BACKGROUND: Blastocystis sp. is a genetically diverse intestinal protist commonly found in humans and animal hosts. The prevalence and subtype diversity in humans have been extensively studied. In contrast, the presence in companion animals and the potential for bidirectional transmission in domestic environments remain less well understood. OBJECTIVES: This review synthesizes current evidence on the prevalence of Blastocystis sp. in dogs and cats, examines reports of shared subtypes between pets and owners, evaluates methodological challenges in confirming transmission, and identifies key research gaps relevant to One Health. KEY FINDINGS: Human-associated subtypes (ST1-ST3) are regularly detected in both pets and owners, but most studies rely on low-resolution small subunit ribosomal RNA (SSU rRNA) subtyping. These approaches cannot distinguish between direct inter-host transmission and shared exposure to contaminated sources such as water, rodents, or soil. Data on cats are underrepresented, and nearly all available evidence is cross-sectional, lacks strain-level resolution, and does not incorporate environmental sampling. Although colonization in pets is usually asymptomatic, co-carriage raises questions for households with vulnerable members. CONCLUSIONS: Current evidence does not confirm that shared subtypes indicate transmission; co-exposure from common sources is at least as plausible. Research gaps include a lack of longitudinal household studies, limited high-resolution genotyping, and an underrepresentation of feline hosts. A One Health approach incorporating improved molecular tools, behavioral risk assessment, and coordinated surveillance is needed to resolve these uncertainties. Companion animals must be considered within the broader framework of Blastocystis transmission dynamics to better inform veterinary and public health policy.

Charles University 3rd Faculty of Medicine Prague Czech Republic

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