Antimicrobial resistance has become an important concern in veterinary medicine. The aim of this study was to describe the rate of antimicrobial resistance in common equine pathogens and to determine the occurrence of multidrug-resistant isolates. A retrospective analysis of all susceptibility testing results from bacterial pathogens cultured from horses at the University of Zurich Equine Hospital (2012-2015) was performed. Strains exhibiting resistance to 3 or more antimicrobial categories were defined as multidrug-resistant. Susceptibility results from 303 bacterial pathogens were analyzed, most commonly Escherichia coli (60/303, 20%) and Staphylococcus aureus (40/303, 13%). High rates of acquired resistance against commonly used antimicrobials were found in most of the frequently isolated equine pathogens. The highest rate of multidrug resistance was found in isolates of Acinetobacter baumannii (23/24, 96%), followed by Enterobacter cloacae complex (24/28, 86%) and Escherichia coli (48/60, 80%). Overall, 60% of Escherichia coli isolates were phenotypically ESBL-producing and 68% of Staphylococcus spp. were phenotypically methicillin-resistant. High rates of acquired antimicrobial resistance towards commonly used antibiotics are concerning and underline the importance of individual bacteriological and anti microbial susceptibility testing to guide antimicrobial therapy. Minimizing and optimizing antimicrobial therapy in horses is needed. Keywords: horse, multidrug-resistant (MDR) bacteria, methicillin-resistant staphylococci (MRS), ESBL-producing

Summary Multidrug‐resistant (MDR) bacteria are an emerging threat in human and veterinary medicine. There are few reports about infections caused by MDR isolates in horses. The aim of this study was to provide an overview of infections caused by MDR bacteria at the Equine Hospital Zurich between 2012 and 2015. Medical records were searched for horses with confirmed MDR bacterial infection. Multidrug resistance was defined according to human guidelines specific for each pathogen. MDR isolates were most commonly isolated from post‐procedural infections (53/110, 48%), followed by musculoskeletal (16/110, 15%) and soft tissue infections (16/110, 15%). Escherichia coli (32/158, 20%) and Staphylococcus aureus (25/158, 16%) were the most common isolates. High resistance rates precluded therapy with commonly used antimicrobial drugs. The overall mortality rate was 20% (22/108) but depended on the localisation of the infection. Antimicrobial treatment prior to development of infection was reported for 89% (91/102) of horses. This study showed that MDR pathogens, mainly MDR E. coli and MRSA, cause a considerable number of infections in horses. A wide range of infections was seen, however, nosocomial infections predominated. These cases are typically hospitalised, pretreated with antibiotics, and suffering from comorbidities putting them at high‐risk for acquiring infections caused by MDR isolates. The mortality of such infections was generally low but depended on site of infection.

Knowledge about local pathogen prevalence is important for guiding initial antimicrobial therapy. The aim of this study was to describe the longitudinal occurrence of bacterial pathogens in hospitalized horses at the University of Zurich between 1988 and 2014. Positive culture results were retrospectively analyzed. Isolates were grouped based on sample site, diagnosis, and year of isolation. The frequency of bacterial isolates over time was compared using the Chi-Square test. Escherichia coli (299/1,723, 17%), Streptococcus equi ssp. zooepidemicus (295/1,723, 17%) and coagulase-positive staphylococci (196/1,723, 11%) were the most common pathogens. Anaerobes were common in dental (18/66, 27%), peritoneal (6/43, 14%) and soft tissue infections (28/208, 13%). Mixed infections were common in skin and dental infections (24/55, 44% and 17/43, 40%, respectively). Frequently isolated bacteria in most organ systems included Gram-positives and Gram-negatives, supporting the need for initial broad-spectrum antimicrobial therapy until culture and susceptibility results become available. AbstractKnowledge about local pathogen prevalence is important for guiding initial antimicrobial therapy. The aim of this study was to describe the longitudinal occurrence of bacterial pathogens in hospitalized horses at the University of Zurich between 1988 and 2014. Positive culture results were retrospectively analyzed. Isolates were grouped based on sample site, diagnosis, and year of isolation. The frequency of bacterial isolates over time was compared using the Chi-Square test. Escherichia coli (299/1,723, 17%), Streptococcus equi ssp. zooepidemicus (295/1,723, 17%) and coagulase-positive staphylococci (196/1,723, 11%) were the most common pathogens. Anaerobes were common in dental (18/66, 27%), peritoneal (6/43, 14%) and soft tissue infections (28/208, 13%). Mixed infections were common in skin and dental infections (24/55, 44% and 17/43, 40%, respectively). Frequently isolated bacteria in most organ systems included Gram-positives and Gram-negatives, supporting the need for initial broad-spectrum antimicrobial therapy until culture and susceptibility results become available.

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Summary A 7‐month‐old Warmblood filly was referred to the equine hospital because of a Salter‐Harris type 2 fracture of the proximal physis of the right hind P1. On admission, the filly had acute grade 3/5 lameness at the walk and diffuse swelling in the area of the right hind proximal phalanx. Radiographs were taken to assess the fracture and plan treatment. Surgical treatment using two 4.5 mm narrow locking compression plates (PIP‐LCPs) resulted in fracture stabilisation and rapid healing. This case report suggests that internal fixation using PIP‐LCPs is useful for the treatment of Salter‐Harris type 2 fractures of the proximal physis of P1 in large foals.