Infracyanine green photodynamic therapy provides short-term disease control, but recurrence is common in equine immune-mediated keratitis

Objective: To evaluate long-term clinical outcomes, recurrence rates, and complications following photodynamic therapy (PDT) in horses with immune-mediated keratitis. Methods: Medical records of horses treated with PDT at the Swedish University of Agricultural Sciences Equine Clinic from May 2018 through October 2025 were reviewed retrospectively. Eyes with a clinical diagnosis of recurrent immune-mediated keratitis that had responded to immunosuppressive therapy and been subjected to PDT therapy were included. Infracyanine green was injected intrastromally, followed by irradiation with an 810-nm diode laser. Recurrence, postoperative complications, and follow-up duration were recorded. Recurrence-free survival was estimated using Kaplan-Meier analysis, and associations with recurrence were explored. Results: 30 eyes from 28 horses were included. Recurrence occurred in 18 of 30 eyes (60.0%); 12 of 30 (40.0%) remained recurrence-free without medication at final follow-up (median, 1,849 days). Recurrence-free survival was 83.1% at 1 year (95% CI, 64.0% to 92.6%) and 49.8% at 2 years (95% CI, 30.2% to 66.6%); median recurrence-free time was 693 days. Postoperative epithelial defects occurred in 22 of 30 eyes (73.3%), and secondary bacterial ulcerative keratitis developed in 4 of 30 (13.3%); all ulcers healed after adjusted treatment. No clear associations between evaluated variables and recurrence were detected. Conclusions: PDT provided medication-free disease control in some eyes, particularly during the first postoperative year, but recurrence was common with longer follow-up. Epithelial defects were frequent, although all resolved without major sequelae. Clinical Relevance: PDT may be useful when prolonged topical treatment is impractical; repeated early fluorescein staining and long-term surveillance are recommended.