Background Although antimicrobial resistance is increasingly common in equine medicine, molecular and epidemiological data remains scarce. Objectives We estimated the prevalence of, and risk factors for, shedding of multidrug resistant (MDR), extended spectrum β‐lactamase (ESBL)‐producing, and AmpC β‐lactamase‐producing, or some combination of these in Escherichia coli in horses in France. We characterized ESBL/AmpC isolates for antimicrobial susceptibility and the presence of virulence and ESBL/AmpC‐associated resistance genes. Animals Fecal samples from healthy adult horses at 41 premises were collected. A questionnaire was completed by each premises manager. A subset of these samples was tested to build 2 bacterial collections. Methods Indicator (without enrichment) and specific (enrichment with ceftriaxone) E. coli tested for antimicrobial susceptibility. Prevalence of isolates nonsusceptible to antimicrobials was estimated at the horse and the premises level. The ESBL/AmpC and virulence genes were identified by PCR. Multivariable logistic regression was used to investigate risk factors for MDR and ESBL/AmpC isolates at premises. Results Approximately 44% of horses shed MDR E. coli. Resistance most commonly was observed to ampicillin, streptomycin, and amoxicillin/clavulanic acid. Twenty‐nine percent of premises housed horses shedding ESBL/AmpC‐producing isolates. The ESBL/AmpC gene most commonly identified was blaCTX‐M‐1. Virulence gene iutA was identified in 1 ESBL/AmpC‐producing isolate. Medical treatment, staff numbers, and activity were identified as risk factors for housing horses shedding ESBL/AmpC‐producing E. coli isolates. Conclusions and Clinical Importance Prevalence of healthy horses harboring ESBL/AmpC genes and MDR isolates in their intestinal microbiota is substantial. Risk factors could be used to elaborate guidelines to prevent their dissemination.

Intestinal strongyles are the most problematic endoparasites of equids as a result of their wide distribution and the spread of resistant isolates throughout the world. While abundant literature can be found on the extent of anthelmintic resistance across continents, empirical knowledge about associated risk factors is missing. This study brought together results from anthelmintic efficacy testing and risk factor analysis to provide evidence-based guidelines in the field. It involved 688 horses from 39 French horse farms and riding schools to both estimate Faecal Egg Count Reduction (FECR) after anthelmintic treatment and to interview farm and riding school managers about their practices. Risk factors associated with reduced anthelmintic efficacy in equine strongyles were estimated across drugs using a marginal modelling approach. Results demonstrated ivermectin efficacy (96.3% ± 14.5% FECR), the inefficacy of fenbendazole (42.8% ± 33.4% FECR) and an intermediate profile for pyrantel (90.3% ± 19.6% FECR). Risk factor analysis provided support to advocate for FEC-based treatment regimens combined with individual anthelmintic dosage and the enforcement of tighter biosecurity around horse introduction. The combination of these measures resulted in a decreased risk of drug resistance (relative risk of 0.57, p = 0.02). Premises falling under this typology also relied more on their veterinarians suggesting practitionners play an important role in the sustainability of anthelmintic usage. Similarly, drug resistance risk was halved in premises with frequent pasture rotation and with stocking rate below five horses/ha (relative risk of 0.53, p < 0.01). This is the first empirical risk factor analysis for anthelmintic resistance in equids. Our findings should guide the implementation of more sustained strongyle management in the field.

introduction that contributed to lower drug resistance risk by 1.75. Premises falling under this 36 typology also relied more on their veterinarians suggesting they play an important role in the 37 sustainability of anthelmintic usage. Similarly, drug resistance risk was halved in premises 38 with frequent pasture rotation and with stocking rate below five horses/ha. This is the first 39 empirical risk factor analysis for anthelmintic resistance in equids, whose findings should 40 guide the implementation of more sustained strongyle management in the field. 41 42

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