Clinical and morphological features of corneal lymphoma in 26 horses (27 eyes)

Abstract Background Little information describing the biologic behaviour and therapies for corneal lymphoma exist. Objectives To characterise histologically confirmed equine corneal lymphoma considering breed, age or sex predisposition, histologic and immunologic features, therapies and outcomes. Study design Retrospective case series. Methods A multicentre retrospective medical record review was used to identify horses that presented with corneal disease confirmed morphologically as corneal lymphoma from 2012 to 2022. Results Mean age at diagnosis was 10.5 years. Males represented 18/26 cases. Warmblood (14) and Thoroughbred (6) breeds were most commonly represented. 25/26 had unilateral ocular involvement. No cases had evidence of multicentric lymphoma. Most eyes (77.7%) had a history of suspected immune‐mediated keratitis (IMMK) or eosinophilic keratitis. Morphological diagnosis confirming lymphoma was available for all cases. B‐cell lymphoma was diagnosed in 19/24 eyes for which immunohistochemistry was performed, with intermediate to large B‐cell lymphoma being most common. Fifteen of 27 eyes received at least one form of adjunctive therapy following keratectomy. Four eyes, which did not receive adjunctive therapy following keratectomy, experienced recurrence. No horses treated with adjunctive therapy following keratectomy experienced recurrence. Malignant transformation of IMMK to lymphoma was morphologically documented in one eye and suspected in three others based on recurrence of clinical signs. All globes were retained and visualised at the last follow‐up. Main limitations Incomplete medical records, owner subjectivity concerning onset of disease and cases being lost to follow‐up. Conclusions Malignant transformation of IMMK to corneal lymphoma likely occurs in horses. Lymphoma should be a differential for nonulcerative keratitis in horses. Keratectomy and histopathology may be considered in progressive or refractory cases of IMMK to rule out neoplastic disease. Recurrence of corneal lymphoma is unlikely following excision and adjunctive therapy. Cases with large corneal surface area or deep stromal involvement may have a likelihood for recurrence.