Lymphoma is the most common feline cancer. Over the years, however, with increased testing and vaccination for retroviruses like feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV), the exact type of lymphoma seen in cats has changed.
Both FeLV and FIV have been associated with the risk of lymphoma in cats. FIV-infected cats have been reported to be at five times higher risk for lymphoma than uninfected cats. Prior to testing and vaccination programs, FeLV infection was associated with a higher risk of developing lymphoma as well—and cats who were FeLV positive had a poorer prognosis if they did develop lymphoma.
Lymphoma involves cells of the lymphatic system. This is a network of channels throughout the body that work in concert with blood vessels to bring oxygen to tissues, collect metabolic waste, and remove other debris that might build up in various organs, such as dead cells, cells that fight infection, and pathogenic agents such as bacteria and viruses. Along with the vessels, the lymphatic system also has lymph nodes where cells of the lymph system are collected and stored. The thymus (in the chest cavity), spleen, bone marrow, and lymphoid tissue lining the intestinal tract are all part of this body-wide system. The liver also has lymphoid tissues and may become involved in lymphomas.
Cats have been shown to suffer from three basic types of lymphoma:
-Mediastinal (in the chest cavity and usually involving the thymus)
-Multicentric (in lymph nodes throughout the body as well as organs with concentrations of lymphoid tissue)
-Alimentary or gastrointestinal (in the lymphatic tissues of the gastrointestinal tract)
The first two types are often associated with FeLV infection and sometimes FIV infection. Since more cats are being tested and isolated if positive or vaccinated if negative, the incidence of mediastinal and multicentric lymphomas in cats has decreased, but intestinal lymphomas have picked up.
Lymphoma tends to be a disease of middle-aged or young senior cats —generally 10 to 12 years of age. An exception, however, involves young Siamese cats who develop intestinal lymphomas (believed to be secondary to inflammatory bowel disease) and Siamese cats who develop mediastinal lymphoma, which tends to strike cats under two years of age. Regular physical examinations, with thorough histories and basic bloodwork can help in early diagnosis.
Your veterinarian will do basic bloodwork, with radiographs or an ultrasound evaluation to look for any abnormalities. Bloodwork may show an increase in liver enzymes, an increase in values associated with kidney function, and/or anemia. Generally, a biopsy of an enlarged lymph node or a section of intestines will be recommended if gastrointestinal lymphoma is expected. The microscopic evaluation of the tissues can help your veterinarian/veterinary oncologist stage the cancer and decide on the best treatment for your cat.
Intestinal lymphomas tend to be small-cell lymphomas. “Small-cell” refers to the type of lymphocytes found in the tumors. Cats with small-cell lymphomas usually have a better prognosis, response to treatment, and longer survival times than cats with large-cell lymphomas. Cats with a lymphoblastic cell type of lymphoma have a more grim prognosis long term.
The gold standard for treatment of lymphomas is chemotherapy. A common starting place for chemotherapy for lymphoma in cats is often a steroid, such as prednisolone, combined with a standard chemotherapy drug such as chlorambucil. Your veterinarian may refer you to a veterinary oncologist to develop a treatment plan for your cat.
Both prednisolone and chlorambucil are pills. This makes it easy (relatively, given the ability of some cats to resist pill taking!) to treat your cat at home. You should wear gloves when handling the chlorambucil. Your cat will still need to visit the veterinary hospital for bloodwork to watch for any side effects of the medications and to track progress in fighting off the cancer cells.
After a month of treatment, most cats (50 to 75 percent) show improvement, with resolution of their clinical signs and a reduction in the cancer volume. At this time, some veterinarians may discontinue treatment altogether if the cat shows enough improvement. Usually, the cancer is “beaten back” but not totally defeated. Observant owners can watch for any signs that the cancer is recurring. At that point a “rescue” protocol of chemotherapy will commonly be instituted.
A rescue protocol may consist of the same original drugs (in the case above, another round of prednisolone and chlorambucil) or your veterinarian may try a different set of medications. That may mean hospital visits for intravenous injections. Rescue medications may include lomustine, cyclophosphamide, or vincristine. One study looked at using low levels of radiation delivered over two days as a “rescue protocol” for cats whose lymphoma was confined to the abdomen. Ten out of 11 cats showed improvement and lived almost a year post treatment.
While no cancer diagnosis is a reason for celebration, feline lymphoma can be responsive to treatment. Most cats tolerate chemotherapy reasonably well. It now appears that many cats can go off chemotherapy after resolution of their clinical signs and still respond to a rescue protocol later if one is needed. In one study of cats with small-cell lymphoma, the survival times averaged over 1,300 days, with at least one cat living for an additional 2,479 days after treatment started. This suggests that lymphoma is, to some extent, a treatable cancer.
How can you best avoid lymphoma risk for your cat? Start by testing any cat or kitten before you bring them home for FeLV and FIV. Then, even if your cat will be strictly indoors, consider vaccinations. At least for the first year of life, FeLV vaccine is usually considered a core vaccine—one recommended for all cats—though FIV vaccine is not (recommended in certain circumstances). Discussions with your veterinarian about these issues is the best way to inform yourself of the risks and benefits of vaccination against these viral diseases. Avoid smoking or allowing anyone else to smoke around your cat, since tobacco smoke is associated with an increased feline lymphoma risk.
Keep your cat as fit and healthy as possible, so that if she does develop lymphoma she is in the best possible condition to fight it.