Equine Lymphoma Study Guide

Overview and Clinical Importance

Lymphoma (also called lymphosarcoma) is the most common hematopoietic neoplasm in horses, accounting for 1.3% to 14% of all equine tumors. It is a malignant cancer arising from lymphocytes that can affect virtually any organ system. Unlike many other species, equine lymphoma presents with significant clinical heterogeneity, making early diagnosis challenging. The disease is typically diagnosed at an advanced stage, which significantly impacts prognosis and treatment options.

Lymphoma affects horses of all ages, breeds, and sexes, with horses aged 5-10 years showing increased predisposition. The median age at diagnosis ranges from 10-13 years. There is no apparent breed or sex predilection, though American Quarter Horses, Thoroughbreds, and Standardbreds are frequently represented in case studies, likely reflecting population demographics.

Etiology and Pathogenesis

The exact cause of equine lymphoma remains poorly understood. Unlike in cattle where bovine leukemia virus (BLV) causes lymphoma, no definitive viral etiology has been established in horses, though associations have been suggested. Recent research has identified Equine Herpesvirus-5 (EHV-5) in some equine lymphoma cases, particularly in TCRLBCL, multicentric, and cutaneous forms. However, a direct causal relationship has not been definitively established.