Lymphoma is a systemic disease that can affect the whole body. You may have heard of it in the human context, but lymphoma can also affect animals, like dogs and cats. In fact, lymphoma in dogs is one of the most common types of dog cancer. If you think your dog could have lymphoma, keep reading to learn more.
What Is Lymphoma?
Lymphoma is a blanket term used by doctors to describe a group of cancers that stem from white blood cells called lymphocytes, which help the immune system fight off infection. Lymphomas are usually concentrated in organs that play a role in the immune system, such as lymph nodes, the spleen, and bone marrow.
While lymphoma isn't limited to these organs, this is where it is most often found. It occurs due to a genetic mutation (or series of mutations) in a lymphocyte. These mutations cause the lymphocytes to become malignant, and they then rapidly divide until they can overwhelm certain organs and body functions. Lymphoma in dogs is like non-Hodgkin's lymphoma in humans. It is so similar, in fact, that vets and human doctors use almost the same chemotherapy protocols to treat lymphoma in their patients.
Types of Lymphoma in Dogs
There are more than 30 different known lymphomas in dogs, and they all differ in how they progress. Some progress rapidly and are life-threatening without treatment, while others are slow-growing and are managed more than chronic diseases. The most common types of lymphoma in dogs that owners should know about:
Multicentric lymphoma is the most common lymphoma in dogs. It mostly affects the external lymph nodes, and the most obvious symptom is the rapid enlargement of the lymph nodes.
Alimentary lymphoma is the second most common form of lymphoma in dogs. It targets the intestines, which is where most of the symptoms occur.
Mediastinal lymphoma is rare. In this form, the thymus or mediastinal lymph nodes in the chest (or both) are affected.
Extra-nodal lymphoma in dogs refers to lymphoma that targets a specific organ, such as the skin, eyes, kidneys, lungs, or central nervous system. The most common extra-nodal lymphoma is called cutaneous lymphoma, which affects the skin.
Causes, Stages, and Treatments for Lymphoma in Dogs
What Causes Lymphoma in Dogs?
Unfortunately, scientists do not know what precisely what causes lymphoma in dogs. Several possible causes have been investigated, such as viruses, bacteria, chemical exposure (like certain pesticides), second-hand smoke, and physical factors (like strong electromagnetic fields), but the cause of the disease remains unclear. Suppression of the immune system is a known risk factor but has not been established as a cause of the disease.
Researchers believe there may be a genetic component to the disease, but genetics alone is not the primary cause. Advanced genetic studies may also help identify any underlying genetic and chromosomal causes and predispositions. Lymphoma is usually found in middle-aged or older dogs (roughly six to nine years in age). Although lymphoma can develop in any breed, some breeds have a higher incidence of lymphoma than others. In particular, lymphoma most commonly affects the following dog breeds:
Symptoms of lymphoma in dogs may depend on the type of lymphoma.
The most common initial symptom of multicentric lymphoma in dogs is firm, enlarged lymph nodes. A lymph node affected by the disease will feel like a hard, rubbery lump under your dog's skin. Dogs with multicentric lymphoma may also exhibit lethargy, fever, anorexia, abnormal thirst, anemia, weakness, and dehydration as the disease progresses.
Since alimentary lymphoma affects the intestines, dogs with this type of lymphoma may have symptoms of vomiting, abdominal pain, anorexia, diarrhea, and weight loss. This lymphoma may also impair the absorption of nutrients from food.
Dogs with mediastinal lymphoma may have difficulty breathing. This may be due to a large mass or accumulation of fluid in the chest. Dogs with mediastinal lymphoma may also show swelling of the face or front legs and increased thirst and urination.
The symptoms of Extra-nodal lymphoma depend on the organs affected. The most common type of Extra-nodal lymphoma is cutaneous lymphoma, which affects the skin. Symptoms of cutaneous lymphoma include individual, raised nodules, plaques, ulcers, or dispersed scaly lesions. In early phases, dogs may also exhibit hair loss and itching. In later stages, the skin may become thick or ooze fluid.
If the extra-nodal lymphoma is in the lungs, the dog may show symptoms of respiratory distress. In the kidneys, lymphoma could cause renal failure. Lymphoma in the eyes can cause blindness, lymphoma in the central nervous system can lead to seizures, and lymphoma in the bones can cause pain or fractures.
