A case of malignant transformation of equine immune-mediated keratitis to B-cell lymphoma

A 13-year-old Thoroughbred/Oldenburg gelding presented as a tertiary referral for a 6-year history of immune-mediated keratitis (IMMK) after the development of corneal and limbal nodules in the presence of medical therapy. A keratectomy extending to the limbus was performed to remove the affected cornea and limbal nodules. Two corneal-limbal samples were submitted for histopathology and initially diagnosed as marked lympho-histiocytic stromal keratitis with epithelial hyperplasia, keratinisation and keratinocyte apoptosis consistent with IMMK. Additional testing with immunohistochemical staining for CD3, CD20, PAX5 and IBA1 revealed large, atypical B-lymphocytes on a background of histiocytes and mature T-lymphocytes, with concern for T-cell-rich large B-cell lymphoma. PCR antigen receptor rearrangement clonality testing was also performed and was diagnostic for large B-cell lymphoma. Polymerase chain reaction (PCR) for equine herpesvirus 5 (EHV-5) was positive. Following keratectomy, a short course of neomycin/polymyxin B/dexamethasone ophthalmic ointment was prescribed; no other treatment nor adjunct therapy was performed. Eighteen months post-operation, the case remains visual and comfortable on no medications.