PLASMA-CELL MYELOMA IN THE HORSE - A CASE-REPORT AND LITERATURE-REVIEW

Plasma cell myelomas in horses have been reported infrequently. Data f rom 10 cases, 9 from the literature and 1 new case, are used to charac terize the disease in the horse. Hot-blooded horses (7/10), specifical ly Quarter Horses (4/10), were most often affected. Median age at diag nosis was 11 years (range, 3 mo-22 yr) and both male (5) and female ho rses (5) were represented equally. Clinical findings included weight l oss (6/8), anorexia (4/8), fever (4/8), limb edema (4/8), pneumonia (3 /8), rear leg paresis/ataxia (3/8), epistaxis (3/8), palpable lymphade nopathy (2/8), and bone pain (2/8). Anemia (8/8) was present routinely , and in three horses, RBCs were macrocytic. Leukopenia (2/8), thrombo cytopenia (2/8), and circulating plasma cells (3/8) were variable find ings. Except for abnormal protein concentrations and hyponatremia (3), abnormal results from serum biochemical analysis including hypocholes terolemia (1), hypercalcemia (1), and azotemia (1) were reported infre quently. Hyperproteinemia (8/9), hypoalbuminemia (7/9), and hyperglobu linemia (8/9) were characteristic but not invariable findings. Monoclo nal proteins (7/7) were detected in the alpha2, beta, or gamma region by serum electrophoresis. The paraprotein's heavy chain, determined in four horses, was a subclass of IgG. Three horses had decreased concen trations of normal immunoglobulins. Variable proteinuria (trace to 4+) was detected by routine urinalysis in four of six horses. Bence Jones proteinuria was detected in one of five horses (heat precipitation) a nd monoclonal proteins were detected in two of three electrophoresed u rine samples. Three of the horses had lytic bone lesions detected radi ographically. Bone marrow aspirates were diagnostic in two of five hor ses. Atypical plasma cells or increased numbers of plasma cells or bot h were present in histologic sections of bone marrow in six of eight h orses. Common extraosseous sites of plasma cell infiltration included lymph nodes (8/8), kidney (5/8), spleen (5/8), liver (3/8), lung (3/8) , brain (2/8), and orbit (2/8). Two horses had intracellular and extra cellular crystalline deposits.