The World Health Organization has classified lymphoma into five stages based upon the degree of metastasis and invasiveness:
Cancer restricted to a single lymph node
Regional lymphadenopathy (either in the front half or back half of the body)
Generalized lymph node enlargement (typically in BOTH the front and back halves of the body)
Involvement of the liver and/or spleen
Involvement of blood, bone marrow, the nervous system, or other organs
The most common stage of lymphoma in dogs is Stage III. Stage I and Stage II diagnoses are much more rare, and dogs that start at one stage can progress to other stages of the disease.
The main purposes for assigning a particular stage to lymphoma in dogs include giving a sense of how far the disease has developed and defining the treatments that might work best in each individual case. Staging the lymphoma may also help give a sense of how effective those treatments may be and help rule out other concurrent medical problems which could affect the dog's prognosis. The further cancer has spread (i.e., the higher the stage), the less effective most conventional treatments may be.
Once cancer is suspected, your vet can get a more definitive diagnosis by taking a biopsy, removing a piece of the lymph node or affected organ. The larger the biopsy sample, the better the chance for an accurate diagnosis of lymphoma.
Besides a biopsy, your vet may also recommend "staging tests" following a lymphoma diagnosis. These "staging tests" can help the vet understand your dog's overall condition and how far the cancer has spread. The more widespread the cancer is, the poorer the dog's prognosis. However, dogs with very advanced lymphoma can still be treated, and their cancer can go into remission. Staging tests also help the vet determine whether there are any other underlying conditions that may affect treatment or prognosis.
The staging tests that your vet recommends may include blood tests, a urinalysis, x-rays, an abdominal sonogram, and a bone marrow aspirate. Organs that appear abnormal on a sonogram can be sampled with a small needle to confirm lymphoma in a test called a fine needle aspirate.
Treatment is usually determined by the stage of the disease. Without treatment, most dogs succumb to lymphoma in approximately four to six weeks.
The most effective therapy for lymphoma in dogs is chemotherapy though your vet may also recommend surgery or radiation therapy. Your vet may also recommend different types of chemotherapy depending on the type of cancer. being treated For instance, the common chemotherapy treatment for multicentric lymphoma in dogs is a 25-week protocol called UW-25, based on a protocol called CHOP that is often used to treat lymphoma in humans. However, the most effective drug for cutaneous lymphoma is lomustine (CCNU).
Dogs tolerate chemotherapy better than humans, as they don't get nearly as sick and they rarely lose their hair during treatment, except for a few breeds like poodles, Old English sheepdogs, and some terriers. The most common side effects of chemotherapy in dogs are mild vomiting and diarrhea, decreased appetite, and decreased activity levels.
The prognosis for lymphoma in dogs depends on the type of lymphoma and the type of chemotherapy used to treat it. The median length of survival of dogs with multicentric lymphoma treated with the UW-25 chemotherapy is between nine and thirteen months.
Remission, or a regression of the cancer, may either be partial or complete. Partial remission means that the overall cancer is reduced by at least 50%. Complete remission means that the cancer has become undetectable to any available screening test. Remission is not the same thing as a cure—total remission means that all signs and symptoms of cancer have disappeared, but that cancer could still be in the body.
Unfortunately, most dogs with lymphoma will have a relapse of their cancer at some point. A second remission is possible, but rarely lasts as long as the first remission. This is because lymphoma cells can adapt to the treatment and become more resistant to the effects of chemotherapy as time goes on. Eventually, lymphoma is fatal.
Living with Lymphoma
While no one wants to hear their pet has cancer, there are steps you can take to help you and your dog after diagnosis. Being proactive about your dog's treatment can improve their quality of life and might extend the time you have to enjoy together. Discuss the availability of treatment options with your veterinarian, including the possibility of hospice care and what you can do to keep your pet comfortable. End of life discussions are never easy, but they only get harder the longer you wait.
Lymphoma is a complex disease, with many factors that can impact your dog's health in various ways. However, treatments are available to help your pet's cancer go into remission. If you suspect that your dog is exhibiting symptoms of lymphoma, talk to your family vet as soon as possible. They are a great resource for you to help your pet live their best life.