Efficacy of Therapeutic UVC Irradiation in Reducing Bacterial Load in Microbial Keratitis: A Retrospective Clinical Study in Veterinary Patients

Objective To evaluate the immediate effect of therapeutic ultraviolet‐C (UVC) irradiation on corneal bacterial load in veterinary patients with microbial keratitis (MK), using a retrospective analysis of clinical data collected at a UK referral ophthalmology hospital. Animals Studied Client‐owned dogs, horses, and cats presented to a referral ophthalmology clinic with MK in 2025. Procedures Clinical records from MK cases were reviewed. For each case, paired corneal swabs were obtained immediately before and immediately after UVC irradiation with a commercially available 265 nm device (PhotonUVC). Treatment consisted of a single 5‐s exposure at 3.5 mW/cm 2 (fluence 17.5 mJ/cm 2 ). Bacterial colony‐forming units (CFU) were quantified by culture. The primary outcome was the percent reduction in CFU; secondary outcomes included proportion of culture‐negative results after UVC, laterality (right eye (OD) vs. left eye (OS)), and species subgroup comparisons. Results A total of 34 eyes from 30 patients (25 canine, 6 equine, 3 feline) met the inclusion criteria. Median (IQR) CFU decreased from 89.5 (30.3–14 400) pre‐UVC to 15 (0–207) post‐UVC. Median percent reduction was 96.5% (88.5–100.0%); mean 92.3 ± 9.6%. A total of 11 of 34 eyes (32%) were culture‐negative after UVC, and 32/34 (94%) showed ≥ 75% reduction. Reductions were observed across all major pathogens, including Staphylococcus, Streptococcus, Pseudomonas, and coliforms. Conclusions and Clinical Relevance In this cohort of patients, a single low‐dose UVC exposure produced substantial immediate reductions in corneal bacterial load in veterinary MK patients. These findings support further controlled trials to define the role of UVC as an adjunct to conventional antimicrobial therapy.