Prevalence, Molecular Characterisation and Zoonotic Potential of Blastocystis sp. in Humans and Domestic Animals: A Cross‐Sectional Study From Northeastern Iran

BACKGROUND: Blastocystis sp. is a common intestinal protist infecting humans and a wide range of animals worldwide. Despite its high prevalence, its genetic diversity and zoonotic transmission remain poorly understood, particularly in regions with close human-animal contact. This study aimed to determine the prevalence, subtype (ST) distribution and phylogenetic relationships of Blastocystis sp. in humans and domestic animals in northeastern Iran. METHODS: In this cross-sectional study (2019-2021), 730 fresh faecal samples were collected from humans (n = 203) and domestic animals (n = 527). Samples were examined using direct smear microscopy and culture. DNA was extracted from culture-positive isolates and subjected to PCR targeting the SSU rRNA gene. PCR-positive amplicons were subsequently sequenced for ST identification, and phylogenetic analysis was performed using the Maximum Likelihood method. RESULTS: Overall, 68 samples (9.3%) were positive for Blastocystis sp., with culture was significantly more sensitive than direct smear microscopy (p = 0.001). The prevalence among humans was 13.3%. Sequence analysis identified eight STs (ST1-ST3, ST5-ST7, ST14 and ST15). ST3 was the predominant ST in humans, while ST7 was mainly detected in poultry. Ruminant-associated ST14 and ST15 were exclusively found in sheep. Phylogenetic analysis revealed clear clustering according to STs and indicated greater intra-ST diversity within ST7. CONCLUSIONS: The findings demonstrate the circulation of genetically diverse Blastocystis sp. STs among humans and domestic animals in northeastern Iran. The presence of shared STs across hosts highlights the potential for zoonotic transmission and underscores the value of molecular approaches for epidemiological studies.