Blastocystis spp. at the human-animal interface: a systematic review and meta-analysis of genetic diversity, subtype concordance, and molecular evidence.

BACKGROUND: The epidemiological significance of Blastocystis spp. concordance between humans and animals remains uncertain, particularly because evidence based solely on shared subtypes (STs) cannot establish direct epidemiological linkage. This systematic review and meta-analysis evaluated Blastocystis spp. prevalence at human-animal interfaces and critically synthesized evidence at the subtype (ST), allele, and sequence levels. METHODS: PubMed, Web of Science, and Scopus were systematically searched from database inception through June 7, 2026, supplemented by Google Scholar and reference-list screening. Eligible studies simultaneously investigated humans and animals within the same epidemiological or ecological setting. Random-effects models were used to estimate pooled prevalence and unadjusted odds ratios (ORs) for source-defined animal-exposed and reference groups. Subgroup analyses examined human exposure category, animal host group, and diagnostic approach. Molecular evidence was separately evaluated at the study-level ST, contact-linked ST, allele, and sequence levels. RESULTS: Thirty-five studies contributing 36 analytical datasets from 22 countries were included, comprising 4038 humans with animal exposure, 347 individuals from source-defined groups without animal exposure, and 4501 animal analytical units. Pooled Blastocystis spp. prevalence was 33.3% (95% CI: 26.9-40.4%) in humans and 30.3% (95% CI: 22.3-39.6%) in animals, with substantial heterogeneity (I2 = 93.1% and 95.9%, respectively). Across nine source-defined comparative datasets, no statistically significant difference in Blastocystis spp. positivity was detected between animal-exposed and reference groups (OR = 1.38, 95% CI: 0.76-2.49; P = 0.293). Human prevalence differed among exposure categories (P = 0.011), while animal prevalence differed among host groups (P < 0.001). Diagnostic-method differences were significant in animals (P = 0.006) but not in humans (P = 0.162). Molecular synthesis identified 11 source-reported STs in humans and 21 formally assigned STs in animals; eight STs (ST1-ST7 and ST23) showed study-level human-animal overlap. Human ST9 was confined to a single mixed ST3/ST9 profile, whereas ST17-like reptile sequences were not counted as ST17 because they were not formally assigned to this ST. Four studies provided allele-level human-animal data, and sequence-level comparisons demonstrated varying degrees of genetic similarity, although individually matched positive human-animal pairs were uncommon. Sensitivity analyses supported the stability of the principal prevalence estimates, and no clear evidence of funnel-plot asymmetry was detected. CONCLUSIONS: Blastocystis spp. was frequently detected in both humans and animals investigated at shared human-animal interfaces; however, similar prevalence estimates and study-level ST overlap do not establish a direct epidemiological relationship. Although allele- and sequence-level concordance provides greater molecular resolution, substantial heterogeneity, limited paired sampling, mixed profiles, partial sequence resolution, and predominantly cross-sectional designs constrain stronger inference. Standardized studies using synchronized sampling of explicitly linked human-animal pairs and higher-resolution molecular characterization are needed to clarify the epidemiological significance of the observed genetic concordance.