Community cats managed under Trap-Neuter-Return as sentinels of human diet-linked chemical exposure across contrasting feeding contexts.

At the local scale, environmental surveillance often overlooks diet-related exposures. In this study, we examined whether community cats managed through Trap-Neuter-Return (TNR) programs could serve as effective sentinels of human-relevant chemical mixtures shaped by local food environments. Intraoperative whole blood samples from 205 free-roaming cats were collected on two islands of the Canary archipelago, La Graciosa (n = 107) and Gran Canaria (n = 98), and analyzed for 55 elements and >360 organic contaminants using validated ICP-MS and micro-QuEChERS LC/GC-MS/MS workflows. The islands displayed distinct chemical profiles. La Graciosa exhibited a coherent "marine/leftovers" signature, with elevated Hg, As, Se, and Sr, and higher composite burdens of persistent organics (ΣPOPs) and polycyclic aromatics hydrocarbons (ΣPAHs). In contrast, Gran Canaria showed higher concentrations of rare-earth and technology-related elements (ΣREEs), and other urban/industrial tracers, whereas Pb and Cd remained low in both cohorts. Although compound-level detections were limited for many organic contaminants, the summed metrics clearly differentiated the two islands and minimized zero-inflation. Fipronil and its metabolite fipronil-sulfide were detected at both sites, consistent with ectoparasiticide use during handling. Second-generation anticoagulant rodenticides were identified exclusively in Gran Canaria, consistent with their routine application in urban pest control. Integrating minimally invasive sampling within routine TNR programs yielded standardized chemical profiles without additional captures, providing a concise indicator panel (Hg, As, Se, Sr, ΣPOPs, ΣPAHs, and ΣREEs) suitable for cross-site screening. Overall, TNR-managed community cats provide practical sentinel signals of diet-linked chemical exposures at the local scale. Because cross-species toxicokinetic differences preclude direct quantitative translation to humans, these results are interpreted as sentinel signals to prioritize follow-up rather than as evidence of human risk. The strength and internal consistency of the La Graciosa signal support targeted human biomonitoring on the island, prioritizing methylmercury and arsenic speciation together with focused dietary and source-apportionment surveys. Broader adoption of this TNR-based framework could enable One Health chemical surveillance across municipalities and seasons.