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]

Alimentary Lymphoma

The alimentary tract comprises any part of the digestive system. The stomach, small and large intestine, and rectum are the areas of the digestive system most commonly affected by lymphoma. [1]

Alimentary lymphoma can manifest in the lymphoid tissue of the digestive tract or directly within gut tissue. The mucosa-associated lymphoid tissue (MALT) and the surrounding lymph nodes associated with the digestive system are most commonly affected. [1][3][5]

Alimentary lymphoma most frequently affects horses younger than 5 years old. Additionally, Arabian horses are at a 4.5 times greater risk of developing intestinal cancers, which encompasses a heightened risk for alimentary lymphoma. [1][5][10][11][12]

Clinical signs can develop quickly after alimentary lymphoma proliferates. These horses might exhibit sudden illness or vague symptoms until specific organs are compromised due to the cancer’s growth. [1][3]

Treatment options for alimentary lymphoma are limited due to poor accessibility of the abdominal cavity. [6][7]

Thymic Lymphoma

The thymus is an organ situated in the lower neck area of horses, extending into the chest cavity. This organ is part of the immune system and plays a vital role during the early development of horses, helping to mature a specific type of white blood cell known as T-lymphocytes, or T-cells, which are crucial for the body’s immune response.

Thymic lymphoma (also called mediastinal lymphoma or thoracic lymphoma) describes the abnormal proliferation of lymphocytes in the thymus. [1][3]

Horses with thymic lymphoma often have difficulty breathing and frequent coughing from the tumor compressing the windpipe, or trachea. These horses may also have a visible mass in the lower throat region. Muffled heart sounds can also occur due to fluid accumulation in the chest cavity. [3][5]

Cutaneous Lymphoma

Cutaneous lymphoma refers to a proliferation of malignant lymphocytes under the skin, typically on the head, limbs, trunk and perineum. This form of cancer is uncommon in horses. [3]

Horses with cutaneous lymphoma present with several small, slow growing masses on the skin, in addition to other clinical signs. These masses may also experience hair loss, turn red, and secrete fluid. [5][13][14]

Cell Types

Lymphoma can be further classified by cell type, based on the specific characteristics of the lymphocytes involved, such as whether they are B-cells or T-cells. A combination of B-cells and T-cells can also be present within the same tumor.

This categorization is done by applying special stains or tests to the tumor cells or blood, which help identify unique markers that define the lymphoma type. [14][15]

Equine alimentary, cutaneous, and multicentric lymphoma typically involve a combination of cancerous B-cells and normal T-cells. [2][14][15] Thymic lymphomas in horses are usually linked to malignancies of T cell origin. [15]

In general, T-cell lymphomas are more aggressive and have a poorer prognosis than B-cell lymphomas. [3]

Other Classifications

In horses, lymphomas are not classified as extensively as in human medicine, but they can still be further categorized by examining the cells under a microscope and observing their appearance and behavior. [15]

Additional classifications of equine lymphomas include the size of the cells, their arrangement in the tissue, and the frequency of cell division, known as the mitotic rate. These factors are important for assessing the aggressiveness of the cancer and can provide insight into the likely outcome or prognosis. [15]

Clinical Signs

The symptoms of lymphoma in horses vary depending on the specific type of lymphoma present. Clinical signs associated with lymphoma include: [1][5][8][11][16][17][18]

  • Poor body condition
  • Weight loss
  • Lethargy or dullness
  • Weakness
  • Intermittent colic
  • Decreased appetite
  • Fever
  • Chronic diarrhea
  • Swelling of the lower abdomen and limbs
  • Swelling of the neck
  • Pale gums
  • Red spots on the gums
  • Eye or nose discharge
  • Itchiness
  • Skin lumps
  • Hair loss
  • Lameness
  • Penile prolapse
  • Difficulty breathing
  • Coughing
  • Increased heart rate

Disease Progression

As of now, the specific cause of lymphoma in horses has not been identified. However, it is widely understood that cancers, including lymphoma, arise from mutations in the DNA of the affected cells. [6]

