Background: Surveillance of antimicrobial resistance (AMR) in horses is important to aid empirical treatment decisions and highlight emerging AMR threats. Objective: To describe the AMR patterns of common groups of bacteria from clinical submissions from horses in the UK during 2018, and to determine how this varies by sample site and type of submitting veterinary practice. Study design: Prospective observational study. Methods: All data on bacterial culture and subsequent antimicrobial susceptibility testing (AST) collected in 2018 from six large equine diagnostic laboratories were included. Resistance patterns were analysed including resistance to 1 or 2 antimicrobial classes, multidrug resistance (MDR), extensively drug resistant (XDR), resistance to highest priority critically important antimicrobials and isolates where there was no readily available treatment for adult horses in the UK. Submitting practices were classified according to whether they treated referral cases or not (first opinion). Comparisons between proportions and resistance for each bacterial group and sample site was performed using Chi squared (or Fisher's exact test). Results: A total of 6018 bacterial isolates from 4038 diagnostic submissions were included from respiratory (n=1555), urogenital (n=1010), skin/hair/wound/abscess (n=753), surgical site infection (SSI) /catheterrelated-infection (CRI) /orthopaedic infections (n=347) and unknown/'other' submissions (n=373). There were 2711 Gram-negative isolates and 3307 Gram-positive isolates. Prevalence of MDR for E. coli was 31.7%, Staphylococcus spp. 25.3% and >25% for the majority of bacterial isolates from SSI/CRI/orthopaedic submissions. For Enterococcus spp. there was no readily available treatment for adult horses in the UK in 30.2% of positive submissions. MDR was significantly higher from referral hospital than first opinion submissions for the majority of pathogens (except Actinobacillus spp. and Pasteurella spp. and β-haemolytic Streptococcus spp.). Main limitations: Since culture and susceptibility results are not systematic analyses based on harmonised methods, selection bias could impact the findings. Conclusions: Ongoing surveillance is essential to understand emerging patterns of resistance. MDR is high in SSI/CRI/orthopaedic infections, which is important for hospital biosecurity and guiding treatment decisions. Harmonisation of diagnostic procedures and interpretation of results amongst veterinary laboratories will improve AMR surveillance and data comparison amongst laboratories.

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