Understanding and minimizing the impacts of host-pathogen-environment interactions in the Pacific oyster Crassostrea gigas

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Bookelaar, Babette E.
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University College Cork
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Globally, Pacific oysters Crassostrea gigas are experiencing significant mortalities especially in warmer summer months, as a result of a complex aetiology and a strong association with pathogens such as ostreid herpes virus-1 microVar (OsHV-1 µvar) and bacteria Vibrio aestuarianus. Such mortalities threaten the future growth and sustainability of this industry worldwide. A better understanding of how these pathogens and diseases are interacting with their host and the environment at culture sites is needed, and new management and practical husbandry techniques are desired. This three-year study investigated the host-pathogen-environment interplay of Pacific oysters by carrying out field trials at the two largest culture sites in Ireland and by performing laboratory trials. The objectives of this study were to: (a) to understand how OsHV-1 µVar is becoming established at C. gigas culture sites. A five month field trial was undertaken in the spring and summer 2015. Green shore crabs Carcinus maenas and edible cockles Cerastoderma edule were collected at the oyster trestles and 500 m on the high shore at two culture sites, and were screened for OsHV-1 µVar by polymerase chain reaction (PCR), quantitative PCR (qPCR) and in situ hybridisation (ISH). OsHV-1 µVar was detected in both species by all diagnostics and was confirmed by direct sequencing. The mean prevalence of infection was 18.3% (n = 121/660) in Dungarvan and 16.3% (n = 155/952) in Carlingford Lough in C. maenas and 14.4% (n = 72/500) in Dungarvan and 13.6% (n = 121/890) in Carlingford Lough in C. edule. Following on from the field screening results, several 14 day laboratory trials were conducted to determine if naturally exposed crabs and cockles from culture sites could transmit OsHV-1 µvar to naïve oysters. OsHV-1 µvar was first detected in oysters cohabiting with exposed crabs on Day 4 and in oysters exposed to the cockles on Day 5. An overall prevalence of 6.5% in oysters held with crabs was observed while 4.4% was observed in the oysters held with cockles. Viral copies remained low (oysters exposed to crabs had an average of 1.0x 102 viral copies μl -1 of genomic DNA, oysters exposed to cockles had an average of 3.0 x103 DNA viral copies μl -1 of genomic DNA) in both trials, but this could be the result of trials being run at a low temperature (14 °C) less than the threshold temperature (16 °C) for replication of the virus. To the best of our knowledge C. maenas is the first non-bivalve species indicating infection of OsHV-1 µvar. (b) investigate the role of crabs, cockles and mussels as carriers or reservoirs for Vibrio spp.. In Ireland, Vibrio aestuarianus has been associated with C. gigas mortalities. V. aestuarianus was found at low infection intensities in C. maenas and C. edule and only at one of the main culture sites in 2015. Mussels Mytilus spp. were sampled from culture (2 hot spots) and non culture sites (<30 km from a culture site) in July, September and October 2017. OsHV-1 µvar was detected in Mytilus spp. in all samples screened and prevalence was significantly higher at the culture site. These results indicate that oyster pathogens and diseases are present in invertebrate species at C. gigas culture sites and extend beyond farms, even when prevalence in C. gigas is low. (c) Determine the role of biotic and abiotic factors as drivers or inhibitors of pathogen and disease development in C. gigas. The effects of air exposure on pathogen development and oyster performance of different families and age classes over a period of 16-month period was investigated. Oyster seed and half-grown “Naïve” Irish and “Resistant” French bred stocks were held at the high shore and subtidally, approximately 6-8 hours difference in air exposure. Oysters were checked after 56 weeks (T1) and 68 weeks (T2) for mortality, growth and pathogen development (OsHV-1 µvar and Vibrio aestuarianus). A significantly higher prevalence of OsHV-1 μVar was detected in oysters 12 months after the trial commenced in June 2017 compared with oysters 16 months post relaying in September 2017, while this was not observed for V. aestuarianus, possibly suggesting a difference in disease development. No clear difference in disease development was observed between oysters located at high shore or lower shore, however especially the “Resistant” French stock performed better in survival and growth at the high shore. The “Resistant” French stock performed better in survival and growth compared to the “Naïve” Irish stock. (d) Assess the effects of seaweed spp. commonly found attached to oyster bags on oyster health and pathogen development. Seaweed extracts, sulphated polysaccharides, have shown to have a positive effect on the immune system of marine invertebrates by their wide variety of immune stimulating, anticancer, antiviral and antibacterial activities. In this study, the effects of natural degrading seaweed spp. on disease development in C. gigas seed when held under an increased temperature regime was investigated. Findings showed that exposure to seaweed spp. had a positive antiviral effect on OsHV-1 μVar development. The results from the testing of a commercial product of sulphated polysaccharides was inconclusive due to the lack of infection in all oysters. Key findings This research will not only give a better understanding of how the pathogens are maintained in the environment, it is also proposing possible consequences within a larger ecosystem perspective. Key findings from this study: - Marine invertebrates, shore crab Carcinus maenas (Chapter 2) and edible cockle Cerastoderma edule (Chapter 3), commonly found at the intertidal area at culture and nonculture sites are carriers of OsHV-1 μVar. - Vibrio aestuarianus was also detected in those two marine invertebrate species along with the blue mussel Mytulis spp., even up to 30 km outside a hotspot of infection (Chapter 4). - Shore holding height (difference in air exposure) of C. gigas can influence oyster performance and disease development for different age classes and oyster families (Chapter 5). - Laboratory experiments indicated possible positive immune effects on seed oysters when exposed to seaweed species or algal derived commercial products (Chapter 6).
Pacific oysters , Crassostrea gigas , Vibrio aestuarianus , Ostreid herpes virus-1 microVa , Host-pathogen-environment interactions
Bookelaar, B. 2018. Understanding and minimizing the impacts of host-pathogen-environment interactions in the Pacific oyster Crassostrea gigas . PhD Thesis, University College Cork.