Tracking Blastocystis sp. in mussels from Northern France in a One Health context: Seasonal dynamics, implications in marine fecal pollution and risk in human health.

Blastocystis sp. is an intestinal protozoan with recognized zoonotic potential that is increasingly investigated in animals closely associated with humans or intended for food production. However, studies on mollusks remain limited, despite their high filtration capacity and ability to accumulate biological contaminants, making them valuable indicators of environmental pollution. Within a One Health framework, this study investigated the seasonal circulation of Blastocystis sp. in a coastal ecosystem in northern France through the analysis of mussels, cattle, and river water samples using real-time PCR. A total of 892 samples were analyzed, of which 14.5% tested positive. Mussels showed a prevalence of 22.7%, with marked seasonal variations suggesting fluctuations in environmental contamination. Cattle exhibited the highest prevalence (38.3%), while 18.8% of river water samples were contaminated. Sanger and next-generation sequencing of positive samples revealed substantial genetic diversity, with 14 subtypes (STs) identified in mussels, 8 in cattle, and 3 in water samples. The lack of host-specific STs, together with the low parasite loads assessed in mussels, suggests that these organisms act as passive accumulators rather than true hosts. Moreover, the broad ST diversity observed highlights ongoing circulation of Blastocystis sp. within this aquatic environment and widespread fecal contamination consistent with human, bovine, and avian sources. Overall, mussels appear to be effective bioindicators of multisource fecal pollution across interconnected ecosystems, emphasizing the potential risk of exposure to Blastocystis sp. through the handling or consumption of raw shellfish and the importance of strengthened environmental and public health surveillance.