Bit & Bridle

Equine Lymphoma

Other Names

  • Lymphosarcoma
  • Malignant Lymphoma
  • Equine Haematological Neoplasia

What is Equine Lymphoma?

Equine Lymphoma is the most common haematopoietic (blood cell-derived) neoplasm in horses and one of the most frequently diagnosed internal cancers in the species. It is a malignant proliferation of lymphocytes (a type of white blood cell) within lymphoid tissues, including lymph nodes, the spleen, bone marrow, and mucosal tissues of the gastrointestinal tract, thoracic cavity, and skin.

The condition is classified into four main forms based on the primary site of involvement: multicentric (generalised, involving multiple lymph node groups), alimentary (gastrointestinal tract), mediastinal (thymic, within the chest), and cutaneous (affecting the skin). Each form has a distinct presentation, disease course, and prognosis.

Equine lymphoma most commonly affects middle-aged to older horses (typically 4–20 years of age), though it can occur at any age. It is a serious and usually ultimately fatal condition, though the rate of progression varies enormously — some horses live comfortably for months to years with appropriate management.

Causes of Equine Lymphoma

The underlying cause of equine lymphoma in most cases is unknown. Unlike in cattle, there is no established viral cause equivalent to bovine leukaemia virus in horses, though research into the role of equine retroviruses continues. Known or suspected contributing factors include:

  • Unknown (idiopathic): The majority of cases have no identifiable cause.
  • Immune dysregulation: Chronic immune stimulation or immune system dysfunction may contribute to malignant lymphocyte transformation.
  • Genetic predisposition: While no specific genetic markers have been identified in horses as in human medicine, certain breeds may have higher incidence, though data remains limited.
  • Age: Risk increases with advancing age, as cumulative cellular mutations accumulate over time.

Symptoms and Signs

Clinical signs depend heavily on which form of lymphoma is present and which organs are involved:

Multicentric (generalised) lymphoma:

  • Progressive weight loss and poor body condition despite adequate feeding
  • Generalised lymph node enlargement — palpable enlargement of the submandibular, parotid, and other peripheral nodes
  • Ventral oedema (swelling under the belly, chest, and lower limbs)
  • Depression and lethargy
  • Intermittent or persistent fever

Alimentary (gastrointestinal) lymphoma:

  • Chronic diarrhoea, often intermittent initially
  • Protein-losing enteropathy: severe weight loss, hypoproteinaemia, ventral oedema
  • Colic episodes
  • Rectal abnormalities on examination in some cases

Mediastinal (thymic) lymphoma:

  • Respiratory signs: dyspnoea (laboured breathing), exercise intolerance
  • Oesophageal compression causing difficulty swallowing (dysphagia)
  • Pleural effusion (fluid in the chest cavity)

Cutaneous lymphoma:

  • Multiple skin nodules, often firm, non-painful, and distributed across the body
  • May be present for months or years before systemic spread occurs
  • Generally associated with a longer survival time than other forms

Diagnosis

Equine lymphoma can mimic many other conditions, making it one of the more challenging diagnoses in equine medicine. A combination of approaches is typically needed:

  • Haematology and biochemistry: Blood tests may show anaemia, elevated white cell count (or paradoxically normal counts), hypoproteinaemia, hypercalcaemia (in some cases), and elevated lactate dehydrogenase. These are supportive but not diagnostic.
  • Cytology: Fine needle aspirate (FNA) of enlarged lymph nodes or skin nodules, examined microscopically, can reveal neoplastic lymphocytes. This is often the most practical first-line diagnostic step.
  • Histopathology: Biopsy of affected tissue provides definitive diagnosis and characterises the lymphoma type — important for prognosis and treatment decisions.
  • Ultrasound: Abdominal and thoracic ultrasound can identify enlarged internal lymph nodes, splenic abnormalities, peritoneal effusion, and pleural effusion.
  • Peritoneal or pleural fluid analysis: Where effusions are present, fluid analysis (cytology and protein content) can reveal neoplastic cells.
  • Bone marrow biopsy: In cases where haematological abnormalities are prominent, bone marrow examination may be warranted.

Treatment

There is no curative treatment for equine lymphoma. Management is aimed at achieving palliation — slowing disease progression and maintaining quality of life for as long as possible:

  • Corticosteroids: Prednisolone (oral) or dexamethasone is the most widely used palliative treatment. Steroids reduce lymphocyte proliferation and inflammation, can cause temporary tumour regression, and significantly improve the horse’s comfort and appetite. Response is variable — some horses respond well for months.
  • Cytotoxic chemotherapy: Multi-drug chemotherapy protocols (adapted from human and small animal oncology) have been used in horses, with variable results. Cost, practicality, and risk of side effects limit routine use. Referral to an equine specialist or teaching hospital is appropriate if the owner wishes to pursue this option.
  • Supportive care: High-quality nutrition (easily digestible, calorie-dense feeds), management of effusions as needed, and pain management are important components of quality-of-life care.
  • Euthanasia: When quality of life deteriorates despite management — severe weight loss, uncontrolled pain, persistent respiratory distress, or complete loss of appetite — humane euthanasia is the most appropriate decision and should be discussed proactively with your vet.

Prevention

There is no known preventive strategy for equine lymphoma at present, given that the underlying cause is in most cases unknown. General health maintenance, good nutrition, and regular veterinary examinations — enabling early detection of lymph node changes — are the most practical measures available.

Prognosis

Prognosis is generally poor to grave for most forms of equine lymphoma. Survival times vary but are often measured in weeks to months from diagnosis, though some horses — particularly those with cutaneous or indolent forms — may live considerably longer with palliative management. Cutaneous lymphoma carries the best prognosis of the four forms. Mediastinal and alimentary lymphoma with severe hypoproteinaemia or pleural effusion carry the poorest outlook.

When to Call the Vet

Contact your vet if your horse shows:

  • Progressive, unexplained weight loss despite adequate feeding
  • Palpably enlarged lymph nodes under the jaw, around the parotid region, or in the groin
  • Ventral oedema (swelling under the belly or limbs) without a clear cause
  • Multiple firm nodules appearing on the skin
  • Chronic diarrhoea or recurrent colic
  • Breathing difficulties or exercise intolerance
  • Persistent fever without an obvious infectious cause

Frequently Asked Questions

Can equine lymphoma be cured?

Current treatment options are palliative rather than curative. Corticosteroids and chemotherapy can slow disease progression and improve quality of life in some cases, but achieving long-term remission or cure is uncommon in horses. Research continues, and specialist oncology centres offer the most up-to-date protocols for owners who wish to pursue aggressive management.

How long can a horse live with lymphoma?

Survival time varies considerably by lymphoma type and individual response to treatment. Horses with cutaneous lymphoma may live for a year or more with good quality of life. Those with multicentric or alimentary forms often have survival times of 1–6 months from diagnosis, though outliers exist. Your vet can provide a more specific prognosis once the form and extent of disease are characterised.

Is equine lymphoma contagious to other horses or humans?

No. Equine lymphoma is not contagious to other horses or to humans. It is a neoplastic (cancer) condition arising from the horse’s own cells, not an infectious disease. There is no evidence of horse-to-horse transmission or any zoonotic risk.

What is the difference between lymphoma and leukaemia in horses?

Lymphoma primarily involves solid tissue (lymph nodes, spleen, gastrointestinal tract, skin), while leukaemia is defined by neoplastic cells circulating in the blood and bone marrow. In practice, the distinction can blur — lymphoma can have a leukaemic phase when cells spill into the bloodstream. Bone marrow examination and haematology help distinguish the two. Both are managed similarly in horses, with the same limited therapeutic options.