Sporulating Haplosporidium nelsoni in Crassostrea gigas in a production bay in Ireland

This report describes the finding of Haplosporidium nelsoni DNA in cohorts of Crassostrea gigas oysters analysed during reoccurring mortality events between 2008 and 2014 in one production bay in SW Ireland (Castlemaine harbour). OsHV-1 mu Var is considered endemic in this bay and Vibrio aestuarianus has been detected since 2006. H. nelsoni DNA was detected by PCR in 43 out of 172 oysters and included the finding by histology and in situ hybridisation of one oyster with sporulating H. nelsoni in the digestive tubule epithelium. This is the first detection of the parasite in this bay and the first observation of a systemic infection with H. nelsoni in Ireland. Although H. nelsoni does not cause disease in C. gigas results from a PCR survey indicated that oysters of all age classes examined over a 7 year period were positive for H. nelsoni DNA and might suggest that H. nelsoni is more common in the environment in this bay than previously thought.