Lymphoma spreads from its primary location through lymph vessels to other areas of the body. This invasion of cancer cells, called metastasis, is the most serious consequence of the disease. [6]

Symptoms arise from the cancer spreading and growing within organs and tissues. For instance, alimentary lymphoma damages the intestinal lining, leading to poor absorption of nutrients. This reduction in nutrient uptake can cause nonspecific symptoms such as weight loss, colic, and a reduced appetite. [11]

Lymphoma can also spread to the spleen, liver, bone marrow, skin, pancreas, lung, kidneys and other areas of the gastrointestinal tract. [5]

Secondary conditions

Depending on where the cancer is located, lymphoma can lead to secondary health issues. Some of these secondary conditions include: [1][4][16][19]

Paraneoplastic Conditions

Paraneoplastic conditions in horses are a group of disorders that occur because of the presence of cancer in the body, but not as a direct result of the physical presence of cancerous tumors.

These conditions are caused by the biological effects of the tumor cells, such as the substances they release, rather than the tumor itself invading or disrupting nearby tissues.

Although rare, equine lymphoma can be associated with a range of paraneoplastic conditions and symptoms, including: [9][17][20][21][22][23]

Diagnosis

Diagnosing lymphoma in a horse typically involves a combination of clinical examination, history, and a series of diagnostic tests. To confirm a diagnosis of lymphoma, your veterinarian will also rule out other conditions that could be causing disease symptoms. [1]

History

Your veterinarian will review your horse’s medical history and any recent changes in health, behavior, or performance. This can provide clues that may point towards lymphoma or rule out other conditions.

When evaluating clinical signs, your veterinarian will also to take into account the horse’s deworming history, the quality of feed available, and whether the horse is consuming an adequate diet. Issues with nutrition and parasite management can lead to vague symptoms that mimic those associated with lymphoma, such as weight loss or lack of appetite. [7]

Physical Examination

In horses with nonspecific symptoms like weight loss, a physical exam is crucial to exclude other causes of disease, such as dental issues. [5][7]

On physical examination of horses with lymphoma, enlarged lymph nodes may be palpated through the skin.

Rectal Palpation

Depending on the location of the cancer, intra-abdominal masses and enlarged lymph nodes may be identified on rectal palpation. [1]

Blood Tests

Lymphoma can sometimes be present in the blood, appearing as abnormalities in blood tests. The most common bloodwork abnormalities in equine lymphoma include anemia (low red blood cells) and altered lymphocyte counts, which can either be higher or lower than normal. [5][9]

Other blood values may also be abnormal if the cancer has spread or is affecting organ function. For example, low protein levels may occur due to poor nutrient absorption and high liver enzymes may be detected if the cancer has reached the liver. [1][8]

Ultrasound

Abnormal masses that are too far forward in the body for rectal palpation may be identified using an ultrasound. Ultrasound is also useful for identifying fluid build-up around the lungs, which accompanies some cases of lymphoma. [1][18]

Cytology

Fluid from the abdomen or cells from a mass can be sampled and analyzed microscopically. The characteristics of cells in these samples may confirm a diagnosis of cancer. [1]

Typically, finding numerous atypical lymphocytes in these samples can indicate lymphoma. These cells can undergo further specialized tests to determine the lymphoma type and help predict the prognosis. [18]

Biopsy

Tissue from enlarged lymph nodes or masses may be collected and further examined under a microscope. Biopsied tissues may be interpreted alongside other diagnostics to understand the behavior of the cancer and disease progression. [1][6]

Special stains may be performed to identify the lymphoma type, which can help in determining the prognosis and guiding treatment more effectively. [1][6]

Endoscopy

An endoscope is a long tube with a camera at the end that is passed into the digestive tract. Endoscopic examination can sometimes assist in locating a tumor or guiding a veterinarian’s choice of site for further investigation. [5]

Surgery

Exploratory surgery is sometimes performed to obtain multiple biopsies for a diagnosis of lymphoma, especially if smaller samples or other diagnostics are not helpful. Surgery may also be indicated to remove tumors early in the disease. [1]

Differential Diagnoses

A differential diagnosis is a systematic method used by veterinarians to identify conditions affecting a horse. Differential diagnosis involves distinguishing a particular disease or condition from others that present with similar clinical signs.

