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Compendium June 2013 (Vol 35, No 6)

Clinical Snapshot: Equine Lymphoma

by Adam Stern, DVM, CMI-IV, CFC, DACVP, Scott M. Austin, DVM, MS, DACVIM

    Case Presentation

    A 12-year-old, 464-kg (1020.8-lb), castrated Quarter horse presented to the University of Illinois Veterinary Diagnostic Laboratory for euthanasia and necropsy. The horse had a 10-day history of an enlarged abdomen, preputial edema, and weight loss. On physical examination, the horse had ventral edema, labored breathing, sunken eyes, and a prolonged skin tent time that suggested 6% to 8% dehydration. The horse’s body condition score was 4/9, with loss of epaxial and gluteal muscle mass. There was no peripheral lymphadenopathy. Ascites was confirmed by brief abdominal ultrasonography. No further diagnostics or treatments were pursued, and the horse was humanely euthanized.

    A postmortem examination revealed approximately 5 L of serosanguineous fluid within the abdominal cavity. The liver was partially effaced by multiple tan masses. The spleen weighed 75 kg (165 lb) and was almost completely effaced by similar tan infiltrates (FIGURE 1).

    1. What is the most likely diagnosis for the masses in the liver and the spleen?

    2. Are there age or sex predilections for this disease?

    3. What clinical signs are common in this disease?

    Answers and Explanations

    1. On gross examination, there were multiple masses within the liver, and the spleen was almost completely effaced (FIGURE 2FIGURE 3). On histopathologic examination, the masses were composed of round neoplastic cells that had a large nucleus:cytoplasm ratio and there was moderate anisocytosis and anisokaryosis. A moderate number of small lymphocytes were scattered within the background of the neoplastic cells. Based on these findings, a presumptive diagnosis of lymphoma was made. To further characterize the neoplasm lymphocytes as T cells or B cells, immunohistochemistry was performed.

    On immunohistochemistry, the neoplastic cells exhibited positive cytoplasmic immunoreactivity for CD20 (a B-cell marker) but did not exhibit immunoreactivity for CD3 (a T-cell marker). The small nonneoplastic lymphocytes scattered within the neoplasm exhibited positive cytoplasmic immunoreactivity for CD3 and were consistent with T-cell lymphocytes. Based on these findings, the neoplasm was consistent with T cell–rich B-cell lymphoma. In one retrospective study of lymphoma in horses, B-cell lymphoma was most commonly diagnosed (77%), and 33% of horses with B-cell lymphoma were classified as having T cell–rich B-cell lymphoma.1,2

    2. There is no sex predilection for equine lymphoma.3 Horses with lymphoma are typically 5 to 10 years of age (range: birth to 25 years of age). In one study, 10% of lymphoma cases involved animals younger than 4 years.1 Lymphoma is the most common malignant neoplasm in horses.2

    3. Physical examination findings depend on the organ system(s) involved. The most common clinical signs associated with lymphoma include depression and weight loss.1,3,4 Less commonly, affected animals have a fever and ventral edema. Peripheral lymphadenopathy is occasionally observed, whereas regional (i.e., intraabdominal) lymphadenopathy is more common.3 In horses with gastrointestinal lymphoma, colic and diarrhea are expected clinical findings. Rectal palpation may reveal splenic enlargement in horses with splenic lymphoma.

    1. Jacobs RM, Messick JB, Valli VE. Tumors of the hematopoietic system. In: Tumors in Domestic Animals. 4th ed. Ames, IA: Blackwell; 2002:157-159.

    2. Durham AC, Pillitteri CA, Myint MS, Valli VE. Two hundred three cases of equine lymphoma classified according to the World Health Organization (WHO) classification criteria. Vet Pathol 2012;Jun 13 (Epub).

    3. Hines MT. Lymphosarcoma. In: The 5-Minute Veterinary Consult Equine. Baltimore, MD: Lippincott Williams and Wilkins; 2002:646-647.

    4. Sanz MG, Sellon DC, Potter KA. Primary epitheliotrophic intestinal T-cell lymphoma as a cause of diarrhea in a horse. Can Vet J 2010;51(5):522-524.

    References »

    NEXT: Feline Small Cell Lymphosarcoma Versus Inflammatory Bowel Disease: Diagnostic Challenges

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