Myeloma in the Horse

Johanna L. Watson • Monica Aleman

Myeloma is a neoplastic proliferation of plasma cells that primarily originates in the bone marrow but may have an extramedullary source.²⁰ Both forms have been reported in the horse.^21-23 Accumulation of neoplastic plasma cells outside the marrow is called extramedullary plasmacytoma.²⁰ Plasmacytomas

■ TABLE 37.2

Monoclonal Antibodies for Specific Equine Leukocyte Antigens (ELAs)

CD, Cluster of differentiation; Eq, equine; IgM, immunoglobulin M; MHC, major histocompatibility complex; WC, white cell.

can form in several organs and tissues, including the skin.

Myeloma is a rare neoplasia of the horse (only 16 cases reported in the literature), with a median age of 14 years and a range of 3 months to 25 years of age.^21-24,26-35 Almost half of the horses were Quarter Horses, but various breeds including draft horses and ponies were represented. There is no apparent gender predilection. Clinical signs vary with the degree of plasma cell infiltration, extent, and location. The most common clinical signs are weight loss, anorexia, fever, pale mucous membranes, lethargy, limb edema, and recur- 21243536

rent infections.^2124,35,36 Bone pain due to osteolytic disease is the most common early sign of myeloma in humans.²⁵ Four horses from the literature were reported to have osteolysis. The clinical signs in these horses included paresis, cervical pain, and lameness.^24,27,29,32

Laboratory features are anemia, hyperproteinemia due to hyperglobulinemia, hypoalbuminemia, and proteinuria. Anemia results from reduced erythropoiesis secondary to myelophthisis, blood loss, and as a result of the osmotic effect of paraproteins. Pancytopenia is observed if myelophthisis develops. Hyper­calcemia has been reported in a few cases.^21,23,24 Significantly different features have been reported (hypoproteinemia, hypoglobulinemia, hypoalbuminemia, hypocalcemia) in a gelding with systemic amyloidosis and multiple myeloma, thought to be the result of severe protein-losing enteropathy.³⁶ Concurrent systemic amyloidosis and multiple myeloma are common in humans but extremely rare in domestic animals, with only one cat and one horse reported in the literature.^36,37 Serum parathyroid hormone-related protein concentration was mildly elevated in one horse with multiple myeloma.²⁴

In secretory myelomas, neoplastic plasma cells are responsible for overproduction of a monoclonal Ig called M protein or paraprotein.

Plasma cells are relatively sparse in normal bone marrow. Involvement of marrow is usually focal rather than diffuse, requiring multiple marrow aspirates in some cases to confirm plasmacytosis. The diagnosis of multiple myeloma in humans is based on the presence of at least 10% of plasma cells in the affected tissue, monoclonal protein in serum or urine, and evidence of end-organ damage (hypercalcemia, renal insuffi­ciency, anemia, or bone lesions). Renal insufficiency has not been reported in horses with myeloma.^21,23 Bleeding and blood hyperviscosity syndrome are common complications of multiple myeloma in humans.²⁵ These complications have been reported in four horses.^21,30

The median length of survival after diagnosis in humans is approximately 3 years.²⁵ Owing to poor prognosis or deteriora­tion of condition, most reported horses were euthanized within a few days to months of diagnosis. However, two horses survived for 1 and 2 years, respectively.^31,34 One horse was euthanized 3.5 years after the diagnosis for deteriorating condition.²³