Diagnostic tests are crucial for ruling out other potential diagnoses and pinpointing the affected tissues or organs in horses with lymphoma. While histology — the study of the microscopic structure of tissues — is the most reliable method for accurately diagnosing lymphoma, it may not always be feasible. [1][5]

To confirm a diagnosis of lymphoma, other conditions with similar clinical signs must be ruled out, including: [5]

Treatment

Treatment options for lymphoma in horses are somewhat limited compared to those available for humans and smaller animals. Treatment strategies should be determined in consultation with a veterinary oncologist and customized to the individual horse’s condition.

When a horse is diagnosed with lymphoma, the treatment plan may include chemotherapy, radiotherapy, surgical removal or corticosteroid medications.

Supportive care is another crucial aspect of treatment, which can include nutritional support, pain management, and other symptomatic treatments to improve quality of life.

The prognosis for horses with lymphoma is generally poor, and the goal of treatment is often to extend a comfortable life rather than to cure the disease.

Chemotherapy

Chemotherapy for equine lymphoma involves the use of anti-cancer drugs to target and kill the malignant lymphocytes. The approach is similar to chemotherapy in humans and other animals, but it is adapted to the specific needs and physiology of horses.

Approximately 50% of treated horses achieve remission within 2-4 weeks of chemotherapy. However, in cases of multisystemic lymphoma, treatment may be required for several months to a year before remission is achieved. [6][18]

While chemotherapy is an option for horses with lymphoma, its use is limited due to the high cost of the treatment. [6]

Radiotherapy

Radiotherapy, also known as radiation therapy, is a treatment that uses high-energy rays or radioactive elements to destroy cancer cells. While it’s a common cancer treatment in humans and small animals, its use in horses, particularly for lymphoma, is not as well-documented due to several practical challenges.

However, radiotherapy may be helpful if the cancer is identified early and if only a single tumor is identified. Case reports suggest radiotherapy may be most useful for cutaneous lymphomas. [3]

Surgical Removal

Surgery may be an effective treatment strategy for tumors that are caught early and remain localized without having spread (metastasized).

However, in equine lymphoma, surgery is rarely recommended because removing a single mass does not cure this malignant cancer, which tends to affect multiple areas of the body. [6][7][14]

In some cases of alimentary lymphoma, removal or resection of part of the small intestine or colon has been effective. [19]

Medications

In some cases of cutaneous lymphoma affecting pregnant mares, the cancer naturally regressed during pregnancy. This indicates that hormone or steroid drugs are a possible therapeutic option in the management of lymphoma. However, more research is needed before this is an established treatment. [3]

Other treatments such as anti-inflammatories (i.e. flunixin meglumine) can also be implemented for symptomatic care. [18]

Prevention

Since the exact cause of lymphoma in horses is not well understood, there are no known preventive measures for this condition. Although there has been a suggested association between bovine leukemia virus and equine lymphoma, no confirmed cases have been reported. [8]

There are no vaccines available to prevent the development of lymphoma in horses.

Prognosis

The prognosis for lymphoma in horses is guarded to poor. While treatments may extend the life of the horse, lymphoma recurrence is high. [6]

Summary

Lymphoma is the most common cancer in horses. While it can be present in horses at any age, it is most commonly diagnosed in middle-aged horses. [5]

Horses with lymphoma can display a wide array of clinical signs depending on disease progression and location of the cancer. Common symptoms include weight loss, fatigue, digestive issues, swelling or lumps under the skin. [1]

Diagnosing lymphoma in horses requires a detailed history, physical examination, blood tests, diagnostic imaging, and tissue samples if a mass is identified. [1][5]

