Feline lymphoma is a malignant cancer of the lymphatic system, the exquisitely structured arrangement of internal organs and tissues that directly or indirectly influences virtually every aspect of a cat’s physical existence. Unfortunately, says Margaret McEntee, DVM, professor of oncology at Cornell University’s College of Veterinary Medicine, it is the most frequently diagnosed among all types of feline cancer.
The feline lymphatic system is a complex network of components that transport life-sustaining substances throughout a cat’s body and help prevent the circulation of harmful agents. Among these components are the thymus gland, the spleen, bone marrow, and what veterinarians refer to as “gut-associated” lymphoid tissue, which lines the surface of various areas of a cat’s body and prevents its exposure to infectious agents.
The distinctive characteristic of the lymphatic system’s anatomy, Dr. McEntee points out, has to do with its role in transporting a life-sustaining fluid (lymph) throughout a cat’s body. Circulation of this fluid is achieved via a network of minuscule, interlaced ducts (lymphatic vessels) that connect with specialized collections of tissue called lymph nodes—tiny, bean-shaped structures situated either deeply within a cat’s body or at various areas on its surface—on the neck, in the groin, and behind the knees.
Included among the functions performed by the lymph fluid and the vessels that transport it are: delivery of oxygen and nutrients to cells; collection of metabolic waste products; absorption of fat from the intestinal tract; and the removal of tissue debris, bacteria, viruses, and other infectious agents from the body.
Decades ago, lymphoma in the chest cavity (mediastinal lymphoma) or spread throughout the lymphatic system (multicentric lymphoma) were the most common sites for a lymphoma diagnosis. However, they are closely linked to infection with the feline leukemia virus (FeLV) and, to a lesser extent, the feline immunodeficiency virus (FIV). These viruses are less frequently seen today, presumably due to testing and isolation of infected cats and the use of vaccines that protect against FeLV. Currently, says Dr. McEntee, feline lymphoma is most often seen in the gastrointestinal tract, and the disease there can be very different in terms of treatment and prognosis than when lymphoma is diagnosed elsewhere.
Clinical signs of lymphoma in the gastrointestinal tract include weight loss, vomiting, diarrhea, and often either a decreased or increased appetite. Physical exam findings may be normal, though thickened intestines or abdominal masses may be felt. Bloodwork is likely to be normal in cats with gastrointestinal lymphoma, and ultrasound of the intestinal tract may show thickening of intestines or intestinal masses, though it may also appear normal. These signs and diagnostics are often indistinguishable from cats with Inflammatory Bowel Disease.
Definitive diagnosis requires biopsies of the intestinal tract. These can be obtained through endoscopy, a non-invasive procedure involving the use of a flexible camera used to visualize the stomach and upper intestines and take small samples of the tissue for microscopic examination. Surgery can also be used to obtain biopsies. Though this technique is more invasive than endoscopy, it can often result in a more accurate diagnosis, since larger samples can be taken from more places in the intestinal tract than can be reached with the endoscope. Microscopic examination of the biopsy will not only diagnose lymphoma in the gastrointestinal system, but it can also distinguish between small cell and large cell lymphoma.
Small cell lymphoma of the intestinal tract is considered an indolent disease, meaning it is much less malignant than other forms of lymphoma. Cats with this disease can be effectively treated at home with a combination of a steroid (prednisolone) and an oral chemotherapy drug (chlorambucil, and over 90% achieve remission of their clinical signs. Regular monitoring is needed, as these drugs can have some side effects, but most cats tolerate these medications well and survival time after diagnosis ranges between two and four years.
Large cell lymphoma in the intestines is a much more malignant disease and carries a significantly worse prognosis than small cell lymphoma. Large cell lymphoma can be associated with a mass in the intestines. In those cases surgery may be recommended, but otherwise treatment and prognosis is similar to that for multicentric lymphoma, described below.
Lymphoma outside the gastrointestinal system
Lymphoma can be present in many parts of the cat's body, as the lymphatic system is spread throughout many organs. Disease present mostly in the peripheral lymph nodes, which can be felt as enlarged lumps in the neck, in front of the shoulder blades, and behind the knees, is much less common in cats than in dogs. The disease can also be found in internal lymph nodes, in the chest cavity (mediastinal lymphoma), the liver, spleen, kidneys, nasal cavity, eyes or central nervous systems.
Depending on where the lymphoma is present, initial signs can range from nonspecific problems such as weight loss, poor appetite, and lethargy, to concerns such as difficulty breathing, nasal discharge, masses on the skin or seizures. If lymphoma is suspected, the ailing animal will undergo a thorough physical examination and testing designed to either confirm or exclude a tentative diagnosis of the disease. This will include a complete blood cell count, blood chemistry panel, urinalysis, chest x-rays, microscopic analysis of lymph node tissue samples, and ultrasound examination of an animal’s abdomen to see whether its liver or spleen—common sites for lymphoma involvement—is enlarged.
Cats of any age can develop lymphoma, although most affected animals are 10 to 12 years of age. Unvaccinated outdoor cats are at greater risk than indoor cats due to their greater exposure to FeLV infection. And recent studies have indicated that cats routinely exposed to tobacco smoke are at elevated risk for gastrointestinal lymphoma.
Treatment for a cat that has been diagnosed with most forms of lymphoma typically centers around chemotherapy. A veterinary oncologist will determine the appropriate combination of chemotherapeutic agents to administer to the cat intravenously, often once weekly for many weeks depending on response to treatment. Monitoring is required to assess the cat for side effects of treatment, though chemotherapy is generally better tolerated in animals than in people. Common side effects include lowered white blood cell count, vomiting, and decreased appetite. Depending on the type of lymphoma, other treatments may be needed. For instance, surgery may be recommended to remove an intestinal mass in the case of large cell lymphoma of the intestinal tract, and radiation therapy can be very effective in the treatment of lymphoma in the nasal cavity. Prognosis can vary depending on the exact location and type of lymphoma present, but with aggressive chemotherapy protocols, between 50-80% of cats will achieve remission of clinical signs for an average duration of between four and nine months.
If chemotherapy or radiation therapy are not an option, cats can be treated palliatively with a steroid (prednisolone) only. This treatment will generally help cats achieve a temporary improvement or remission of disease for between two and four months. While the prognosis for cats receiving steroid-only treatment is significantly worse than cats receiving more aggressive chemotherapy protocols, it can be administered at home, significantly reducing the number of trips to the veterinarian and the cost of treatment.
While lymphoma cannot be prevented, Dr. McEntee notes, the chances that a cat will develop the disease may be reduced through vaccination against FeLV, by preventing contact with FIV or FeLV infected cats, and by making sure it does not live in an environment contaminated by tobacco smoke. Since early detection of the disease may improve an animal’s chances for survival, Dr. McEntee also recommends that all cats seven years of age and older undergo twice-yearly physical examinations that include blood chemistry testing and thorough palpation of the patient’s body.