Occurrence and subtyping of the intestinal protozoan Blastocystis sp. in French oyster areas reveal potential fecal contamination routes and associated risks to consumer health

Blastocystis sp. is the most frequently detected intestinal protozoan in humans and recognized as a zoonotic eukaryote. Therefore, raw-consumed mollusks such as oysters may act as potential reservoirs for contamination, posing a risk to consumer health. However, data on the prevalence and subtype (ST) distribution of this parasite in mollusks remain scarce. This study aimed to detect Blastocystis sp. by real-time PCR in 360 wild oysters collected in 2021 from three major French oyster areas (Arcachon, Bourgneuf, and Marennes-Oléron bays) and in 210 surrounding water samples (nano- and micro-mesoplankton fractions). Additionally, 120 oysters collected in 2024 from Marennes-Oléron were screened to complement the analysis. The prevalence observed in oysters was 1.4% in 2021 and increased to 19.2% in 2024, while only 1.9% of environmental samples tested positive. Sanger and next-generation sequencing of PCR products from positive samples identified ten STs in oysters (ST1, ST2, ST3, ST4, ST6, ST7, ST21, ST26, ST30, ST44), four in nanoplankton (ST4, ST6, ST7, ST28), and one in micro-mesoplankton (ST7), indicating circulation of the parasite in the marine environment while highlighting the role of oysters as sentinels of microbiological quality in coastal ecosystems. The ST diversity and absence of a predominant ST reported in oysters suggest that these bivalves are not natural hosts but rather passive carriers of Blastocystis sp. The ST profiles suggest fecal contamination originating from both humans and animals (bovines and birds). Since most STs identified are transmissible to humans, the consumption of raw contaminated oysters may represent a potential public health risk. • Blastocystis sp. is frequently detected in French oyster production areas. • Blastocystis sp. STs analysis suggests oysters are not natural hosts of the parasite. • Contamination of oysters is related to fecal pollution from human and animal sources. • Oysters serve as reliable sentinels of the microbiological quality of their ecosystem. • Consumption of raw contaminated oysters may pose a risk to human health.