Prevalence and subtyping of Blastocystis sp. parasite in French oysters and production areas
Blastocystis sp. is an intestinal protozoan with recognized zoonotic potential and is currently the most frequently unicellular eukaryote found in humans. Raw-consumed mollusks such as oysters may serve as potential reservoirs of contamination and thus pose a potential risk to consumers health. However, data on the prevalence and subtype distribution of this parasite in mollusks remain scarce. This partly retrospective study aimed to detect Blastocystis sp. by real-time PCR in 120 wild oysters collected in 2021 from three major French oyster production areas (Arcachon, Bourgneuf, and Marennes-Oleron bays) and in 210 associated nano- and micro-mesoplankton samples. Additionally, 120 oysters collected in 2024 from Marennes-Oleron were screened to evaluate the presence of Blastocystis sp. in different organs (gills and digestive gland) in order to complement the analysis. The prevalence observed in oysters was 2.5% 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 subtypes 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 active circulation of the parasite in the marine environment while highlighting the role of oysters as sentinels of microbiological quality in coastal ecosystems. The subtype diversity and absence of a predominant subtype reported in oysters suggest that these bivalves are not natural hosts but rather passive carriers of Blastocystis sp. The subtype profiles strongly suggest fecal contamination originating from both humans and animals, including bovines and birds. Moreover, since most subtypes identified are transmissible to humans, the consumption of raw contaminated oysters may represent a potential public health risk.