Detection of the protistan parasite, Haplosporidium costale in Crassostrea gigas oysters from the French coast: A retrospective study

•A specific real-time PCR protocol was developed for faster detection of H. costale.•In France, H. costale was detected in adult oysters since 2005 and spat since 2008.•The H. costale prevalence in spat (14.59%) appeared higher than in adults (6.50%).•Actin gene sequencing showed 2 H. costale strains based on 3 sequence mutations.•The strain 1 was found in hatchery spat while the strain 2 was found in wild spat.