Lymphoma, also known as lymphosarcoma, is the most common malignant cancer in horses. Lymphoma is a cancer that forms when lymphocytes, a type of white blood cells, become malignant and grow uncontrollably. [1][2][3]

Clinical signs of lymphoma in horses vary widely, but can include weight loss, colic, poor appetite and visible skin bumps. Symptoms may vary depending on the location of the cancer cells. [1][4]

Diagnosing lymphoma in horses can be challenging for veterinarians. Several diagnostic tests are used to rule out other underlying conditions, and to confirm a lymphoma diagnosis. [1][5]

Overall, lymphoma has a very poor prognosis with limited available treatment options in horses. [1][6]

Lymphoma in Horses

Lymphoma or lymphosarcoma refers to a type of cancer that originates in the lymphocytes. The first case of lymphoma in horses was described in 1858. [5]

Lymphocytes are a type of white blood cell that play a crucial role in the horse’s immune system, helping the body fight off infections and diseases. These immune cells are produced and develop in special areas of the body known as lymphoid tissue.

Lymphoid tissue is part of the lymphatic system and includes the spleen, lymph nodes, thymus, bone marrow and special immune tissue located in the digestive system known as mucosa-associated lymphoid tissue (MALT). [1][3][5]

Lymphoma is always malignant, meaning it is made up of cancer cells that grow rapidly and can spread to nearby tissues. Unlike benign tumors, which are non-cancerous and don’t usually spread, lymphoma invades other parts of the body. [1][7]

Prevalance

Lymphoma is the most common malignant cancer among equines worldwide, but the overall prevalence of this condition is low. [5][8]

Horses diagnosed with lymphoma are usually middle-aged, commonly between 8 to 10 years old. Nonetheless, cases of lymphoma have been reported in horses as young as 2 months and as old as 31 years. [2][5][8][9]

Any breed of horse can develop lymphoma, but Quarter horses, Thoroughbreds, Standardbreds, and Arabians are most commonly affected. [2][10]

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Types of Lymphoma

Lymphoma can develop in nearly any part of the body, but in horses, it is typically classified into four main types based on the location it affects. These types are: [2][5]

  • Multicentric lymphoma, which involves multiple lymph nodes and organs.
  • Alimentary lymphoma, which affects the gastrointestinal tract.
  • Thymic lymphoma, which originates in the thymus gland.
  • Cutaneous lymphoma, which appears on the skin.

Multicentric Lymphoma

Multicentric lymphoma refers to cancer present in multiple lymph nodes and organs. Multicentric lymphoma is the most common form of lymphoma in horses and typically affects the liver, spleen, intestine, kidney and bone marrow. [3]

Horses with multicentric lymphoma can have a variety of symptoms, often determined by the location of the cancer. The most common symptoms in affected horses are weight loss and swelling of the ventral abdomen, in addition to other systemic symptoms. [5][9]