Multiple myeloma presenting as blepharitis in a horse

Myeloma-related disorders, including multiple myeloma, extramedullary plasmacytoma, and solid osseous plasmacytoma, are rare in horses. Clinical complaints for myeloma-related disorders are nonspecific, and when present, M-protein location is more variable on protein electrophoresis in horses relative to dogs and cats. Here, we describe a case of a 15-year-old Thoroughbred mare who presented with recurrent blepharitis. Marked hyperglobulinemia was an incidental finding on routine hematologic and biochemical testing. Bone marrow aspiration consisted of >30% plasma cells, and serum protein electrophoresis demonstrated a monoclonal gammopathy in the alpha 2 fraction leading to a diagnosis of multiple myeloma. Immunofixation and radial immunodiffusion confirmed the presence of an IgG M-protein. Based on a restricted peak in the alpha 2 location, the specific M-protein is suspected to be IgG(T), an IgG isotype unique to horses. M-protein migration in horses is variable relative to dogs and cats, yet immunofixation can still be used to identify equine IgG M-protein isotypes. The unique clinical presentation in this case also serves as a reminder to consider neoplasia in horses with unusual or nonspecific clinical signs.