Lymphoma is a cancer originating from a white cell called a lymphocyte that is found in lymphoid tissue.
Lymphoma can be nodal (found in the lymph nodes also known as glands) or extranodal (in the thymus, spleen, mucosa, conjunctiva, or skin-associated lymphoid tissue).
Lymphoma is one of the most common cancers of the horse. Representing around 1.5-14% of all equine tumours.
Lymphoma is the most common malignant tumour of the equine gastrointestinal tract and of the thorax (chest).
Lymphoma affects horses of all ages, all breed and both genders.
Lymphoma is always malignant
Types of Lymphoma
Lymphoma is classified using three factors:
Morphology (appearance) of the cell types involved: small-cell, large-cell, or mixed
Cell Type involved: B-cell, T-cell, mixed B- and T-cell, or NK-cell.
T cell lymphoma is usually more aggressive.
Multicentric/generalized lymphoma (the most common form),
Alimentary (intestinal tract and stomach)
Thoracic (inside the chest)
Solitary (single location)
Cutaneous (in the skin)
Clinical Signs of Lymphoma
The clinical signs vary depending on where the lymphoma is located and its stage (the extent of disease within the body).
The most common signs of lymphoma are nonspecific – weight loss, apathy, loss of appetite and elevated heart rate. A high temperature, ventral oedema (fluid accumulation on the tummy and limbs), recurrent colic and diarrhoea are frequently seen.
At the end-stage of lymphoma, the signs reflect dysfunction of organs involved.
Paraneoplastic signs are common – these include weight loss, anaemia, and significant changes in blood composition such as high blood calcium and low blood glucose.
Above: A 22 year old WB horse with generalized lymphoma showing weight loss, loss of muscle mass and paraneoplastic hypotrichia (hair loss)
Multicentric (Generalized) Lymphoma
Multicentric lymphoma can involve any cell type and can be found in the lymph nodes (glands) or in any lymphoid tissue throughout the body (extranodal)
Clinical signs include: Lethargy, Loss of appetite, Elevated temperature, Weight loss, Oedema (fluid accumulations), Enlarged lymph nodes and sometimes eye symptoms
This type of Lymphoma can progress to involve a variety of organs or to the development of leukaemia (bone marrow infiltration) at the end stage of disease
Alimentary or Gastrointestinal Lymphoma
Lymphoma can occur throughout the intestinal tract, however the small intestine is most frequently affected. T-cell lymphoma is most common at this site however , B-cell or T-cell rich can also occur.
Clinical Signs include: Lethargy, Loss of appetite, Weight loss, Oedema (fluid accumulations), Recurrent colic and Diarrhoea
The lymphoma can progress to involve other organs
Thoracic (Mediastinal or Thymic) Lymphoma
This type of lymphoma occurs in the lymph nodes (glands) within the chest (mediatsinal lymph nodes) and in the thymus (a lymphoid organ in front of the heart). the T cell form is more common at these sites, however B cell or mixed lymphomas are also possible.
Clinical signs include: Respiratory signs (nasal discharge, cough, respiratory distress), Oedema (fluid accumulation), Jugular vein distension, Enlarged lymph nodes
This type of lymphoma can progress to involve other organs
This type of lymphoma is found in the skin and just below the skin (subcutaneous tissue). It usuaully involves the B-cell type (T-cell rich B -cell type) but T-cell is also possible
Clinical signs include: Single or multiple subcutaneous, firm, non-painful (1-20cm diameter) masses. The lumps may wax and wane in size due to sex-hormones, season of the year or steroid therapy
Cutaneous lymphoma rarely involves the lymph nodes or spreads to other organs but it is possible
Diagnosis of Lymphoma
There are a number of changes on samples collected form teh bosy that will be suggestive of or confirm lymphoma.
Bloods may show:
Anaemia, high neutrophils (white cell that fights infections), high fibrinogen and high gamma globulin and low albumin (all signs of active inflammation), without other signs of infectious disease. Occasionally you wil find circulating lymphoma cells (lymphocytic leukemia) but this is rare.
Ultrasound examination of the chest or abdomen may show evidence of free fluid and masses in the lymph nodes or organs. the appearance of infiltrated organs will be affected
Cytology of the cells within any fluid (abdomen or thorax) may identify the presence of cancerous lymphocytes. Fluid in the chest usually provides a clear result but in the abdoemn the fluid may appear negative in horses with lymphoma.
Aspirates or biopsy of affected tissues is usually diagnostic
Above: Liver histopathology of generalized lymphoma. The normal liver architecture is obliterated by tumour cells – note the wide variation in size and morphology of the tumour cells
Immunostaining can be used if it is hard to differentiate lymphoma from chronic inflammation with reactive lymphocytes. A variety of types of lymphocytes (polyclonal population) supports inflammation, whilst a single type (monoclonal population) supports neoplasia
Treatment Options and Prognosis
The form of lymphoma will dictate the options available for treatment.
treatment is only palliative (prolonging life) rather than curing the disease. However in solitary and some cutaneous lymphomas where surgical removal is possible a cure may be acheivable, assuming there is no other disease elsewhere.
If end-stage/advanced lymphoma is present then there are limited options available
Surgical excision of solitary tumours
The unusual form of lymphoma that occurs in the conjunctiva of the eye region is best treated with surgery
Lynphoma can be treated with multi-drug protocols or with corticosteroids alone.
Some lymphomas are responsive to progesterone therapy (especially cutaneous forms of lymphoma).
Prognosis in the short-term is fair to poor with therapy, however in the long-term it is extremely poor. In long-term cases of lymphoma, death or euthanasia is inevitable.
We are grateful to Barbora Bezděková, DVM, PhD., DECEIM and Marta Barba, DVM, PhD, DACVIM, MRCVS for their help with this section.