Study clarifies corneal effects of equine photodynamic therapy

Photodynamic therapy with infracyanine green appears to cause substantial, intentional stromal cell loss in the healthy equine cornea, according to a new Equine Veterinary Journal study from researchers at Cornell University. In six healthy university-owned horses, unilateral treatment with intrastromal infracyanine green followed by 810 nm diode laser activation caused corneal ulceration in every treated eye, immediate keratocyte destruction on confocal microscopy, keratocyte depopulation through days 5 and 15, and stromal keratitis that persisted through day 103. Slow repopulation was seen by days 33 and 103, and no horses developed blinding complications. The authors propose that this keratocyte depopulation may help explain why the approach has shown promise in horses with immune-mediated keratitis. (madbarn.com)

Why it matters: For veterinary ophthalmologists and equine practitioners, the study adds an important safety and mechanism signal to a therapy that's already gaining traction in immune-mediated keratitis. A separate 2026 Veterinary Ophthalmology report described infracyanine green-based photodynamic therapy in horses with immune-mediated keratitis using cases from Auburn University and Equine Clinic Munich-Riem, suggesting the technique is moving into broader clinical use. This new work suggests the treatment effect may come with predictable epithelial injury, prolonged stromal inflammation, and months-long dye retention, all of which could shape case selection, pet parent counseling, follow-up intervals, and expectations around healing. (ovid.com)

What to watch: The next key question is whether larger clinical studies can define which immune-mediated keratitis cases benefit enough from photodynamic therapy to justify the ulceration and prolonged stromal changes seen in normal corneas. (madbarn.com)