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To determine the prevalence and diversity of Borrelia burgdorferi sensu lato (s.l.) in an endemic Nature Reserve (Sierra del Sueve) in North-Western Spain, and the risk of human exposure to infected ticks in Asturias, 1013 questing ticks and 70 small mammals were collected between 2012 and 2014. A retrospective descriptive analysis was also carried out on human Lyme borreliosis (LB) cases reported to the local hospital (Cabueñes). Samples were screened for B. burgdorferi s.l. presence by a nested PCR assay, and genospecies were confirmed by sequencing. B. burgdorferi s.l. was detected in 1.4% (12/845) of I. ricinus questing nymphs, 9.1% (2/33) of questing adults, and 12.9% (9/70) of small mammals, as well as in the other tick species. PCR positive samples of 17 questing tick and 6 small mammals were sequenced. Four genospecies were identified: B. afzelii, B. garinii, B. lusitaniae, and B. valaisiana. Phylogenetic analyses based on the flaB gene showed the heterogeneity of B. afzelii in this area. The detection of B. burgdorferi s.l. among questing ticks and small mammals in the study area, as well as the abundance of ticks and of large wild and domestic mammals, indicate a high risk of infection by B. burgdorferi s.l. in the area. Reporting of LB cases to the local hospital support this, and shows the need of thorough monitoring of B. burgdorferi infection in ticks and hosts in the area. More investigations are needed to assess the role of different wildlife species and the risk of transmission to humans., (Copyright © 2016 Elsevier España, S.L.U. and Sociedad Española de Enfermedades Infecciosas y Microbiología Clínica. All rights reserved.)