Crossing the species barrier: Bidirectional transmission of

Background: Objectives: This review synthesizes current evidence on the prevalence of 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