this are presently unclear and merit further study.
Crassostrea gigas , Mytilus spp. , Transmission , Primer Pair , PCR , qPCR , Salinity , Temperature , Nutrients , Vibrios , OsHV-1 μVar
O'Reilly, A. 2018. Understanding host, pathogen, environment dynamics in the marine environment with a particular focus on the Pacific oyster crassostrea gigas and its pathogens. PhD Thesis, University College Cork.
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