First molecular detection of Blastocystis sp. in Mediterranean mussels Mytilus galloprovincialis from the Western coast of Italy: seafood safety and environmental contamination implications

Increasing anthropogenic pressure on coastal marine environments has raised concerns about the introduction and persistence of waterborne pathogens, including emerging protozoan parasites. This study investigated the occurrence of Blastocystis sp., an intestinal protist of vertebrates with zoonotic subtypes (STs), in Mediterranean mussels (Mytilus galloprovincialis). Samples were collected from 12 coastal sites in central and southern Italy between 2022 and 2023. A total of 1177 Mediterranean mussels were sampled and screened using real-time PCR targeting the SSU rDNA gene. Blastocystis sp. DNA was detected in 10.3 % of the specimens, with site-specific prevalence ranging from 0 % to 42 %. High prevalence rates were observed in mussels from the Gulf of Naples and from sites along the Lazio coast, particularly near the mouth of the Tiber River. Among the positive samples, 35 were successfully sequenced, revealing the presence of ST3. This represents the first molecular evidence of Blastocystis sp. in Mediterranean mussels from the Mediterranean Sea. The detection of Blastocystis sp. suggests that mussels may serve as passive carriers and potential reservoirs of this protist, potentially posing a risk of transmission to humans. This study also highlights the need for a continuous surveillance of protozoan pathogens in bivalve molluscs. These organisms can serve as effective sentinels for seafood safety and for environmental contamination with faeces from terrestrial vertebrates, especially in areas affected by urban and intensive farming run-off.