Cancer is relatively rare in horses. However, one of the most common equine types is called lymphosarcoma, a serious cancer with a poor prognosis. This is a cancer of the lymphatic system, lymphocytes in the internal forms and histiocytes in some cases with the skin form. The disease may take several forms, each with different signs:
• Cutaneous: This form appears as lumps under the skin. They may be single or multiple. These lumps usually stay covered with skin and hair but may ulcerate through the skin. The masses may be mistaken for hives, bite/sting reactions or benign tumors. As time goes on, local lymph nodes become enlarged and the tumors may also invade underlying muscles.
The cell type in this form of lymphoma is different from the internal lymphosarcomas and slower to spread. Horses may live for years after this disease is diagnosed. Interestingly, cutaneous lymphoma may be associated with granulose-cell tumors of the ovary, which should also be checked.
• Intestinal: The malignant tissue is within the wall of the intestinal tract, which has a rich supply of lymphatics and lymph nodes. A universal finding is unexplained weight loss. If the disease involves only the small intestine, the horse may not have diarrhea. With large intestinal involvement, there will be diarrhea. Colic may be seen. The disease may be either well localized to one segment of bowel and the adjoining lymph nodes, or be present in multiple locations and organs inside the abdomen.
• Thoracic: In the chest. The thymus gland or lymph nodes of the chest may be involved.
• Multicentric: Multicentric lymphosarcoma refers to disease in more than one location or organ. It can be found anywhere, from nasal cavity to the chest to abdominal organs. Enlarged lymph nodes around the trachea may cause difficulty breathing or swallowing. Bone, heart and even nervous system involvement can occur.
The cause of lymphosarcoma is unknown but viruses have been implicated in other species and may be the cause in horses as well. In rare cases where horses under the age of 4 develop this disease, an immunodeficiency may be found in the horse.
It’s not clear whether the immunodeficiency is secondary to the same virus that causes lymphosarcoma, or if there is a genetic immunodeficiency that forces other arms of the immune system to work harder or become unregulated, resulting in the cancer.
• Unexplained significant weight loss and poor appetite.
• Depression and lethargy, often associated with periods of fever (“tumor fever”).
• Edema of the legs, chest or belly. The edema is caused by enlarged lymph nodes restricting drainage of venous blood and lymph.
• Fluid collection in the chest, pericardial sac or abdomen.
• Cough and poor exercise tolerance with lung involvement.
• Colic and/or diarrhea with intestinal forms.
• Anemia and/or clotting problems with bone marrow involvement.
• Rectal exam to check for intestinal thickening, enlarged lymph nodes, enlarged abdominal organs. These findings would be suspicious but not diagnostic.
• Abdominal ultrasound to look for same.
• Chest X-ray and chest ultrasound if there are respiratory symptoms or swallowing difficulty. Again, findings would be suspicious but not diagnostic.
• Glucose or xylose (another form of sugar) absorption tests may be abnormal with involvement of the small intestine.
• Serum protein. Albumin may be decreased in intestinal form. Globulin normal to somewhat increased. Suspicious but not diagnostic.
• Serum chemistry. May see elevated calcium, abnormal liver or kidney function tests. These are not common, and not diagnostic.
• Blood counts: Mild anemia is common, severe with bone marrow involvement. Neutrophils may be elevated. Lymphocytes and white blood count are rarely elevated.
• Biopsy. This is the only truly diagnostic test. Biopsy of thickened bowel, skin nodule, an enlarged organ (e.g. spleen, liver) or an enlarged lymph node will reveal if there is lymphosarcoma.
Currently, we have no truly effective form of treatment for lymphosarcoma in the horse. Steroids are sometimes used with perhaps a short-lived improvement, but anticancer drugs that perform well in other species, like cyclophosphamide and vincristine, don’t work well in horses, have significant side effects and are fairly expensive.
Generally six to 12 months for all but the cutaneous forms of lymphosarcoma. If the skin tumors aren’t being caused by spread from an internal area, the horse may live for two to six years before starting to go downhill rapidly.
If the horse is a mare and the skin form involves histiocytes, the ovaries should be checked for a granulosa cell tumor. Removal of that can significantly improve the skin cancer but usually won’t cure it. Eventually, most horses are euthanized for humane reasons related to pain, weakness and severe weight loss.
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