Phage Therapy for a Refractory and Multidrug-resistant Equine Surgical Site Infection

A 14-year-old American Quarter Horse mare developed a refractory deep surgical site infection following emergency celiotomy for acute right dorsal large colon displacement with 180° volvulus. Initial cultures revealed a high-burden polymicrobial infection with Escherichia coli and Enterococcus faecium, associated with delayed healing, and later followed by multidrug-resistant Pseudomonas aeruginosa, further limiting therapeutic options. Phage therapy was initiated using a customized lytic phage cocktail administered topically on a daily basis over a 35-day period, with cocktail composition guided by daily wound culture results. Phage therapy markedly reduced colony-forming units of targeted pathogens and promoted progressive healing. Wound microbial communities varied during treatment, demonstrating selective pathogen control while preventing residual populations from recolonizing the tissue. By the time of hospital discharge, the wound surface area had decreased by more than 85%, and the body wall had healed without evidence of acute herniation. This case highlights the feasibility, safety, and clinical potential of adaptive phage therapy for managing refractory multidrug-resistant equine wound infections.