Recently, a 14-year old mare was presented to Willamette Valley Equine on referral for evaluation of lumps around her vulva that had been developing over the past several years. In addition, she had swelling above her eyes and near her right hind fetlock. The lumps had been increasing in size, but did not appear to bother the mare, and she showed no signs of illness. We surgically removed the masses near the vulva and submitted them for testing. The diagnosis from the pathologist was lymphoma.

Lymphoma is the most common malignant neoplasm in the horse, although it is still not frequently diagnosed. The onset of lymphoma is usually between the ages of 4 and 10 years, and can affect any breed, and both sexes equally. Clinical signs of lymphoma vary depending on the tumor site, but may include weight loss, subcutaneous edema (fluid accumulation), enlarged lymph nodes, fever, decreased appetite and depression.

There are four main classifications of lymphoma: generalized or multicentric, gastrointestinal, mediastinal, and extranodal. The generalized form is the most common, making up half of lymphoma cases. In the generalized form, lymph nodes throughout the body are involved, as well as internal organs such as the liver, spleen, kidneys or lungs. Gastrointestinal lymphoma affects the small intestine and associated lymph nodes, and leads to clinical signs such as diarrhea, abdominal pain, and weight loss. Mediastinal lymphoma indicates lymphoma of the lymph nodes within the thorax, and can cause fluid build-up within the chest cavity, difficulty breathing, and an increased respiratory rate. Extranodal lymphoma occurs with tumor development at any of the following sites: skin, upper respiratory tract, eyes or the tissue surrounding the eyes and the central nervous system. Many cases of lymphoma may involve more than one of these classification types.

Cutaneous lymphoma is the least common form of lymphoma. The onset is usually noticed as a progressive, slow development of masses within the skin or in the subcutaneous tissue. The masses are usually well circumscribed yet nonpainful and may get larger then dissipate before later returning. The skin over the masses is often normal, but may be hairless or develop sores. Common areas for these masses to occur include the shoulder, around the anus, or on the chest. To obtain a diagnosis, biopsy samples must be collected. The most effective method is by an excisional biopsy, where the mass is surgically removed. Then the entire mass is submitted for histopathologic evaluation. Cutaneous lymphoma may be an indication of internal malignancy, therefore a thorough work-up is critical. Further diagnostics that may be warranted include rectal palpation to evaluate lymph nodes, bloodwork, aspirates of enlarged lymph nodes, evaluation of fluid from the abdominal or thoracic cavities and ultrasound to evaluate internal organs. If there is no internal organ involvement, the cutaneous form of lymphoma can remain stable for several years, even without treatment. Several forms of treatment have been used for equine lymphoma, including systemic chemotherapeutic agents. Concerns with the use of chemotherapy include cost and possible toxicity to the horse. Besides chemotherapy agents, corticosteroids may be helpful to reduce the size of the tumors while also reducing secondary problems that can occur with lymphoma. Daily treatment with dexamethasone can be initiated, and then the tumors should be monitored for reduction in size to allow adjustment of the dosage.

Since the surgical removal we have been treating our cutaneous lymphoma patient with dexamethasone for approximately two months. She has been doing well and her masses have decreased in size.