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

Summary 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.