Study clarifies corneal effects of equine photodynamic therapy: full analysis
A new equine ophthalmology study is putting clearer boundaries around a treatment that's been gaining momentum in referral practice. In Equine Veterinary Journal, Cornell investigators reported that photodynamic therapy with intrastromal infracyanine green and an 810 nm diode laser caused corneal ulceration, keratocyte destruction, prolonged stromal keratitis, and long-lasting dye retention in healthy horse corneas, while stopping short of blinding complications in the six-horse study. The authors say the findings offer a plausible mechanism for the therapy's benefit in immune-mediated keratitis: stromal cellular depopulation. (madbarn.com)
That matters because photodynamic therapy is no longer just theoretical in equine ophthalmology. Cornell lists the project as a 2024-2025 funded study supported by ACVO Visions for Animals, noting that photodynamic therapy has already been used in horses for adnexal and ocular surface neoplasia, Descemet’s membrane detachment, and immune-mediated keratitis, even though its safety profile in the normal equine cornea had not been described. Earlier veterinary literature has also positioned photodynamic therapy as a potentially useful ophthalmic tool across species, and the new paper builds on that translational interest by testing what the treatment actually does to healthy corneal tissue. (vet.cornell.edu)
In the current study, six university-owned horses each underwent unilateral corneal photodynamic therapy with intrastromal EmunDo® and diode laser photoactivation at 500 mW for 2.5 minutes, for a total of 75 J. The team followed horses with serial ophthalmic exams, digital and infrared photography, ocular thermography, in vivo confocal microscopy, and ultrasound biomicroscopy before treatment and on days 1, 5, 15, 33, and 103. Every treated eye developed corneal ulceration. Confocal microscopy showed destroyed keratocytes immediately after treatment, followed by keratocyte depopulation on days 5 and 15, with slow repopulation by days 33 and 103. Stromal keratitis was present by day 5 and persisted through day 103, and the dye remained visible at day 103 in five of six horses. (madbarn.com)
The paper also lands alongside a separate 2026 Veterinary Ophthalmology report on infracyanine green-based photodynamic therapy for immune-mediated keratitis in horses. That study drew on cases from Auburn University and Equine Clinic Munich-Riem from 2014 to 2020, underscoring that clinicians are already using the technique in affected horses, not just discussing it experimentally. Taken together, the two papers suggest the field is moving from anecdotal or early clinical adoption toward a more defined understanding of both outcomes and tissue effects. (ovid.com)
Direct outside commentary on the new study was limited in publicly available sources, but the broader clinical context is clear: immune-mediated keratitis in horses can be chronic, vision-threatening, and frustrating to manage, often requiring prolonged topical therapy and close monitoring. Prior literature cited by the EVJ paper describes immune-mediated keratopathies as inflammatory corneal disease, and newer regenerative medicine reviews continue to frame equine immune-mediated keratitis as a meaningful therapeutic challenge. In that setting, a treatment that may reduce pathologic stromal cellular activity is understandable attractive, even if it produces controlled collateral injury. (madbarn.com)
Why it matters: For veterinary professionals, this study shifts photodynamic therapy from a promising intervention to a treatment that now carries a better-defined biologic tradeoff. If keratocyte depopulation is indeed part of the therapeutic mechanism, then some degree of tissue injury may not be an adverse byproduct so much as part of the intended effect. But that also means clinicians should be careful about extrapolating use across case types, especially where epithelial integrity, stromal reserve, or follow-up compliance may already be concerns. The findings may also influence how referral centers discuss expected healing time, post-procedure discomfort, secondary ulcer risk, and the possibility of persistent corneal changes with pet parents. (madbarn.com)
What to watch: The next step is likely larger, disease-specific clinical work that links these morphologic changes to visual outcomes, recurrence rates, and comfort in horses with immune-mediated keratitis, while helping define dosing, candidate selection, and whether the prolonged stromal effects seen in healthy corneas are acceptable in routine practice. (madbarn.com)