RESUMO
Background: Although relatively uncommon, lymphoma is the most prevalent haematopoietic neoplasia in horses, and multicentric lymphoma remains the most common presentation of the disease. The pathogenesis of equine lymphoma is still poorly understood and the diagnosis is usually confirmed at an advanced stage of the disease, compromising the prognosis. This study investigated the clinical, pathological, and molecular features of a case of equine multicentric lymphoma. Case Description: An apparently healthy 5-year-old crossbreed mare hospitalized at the Centre of Animal Reproduction of Vairão, Portugal, suddenly presented clinical signs of supraorbital oedema and mandibular lymph node enlargement, developing fever, facial oedema, and generalized lymphadenopathy. The mare ended up dying twenty-four days after the first clinical signs due to multisystem organ failure. Haematological and biochemical analyses, necropsy, and microscopic and molecular evaluation of affected tissues were performed. At necropsy, the main findings were multiple multinodular lesions, distributed along the serous surface of oropharynx, trachea, pericardium, gastrointestinal tract, and mesentery. Microscopically, these consisted of solid proliferations of neoplastic round cells that exhibited immunopositivity for CD3 (T cells). Based on these findings, a medium-grade multicentric T-cell lymphoma was diagnosed. Conclusion: There is still very little research regarding the molecular characterization of lymphoma in horses. As an entity itself is quite heterogeneous, it is important to describe the interspecies particularities to understand its development and behaviour.