First Detection of the mcr-1 Gene in a Wild Red Fox (Vulpes vulpes), from a High-Density Livestock Area.

Background/Objectives: Antimicrobial resistance (AMR) can be influenced by the interconnection of human, animal and environmental compartments, making wildlife-based surveillance an important component of One Health surveillance. The red fox (Vulpes vulpes), due to its synanthropic behaviour and wide distribution, can be a valuable indicator of environmental AMR. Mobile colistin resistance (mcr) genes, which encode resistance to a highest-priority critically important antimicrobial (HPCIA), have never been identified in free-ranging foxes. Case presentation: A red fox faecal strain isolated using selective media was PCR-screened for major AMR genes, underwent antimicrobial susceptibility testing and whole-genome sequencing (WGS). This led to the identification of the first mcr-1-positive Escherichia coli from a wild red fox. The fox inhabited an area with a small human population but one of the highest cattle densities in Italy and high antimicrobial use (AMU), as established by comparing local livestock density and AMU with surrounding areas. Despite being borderline phenotypically susceptible to colistin, the isolate displayed resistance to other HPCIAs and antimicrobial classes commonly used in livestock. WGS revealed that mcr-1 was chromosomally integrated. Discussion: The study area showed near-zero colistin use in livestock alongside high overall AMU, suggesting that the mcr-1-positive isolate may have evolved through co-selection rather than direct colistin pressure. mcr-1 can persist and circulate in the environment, even where colistin use is minimal, and foxes may represent effective sentinels for tracking such events. Our results emphasise the importance of including wildlife in One Health surveillance frameworks to monitor AMR at the human-animal-environment interface.