Molecular Prevalence and Genetic Diversity of <i>Blastocystis</i> sp. in Slaughtered Ruminants in Qazvin Province, Iran: A Zoonotic Concern

BACKGROUND: Blastocystis sp. is a genetically diverse intestinal protist with global distribution in humans and animals. Despite increasing evidence of its zoonotic potential, data on its prevalence and subtype (ST) diversity in Iranian ruminants remain limited. This study aimed to investigate the molecular prevalence and ST distribution of Blastocystis sp. in cattle, sheep and goats in Qazvin Province, Iran. METHODS: A total of 142 faecal samples were collected from slaughterhouses in Abyek, Buin Zahra and Takestan counties in 2025, including 84 cattle, 26 sheep and 32 goats. Genomic DNA was extracted using the QIAamp DNA Stool Mini Kit. PCR amplification was performed based on the SSU rRNA gene, and products were visualized on 1.5% agarose gels. Positive amplicons were randomly selected for sequencing, and STs were assigned by BLAST comparison with GenBank references. Phylogenetic relationships were constructed using MEGA X with the Neighbour-Joining method. Statistical associations between infection and host variables were evaluated using chi-square and Fisher's exact tests. RESULTS: Blastocystis sp. DNA was detected in 32 of 142 ruminant samples (22.5%). Prevalence was 19% (16/84) in cattle, 27.3% (7/26) in sheep and 28.1% (9/32) in goats, with no significant difference among species (p = 0.486). Infection was significantly higher in females (30.1%) than in males (14.5%) (p = 0.026). Further analysis revealed no significant correlation between infection status and age group (p = 0.454) or slaughterhouse location (p = 0.698). Sequencing 16 randomly selected PCR-positive isolates revealed five zoonotic STs: ST1 (n = 2), ST5 (n = 3), ST7 (n = 1), ST10 (n = 9) and ST14 (n = 1). ST10 and ST14 are zoonotic and ruminant-associated, while ST1, ST5 and ST7 are frequently reported zoonotic STs with broad host ranges. CONCLUSIONS: This study demonstrates a moderate prevalence of Blastocystis sp. in ruminants of Qazvin Province, with a diverse set of zoonotic STs including both host-adapted and common zoonotic STs. These findings align with global prevalence patterns and ST distributions in ruminants, while also complementing recent data from ruminants in Iran. Although the identification of zoonotic STs suggests possible public health implications, definitive evidence of interspecies transmission requires further One Health studies integrating livestock, humans and environmental sources.