Modification of empirical antimicrobial regimens in large animal medicine

Background Empirical antimicrobial regimens can be modified following new diagnostic information or when empirical treatment fails. Little is known about the frequency or clinical context in which these modifications occur. We characterised these modifications in a large animal hospital to identify when antimicrobial use could be optimised. Methods Chart reviews were performed for all inpatients and outpatients administered antimicrobials at a large animal veterinary referral and teaching hospital in 2017-2018 (n=1163 visits) to determine when and why empirical regimens were modified. Multinomial logistic regression was performed to identify factors associated with reasons for modification. Results Empirical antimicrobial regimens were modified in 17.3 per cent of visits. The main reasons were parenteral-oral conversions in horses and failure of disease prevention or treatment in ruminants. Empirical therapy for disease prevention was more likely to be modified because of complications in ruminants and in animals on the emergency/critical care service. Empirical therapy for disease treatment was more often modified for reasons other than de-escalation in ruminants and in animals with longer lengths of stay. Conclusions Empirical antimicrobial regimens were modified infrequently and mostly for purposes of parenteral-oral conversion in horses and lack of response in ruminants. De-escalation of antimicrobials administered for disease treatment, when guided by diagnostics, is a major tenet of judicious antimicrobial use. However, more research is needed to determine when and how antimicrobial regimens administered for disease prevention should be modified.