Lymphocytes are an important component of the equine immune system. Like all cells within the body, lymphocytes have the potential to undergo neoplastic (cancerous) transformation that results in uncontrolled regulation and growth. Lymphosarcoma is the proliferation of neoplastic lymphocytes.
Equine lymphosarcoma is relatively common, but the exact incidence is unknown. The majority of cases occur in horses aged 4-15 years, but cases have been described in horses of all ages. Epidemiologic studies suggest no apparent gender or breed predisposition. Like many equine cancers, the cause of lymphosarcoma is rarely identified, but certain bacteria and viruses have been implicated in its development.
Four anatomical categories are frequently utilized for classification of lymphosarcoma. These categories include: multicentric (generalized or within multiple locations), thymic (mediastinal, within the chest cavity), alimentary (intestinal), or cutaneous (skin or extranodal).
Clinical signs may develop abruptly or over several months. Signs can develop due to organ dysfunction directly related to infiltration by neoplastic lymphocytes; physical obstruction caused by neoplastic masses; or from neoplastic byproducts (paraneoplastic syndrome). Depression, weight loss, subcutaneous edema, fever, anemia, and lymphadenopathy (swollen lymph nodes) are the most commonly observed clinical signs, but signs can vary based on the affected organs. Multicentric, thymic, and cutaneous forms can compress the airways and esophagus and result in respiratory or swallowing abnormalities. The intestinal form can result in colic, diarrhea and weight loss. Cutaneous nodules may be observed in or under the skin; these masses can be influenced by hormones, thus may wax and wane in size. Various paraneoplastic syndromes have been described in horses and include: hypercalcemia, pseudohyperparathyroidism, pruritus and alopecia (itching and hair loss), and immune-mediated hemolytic anemia and thrombocytopenia.
Veterinarians may suspect a diagnosis of cancer after visualization of cutaneous nodules, transrectal palpation of abdominal masses, or detection of masses by radiology, ultrasonography or surgery. Clinical differentiation of neoplasia from non-neoplastic lesions is difficult. A definitive diagnosis of lymphosarcoma is made by microscopic visualization of neoplastic lymphocytes in body fluids, fine needle aspirates, surgical biopsies or necropsy samples.
The majority of horses diagnosed with lymphosarcoma either die or are humanely euthanized within months after developing clinical signs. Horses with the cutaneous form typically have longer survival times in comparison to those with other forms. Treatment is infrequently attempted, but temporary improvement may occur following surgical excision, or treatment with hormones, chemotherapeutics, immunomodulators and corticosteroids.
The University of Kentucky Veterinary Diagnostic Laboratory diagnosed 57 cases of equine lymphosarcoma from September 2009 to September 2015. Diagnoses were made from 30 surgical biopsies, 23 necropsies and four cytologic examinations. These cases represented 51 horses of seven different breeds. The age of affected animals ranged from a fetus at 300 days of gestation to a 27-year-old gelding; the mode was three years of age and included six cases. Cases were composed of 21 multicentric, 15 cutaneous, 13 lymphoid (lymph node, spleen, or thymus), and five alimentary lymphosarcomas. Additionally, one case was diagnosed from thoracic effusion (fluid) and two cases from abdominal effusions.
This article was written by Dr. Alan Loynachan of the University of Kentucky’s Veterinary Diagnostic Laboratory. You can contact him at [email protected].
This article is from the Equine Disease Quarterly, published by the University of Kentucky College of Agriculture, Food and Environment Department of Veterinary Science and sponsored by Lloyd’s of London and its Kentucky agents. You may subscribe to this publication for free.