Treatment options for equine lymphoma are limited, and complete remission is less common in horses compared to smaller animals and humans. Treatment primarily aims to keep the horse comfortable and extend their quality of life. [1][6]

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References

  1. Reed S.M. et al., Equine internal medicine. 3rd ed. Saunders Elsevier. 2010.
  2. Durham A.C. et al., Two Hundred Three Cases of Equine Lymphoma Classified According to the World Health Organization (WHO) Classification Criteria. Vet Pathol. 2013. View Summary
  3. Taintor J. and Schleis S., Equine Lymphoma. Equine Veterinary Education. 2011.
  4. Firth E.C., Horner’s Syndrome in the Horse: Experimental Induction and a Case Report. Equine Veterinary Journal. 1978.View Summary
  5. Van Den Hoven R. and Franken P., Clinical Aspects of Lymphosarcoma in the Horse: A Clinical Report of 16 Cases. Equine Veterinary Journal. 1983. View Summary
  6. Mair T.S. and Couto C.G., The Use of Cytotoxic Drugs in Equine Practice. Equine Veterinary Education. 2006.
  7. East L.M. and Savage C.J., Abdominal Neoplasia (Excluding Urogenital Tract). Veterinary Clinics of North America: Equine Practice. 1998. View Summary
  8. Neufeld J.L., Lymphosarcoma in the Horse: A Review. Can Vet J. 1973. View Summary
  9. Meyer J. et al., Clinical, Laboratory, and Histopathologic Features of Equine Lymphoma. Vet Pathol. 2006. View Summary
  10. Taylor S. d. et al., Intestinal Neoplasia in Horses. Journal of Veterinary Internal Medicine. 2006. View Summary
  11. Roberts M.C. and Pinsent P.J.N., Malabsorption in the Horse Associated with Alimentary Lymphosarcoma. Equine Veterinary Journal. 1975. View Summary
  12. Platt H., Alimentary Lymphomas in the Horse. Journal of Comparative Pathology. 1987. View Summary
  13. Valli V.E. et al., Tumors of the Hemolymphatic System. In: Tumors in Domestic Animals. John Wiley & Sons, Ltd. 2016.
  14. Miller C.A. et al., Classification and Clinical Features in 88 Cases of Equine Cutaneous Lymphoma. J Vet Diagn Invest. 2015.
  15. Kelley L.C. and Mahaffey E.A., Equine Malignant Lymphomas: Morphologic and Immunohistochemical Classification. Vet Pathol. 1998. View Summary
  16. Matsuda K. et al., Jejunal Intussusception Associated with Lymphoma in a Horse. Journal of Veterinary Medical Science. 2013.
  17. Finley M.R. et al., Paraneoplastic Pruritus and Alopecia in a Horse with Diffuse Lymphoma. J Am Vet Med Assoc. 1998.
  18. Saulez M.N. et al., Use of Chemotherapy for Treatment of a Mixed-Cell Thoracic Lymphoma in a Horse. Journal of the American Veterinary Medical Association. 2004.View Summary
  19. Dabareiner R.M. et al., Large Colon Resection for Treatment of Lymphosarcoma in Two Horses. J Am Vet Med Assoc. 1996.
  20. Adams R. et al., Malignant Lymphoma in Three Horses with Ulcerative Pharyngitis. J Am Vet Med Assoc. 1988.
  21. La Perle K.M. et al., Multisystemic, Eosinophilic, Epitheliotropic Disease with Intestinal Lymphosarcoma in a Horse. Vet Pathol. 1998.
  22. Reef V.B. et al., Lymphosarcoma and Associated Immune-Mediated Hemolytic Anemia and Thrombocytopenia in Horses. J Am Vet Med Assoc. 1984.
  23. Mendes L.C.N. et al., Clinical, Histological and Immunophenotypic Findings in a Mare with a Mammary Lymphoma Associated with Anaemia and Pruritus: Mammary Neoplasm. Equine Veterinary Education. 2011.