Enterococcus Infection and Susceptibility to Antibiotics in Animals in Oregon, USA, from 2021-2025

Abstract

Introduction : Enterococcus sp. are commensal and pathogenic bacteria that colonize humans and a wide range of animals. Those bacteria are encountered in many habitats including the intestinal tract microbiome of humans and animals. The widespread application of antibiotics in hospitals and agriculture has contributed to the emergence of antibiotic resistant Enterococci, including vancomycin resistant Enterococcus faecium . Because animals and humans live in close contact, we analyzed the infections caused by Enterococci in animal species in Oregon from 2021 to 2025. Methods : Specimens delivered to the diagnostic laboratory were cultured on blood agar and additional media as required. Bacterial colonies were identified using Mass spectrometry MALDI-TOF (Matrix - Assisted Laser Desorption/Ionization — Time-of - Flight) technology. Those identified colonies were then used to determine antibiotic susceptibility using either disc diffusion method or broth microdilution according to the CLSI (Clinical Laboratory Standards Institute) . Results : Enterococcus sp. isolated from 2021 to 2025 were included in the analysis. In large animals, eleven infections were caused by E. faecalis , five by E. faecium , and four by Enterococcus sp. In addition, ten infections belong to other species such as E. avium , E. casseliflavis , E. mundtii , and E. gallinarium . Those Enterococci isolated were from caprine (5 cases), feline (1 case), porcine (1 case), alpaca (1 case), and equine (2 cases). E. faecium was isolated from surgical incisions, urine and wound. Most of the strains were resistant to the majority of the antibiotics tested except for vancomycin. E. faecalis was susceptible to amoxicillin/clavulanate and vancomycin, with only one isolate from a horse wound showing resistance to the antibiotics. In dogs and cats, E. faecalis was isolated from 71 infections, while E. faecium was identified as the pathogen in 37 infections. The most common sites of infection by both Enterococcus species in dogs was bile, ear, urine and surgical incision. E. faecium was isolated from two pleural fluids. All E. faecalis were susceptible to ampicillin and vancomycin. In the case of E. faecium, approximately 40% were resistant to aminopenicillins and one isolate from a dog’s urine was vancomycin resistant. Among 12 canine and one cat case(s), of Enterococci, one E. casseliflavus isolated from a foot infection, was vancomycin resistant. Conclusion : The number of infections caused by Enterococcus sp. were commonly seen in diverse infection sites of domestic animals, being more frequently isolated from incision or surgical infections in horses and GI tract and surgical sites in dogs and cats. Over the period of 5 years very few isolates showed to be resistant to vancomycin, although resistance to other antibiotics were common.