Lymphomas are cancers that arise from lymphocytes (white blood cells that fight disease). Normally, these cells travel through the body in the blood stream and in another network of thin “tubes” called lymph vessels, which interconnect organs of the lymphoid system (spleen, lymph nodes, thymus). Lymphocytes spend much of their time in these organs, where most of their disease-fighting activity takes place. When a lymphocyte becomes cancerous, it divides out of control and produces large numbers of cells just like it, which crowd the lymph nodes and make them swell. That is why the lymph nodes (and sometimes other lymphoid organs) appear enlarged in cases of lymphoma.
Lymphoma is one of the most common cancers seen in dogs. In fact, lymphoma occurs about 2 to 5 times as frequently in dogs as in people and it is estimated that approximately 1 of every 15 dogs born today will get lymphoma at some point in his life, most likely during late adulthood (between the ages of 8 and 11). There are breeds where the lifetime risk is higher (and lower); still, lymphoma can affect any dog of any breed at any age.
As noted above, most cases of lymphoma appear as swollen “glands” or lymph nodes that can be seen or felt under the neck, in front of the shoulders, or behind the knee. Occasionally, lymphoma can affect lymph nodes that are not visible or palpable from outside the body, such as those inside the chest or in the abdomen. In these cases, dogs may accumulate fluid in the chest that makes breathing difficult, or they may have digestive problems (diarrhea, vomiting, or apainful abdomen). If left untreated, dogs with lymphoma will generally die from their disease within 3 to 4 weeks. Treatment with prednisone (a corticosteroid) alone generally can induce short-lived remissions (usually less than 8 to 12 weeks), but this treatment can make the disease resistant to other treatments.
Long-lasting remissions can be achieved for dogs with lymphoma, so the disease is considered “treatable.” Multi-agent chemotherapy (medicines that kill cancer cells) can produce remissions of 12 to 18 months in many cases, and occasionally longer. Some of these drugs can be given orally; others must be injected into a vein or muscle. The goal of treatment for canine patients is to maintain quality-of-life during their remission, so in most cases, drug doses used to treat dogs will be lower than those used for comparable diseases in children or in adult humans. This minimizes side effects, but in some cases, chemotherapy can still make dogs feel sick and weak. These effects are often temporary and usually go away once chemotherapy treatments are finished or if they are discontinued. Veterinarians can prescribe medicine to minimize or control these side effects. Sometimes, radiation therapy may also be used to kill the cancer cells, and recent work suggests that some dogs could benefit from bone marrow transplants, although this type of treatment remains experimental and is only available through very few specialized centers. We must note that there are subtypes of lymphoma that exhibit different behaviors, and some of the more aggressive types are unresponsive to any available treatment.
Unfortunately, even those lymphomas that respond to treatment will eventually recur in most cases. Cures are rare, and most dogs that get lymphoma die from causes related to their disease. Therefore, research into prevention is as important as research to develop new and more effective treatments. Presently, we do not know precisely what causes lymphoma. While we know that in cats, cows, mice, and people there are certain viruses that can lead to lymphoma, no such viral agents have been identified in dogs. Similarly, no single environmental agent or toxin can be blamed for lymphoma (although it is possible that some may increase the risk of the disease). Lastly, heritable risk for lymphoma cannot be assigned to a single gene, but rather, it seems to follow complex patterns that indicate involvement and interaction among many genes. Still, the observation that the lifetime risk and incidence of lymphoma are different among distinct dog breeds provides a unique opportunity to identify heritable factors that could be manipulated to reduce this risk (and which would be difficult or impossible to identify in other species).
A recently published study supported in part by the AKC Canine Health Foundation showed that the oldest breeds, including Spitz dogs (Siberian Husky, Alaskan Malamute, Chinese Shar-Pei, etc.) and small Asian “lap dogs” (Shih Tzu, Lhasa Apso, etc.) share a predisposition for excess lymphomas that arise from cells called “T-lymphocytes”, suggesting these breeds retain inherited risk factors that arose ancestrally. In contrast, some recently derived European breeds such as Basset Hounds and Cocker Spaniels are predisposed to excess lymphomas that arise from cells called “B-lymphocytes”, suggesting these tumors may stem from different risk factors, or combinations of risk factors, that arose during the process of breed derivation and selection. Finally, Boxers and Golden Retrievers show an approximately equal number of occurrences of lymphomas that arise from B-lymphocytes and from T-lymphocytes, and in Goldens, each of these tumor types has unique genetic characteristics. These exciting results provide the first level of insight that will allow scientists to identify heritable factors that influence the risk of lymphoma in both dogs and people.
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