Unusual IgM-secreting multiple myeloma in a dog.

A 4-year-old castrated male dog was evaluated because of multiple-limb lameness. Signs of pain were elicited during palpation of the regions of the proximal tibial metaphyses and distal left radial diaphysis. Radiography revealed osteolytic lesions of the long bones. Blood analyses revealed hypercalcemia and transient cytopenias. Serum protein electrophoresis did not reveal a monoclonal gammopathy; however, urine protein electrophoresis revealed Bence Jones proteinuria. Serial diagnostic sampling of bone lesions, immunohistochemical staining methods, and serum and urine protein immunoelectrophoresis were required to establish a diagnosis of multiple myeloma. Two IgM components were identified via serum protein immunoelectrofixation. The dog improved clinically after initiation of chemotherapy with melphalan and prednisone; however, the dog ultimately was euthanatized because of pathologic fracture. The case was unique because there was lack of vertebral involvement, an unusual gammopathy, and difficulty in identifying myeloma cells via serial sampling.