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

Overview

• This study evaluates the long-term effectiveness and safety of infracyanine green photodynamic therapy (PDT) in treating immune-mediated keratitis in horses, focusing on disease recurrence and postoperative complications.

Background and Purpose

• Immune-mediated keratitis (IMK) in horses is a recurrent eye disease characterized by inflammation that often requires immunosuppressive therapy.
• Photodynamic therapy (PDT) using infracyanine green and laser irradiation has been proposed as an alternative treatment to improve disease control and reduce reliance on medication.
• The study aimed to assess the long-term clinical outcomes, recurrence rates, and complications after PDT in horses with recurrent IMK that had responded to prior immunosuppressive treatment.

Study Design and Methodology

• Retrospective review of medical records from the Swedish University of Agricultural Sciences Equine Clinic (May 2018 – October 2025).
• Included 30 eyes from 28 horses diagnosed with recurrent immune-mediated keratitis, all previously responsive to immunosuppressive therapy and treated with PDT.
• PDT Procedure:
  • Injection of infracyanine green dye directly into the corneal stroma (intrastromal injection).
  • Subsequent irradiation with an 810-nm diode laser to activate the dye and induce photodynamic effects.
• Data collected:
  • Recurrence of keratitis after PDT.
  • Postoperative complications, such as epithelial defects and bacterial ulcerative keratitis.
  • Duration of follow-up for each case.
• Statistical analysis:
  • Recurrence-free survival was estimated using Kaplan-Meier analysis.
  • Exploration of associations between various clinical variables and keratitis recurrence.

Key Findings

• Recurrence rates:
  • 18 out of 30 eyes (60%) experienced recurrence of keratitis after PDT.
  • 12 out of 30 eyes (40%) remained recurrence-free without any medication at final follow-up.
  • Median follow-up duration was 1,849 days (approximately 5 years).
• Recurrence-free survival rates:
  • 83.1% at 1 year post-treatment.
  • 49.8% at 2 years post-treatment.
  • Median recurrence-free period was 693 days (~1.9 years).
• Postoperative complications:
  • Epithelial defects (damage to the outer layer of the cornea) occurred in 22 of 30 eyes (73.3%).
  • Secondary bacterial ulcerative keratitis developed in 4 of 30 eyes (13.3%).
  • All ulcers healed after adjusted treatment, with no major long-term adverse effects reported.
• No clear associations were identified between evaluated clinical variables and the likelihood of disease recurrence.

Conclusions and Clinical Implications

• PDT with infracyanine green provides effective, medication-free disease control in some horses, particularly during the first year after treatment.
• Recurrence of immune-mediated keratitis remains common when followed over longer periods (beyond 1-2 years).
• Although epithelial defects are frequent immediately postoperatively, they generally resolve without serious complications.
• PDT could be a useful treatment option in situations where prolonged topical immunosuppressive therapy is impractical or undesirable.
• Recommendations include:
  • Careful and repeated early postoperative monitoring using fluorescein staining to detect corneal defects.
  • Long-term surveillance to promptly identify and manage disease recurrence.

Significance for Veterinary Medicine

• This study contributes valuable long-term data on PDT as an adjunct or alternative therapy for equine immune-mediated keratitis.
• It emphasizes both the potential benefits in reducing medication burden and the limitations related to recurrence risk.
• The findings support cautious optimism in the use of PDT but underscore the need for continued post-treatment monitoring and potential retreatment strategies.