1.0 Introduction
Lymphoma in the horse is uncommon but is the most common hematopoietic neoplasm in equids accounting for 1-14% of all equine neoplasms with a prevalence of 0.02 – 0.5% in the equine population (Savage 1998; Schneider 2003; Knowles et al. 2016). There is no sex, breed or age predilections and lymphoma has been reported in all stages of life from fetal to geriatric (Haley and Spraker 1983; Meyer et al. 2006). Lymphoma can be expressed in various forms including multicentric, alimentary, cutaneous, mediastinal or solitary extranodual tumors and the immunophenotype is important for classification and treatment planning (Sheard and McGovern 2021).
A retrospective study of 203 equine lymphoma cases utilized the World Health Organization (WHO) Classification System based on immunophenotyping. This retrospective study identified T-cell-rich, large B-cell (TCRLBCL) as the most common immunophenoytype representing 43% of cases and diffuse large B-cell (DLBC) the 3rdmost common accounting for 26 or 12.5% of the 187 reported cases with immunohistochemistry (Durham et al. 2013). The multicentric form of the disease is the most common and is often found in younger horses aged 4 to 10 years old, whereas the alimentary form is more commonly found in horses greater than 16 years old (Schneider 2003; Meyeret al. 2006; Taintor and Schleis 2011; Durham et al. 2013). Organs most typically affected with the multicentric form include the lymphatic system, spleen, liver, intestine, kidney, lung and occasionally bone marrow (Sheard and McGovern 2021). Clinical signs are insidious and non-specific, varying with the affected organs but most often include depression, edema of distal limbs or ventral abdominal wall, weight loss, pyrexia and cachexia (Meyer et al. 2006; Muñozet al. 2009).