Assessment of the impact of a pathogen, Bonamia ostreae, on Ostrea edulis oyster stocks with different histories of exposure to the parasite in Ireland

The protozoan parasite Bonamia ostreae has decimated Ostrea edulis stocks throughout Europe over the past four decades. A study of two stocks of O. edulis in Ireland with varying periods of exposure to B. ostreae, 5. years and 22. years, was undertaken. The objective of the study was to determine if varying lengths of exposure would translate into observations of differing susceptibilities to B. ostreae. A number of oyster beds within each area were screened. The study was carried out over 13. months to investigate seasonality and the role of environmental parameters, population density and size on disease development. Of particular interest was the fact that prevalence of infection in both stocks was very similar. The stock that had been exposed for 22. years had a similar prevalence, intensity and seasonality of infection as the stock infected for 5. years.B. ostreae was detected in both stocks throughout the year with the highest prevalence in spring, possibly related to the increase in water temperature and/or oysters directing their energy towards gametogenesis. The study indicated that oyster stocks can maintain themselves over extended periods of time in B. ostreae endemic areas. However, prevalence of B. ostreae will remain relatively stable within the stock without some intervention to improve resistance levels e.g. by breeding for resistance over a number of years. Some natural resistance to infection will build up in individual oysters but in natural populations this will continually be diluted by cross fertilisation with more susceptible oysters.