A high proportion of bacterial isolates from septic neonatal foals in Ontario express multidrug resistance and low susceptibility to first-line antimicrobials.

Abstract: To describe common bacterial isolates cultured from sick neonatal foals and their antimicrobial susceptibility and resistance patterns. Unassigned: Medical records of foals ≤ 30 days of age, admitted to the Ontario Veterinary College from 2020 to 2023, and with a positive aerobic bacterial culture and susceptibility testing performed (Kirby-Bauer method) were included. A descriptive analysis of species isolated and antimicrobial and multidrug resistance profiles was performed. Unassigned: 62 samples from 60 predominantly Thoroughbred (42% [25 of 60]) neonatal foals (median age, 8 days) yielded 104 isolates of 34 bacterial species and genera. Enterococcus spp (22% [23 of 104]), Streptococcus equi subsp zooepidemicus (17% [18 of 104]), Escherichia coli (12% [12 of 104]), Actinobacillus equuli subsp haemolyticus (8% [8 of 104]), and Staphylococcus aureus (8% [8 of 104]) were most frequent. Antimicrobial susceptibility was highest to chloramphenicol (74% [75 of 102]), ceftiofur (68% [71 of 104]), and enrofloxacin (64% [66 of 103]). Gram-positive isolates were most susceptible to chloramphenicol (87% [53 of 61]), ampicillin (75% [38 of 61]), and rifampin (70% [43 of 61]). Gram-negative bacteria were most susceptible to enrofloxacin (81% [34 of 42]) and ceftiofur (71% [30 of 42]). An amikacin plus ampicillin combination had the highest susceptibility (78% [70 of 90]). Multidrug resistance for the 104 isolates was 78% (81 of 104): 66% (41 of 62) for gram-positive isolates and 93% (39 of 42) for gram-negative isolates. Unassigned: S equi subsp zooepidemicus was the most common species and Enterococcus was the most common genus, but a breadth of equine-associated and environmental species were cultured. There was a high proportion of isolates expressing multidrug resistance and low susceptibility to first-line antimicrobials. Unassigned: Ongoing susceptibility monitoring and implementation of an effective antimicrobial stewardship program for this patient population is required.