Lymphoma is a common cancer in people and dogs that affects the lymph nodes and lymphatic system. The lymphatic system is made up of organs, tissues, and vessels. It is responsible for fighting infection and keeping a healthy level of fluids throughout the body.
While older dogs tend to be more predisposed to developing cancer, younger dogs can also be at risk. Understanding lymphoma can help owners come to terms with their dog’s condition, assisting them in making more informed decisions about their dog’s health.
What Is Lymphoma?
Scientifically speaking, lymphoma is a blanket term doctors use to describe a group of cancers that occur due to a change or overgrowth of lymphocytes. Lymphocytes are a type of white blood cell that helps the immune system fight off infection and are highly concentrated in organs that play an important role in the immune system. Organs like the lymph nodes, the spleen, and bone marrow are heavily affected by changes in lymphocyte levels. Most lymphoma cancers are found in these organs, but lymphoma can affect any organ in the body.
Veterinarians and human doctors use similar chemotherapy protocols to treat lymphoma in dogs and people. There are also similarities between dogs and humans in the way lymphoma is diagnosed and classified. However, precursor diseases, which are the presence of abnormal cells that progress to disease and increase the likelihood of illness, are more common in dogs than in people. These precursor diseases are tied to different types of lymphoma in dogs. For example, if they originate from cells in bone marrow, the cancer will involve blood or bone marrow.
Types of Lymphoma in Dogs and Their Symptoms
There are more than 30 different types of known canine lymphomas, all of which vary in aggressiveness, survival rates, and clinical signs. The symptoms of lymphoma in dogs aren’t always the same, which can make it difficult to diagnose at first.
The four most common types of lymphoma in dogs are:
- Multicentric lymphoma
- Alimentary lymphoma
- Mediastinal lymphoma
- Extranodal lymphoma
Multicentric lymphoma accounts for approximately 80% to 85% of lymphomas in dogs. This type of cancer affects the lymph nodes. In most cases, the most obvious sign for veterinarians is the lymph nodes rapidly getting larger. Lymph nodes are found in different parts of the body, such as the neck, chest, armpits, behind the knees, and groin, and any of these areas can be affected. It’s common for dogs with lymphoma to have enlarged lymph nodes, between three to 10 times larger than normal. This swelling isn’t painful and feels like a firm, rubbery lump that moves freely beneath the skin. Dogs with multicentric lymphoma may also develop lethargy, fever, anorexia, weakness, and dehydration as the disease progresses.
A pilot study on multicentric lymphoma found that several types of gut bacteria were found in greater amounts in dogs with multicentric lymphoma. This suggests their presence could potentially act as biomarkers for multicentric lymphoma, which are indicators of what’s going on in a cell at a given time. By recognizing possible biomarkers of multicentric lymphoma, scientists may be able to more effectively track the disease’s progress in the future.
The second most common form of lymphoma is called alimentary lymphoma, which accounts for less than 10% of canine lymphomas. Alimentary lymphoma targets the intestines, which is where the majority of symptoms occur. Dogs with alimentary lymphoma, which affects the intestines, may experience vomiting, abdominal pain, anorexia, diarrhea, and weight loss.
Mediastinal lymphoma is rare. It is named for the mediastinal lymph nodes, which are the ones located in the chest area. This can be the location of this lymphoma, or the thymus, the gland that produces white blood cells. Either or both may become enlarged. This is caused by high-grade malignant T lymphocytes, which grow and spread quickly and don’t look normal under a microscope.
As the disease progresses, dogs with mediastinal lymphoma may experience shortness of breath or have difficulty breathing. This can happen when fluids accumulate in the chest, creating additional pressure on the lungs and possibly even blocking veins that carry blood away from the heart. Affected dogs may also drink and urinate more frequently when calcium builds up in the blood.
Extranodal 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 extranodal lymphoma affects the skin and is called cutaneous lymphoma. The symptoms depend on the organs affected. Telltale signs of cutaneous lymphoma are individual, raised nodules or generally dispersed scaly lesions. Cutaneous lymphoma may also affect the gums, lips, and the roof of the mouth.
If extranodal lymphoma is located in the lungs, symptoms of respiratory distress are likely. Similarly, extranodal lymphoma in the kidneys 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.
What Causes Lymphoma in Dogs?
Lymphoma has only recently been properly recognized as a group of diseases with distinct subtypes. Still, there is a lot we don’t know about what causes lymphoma in dogs. Lymphoma accounts for approximately 7% to 24% of cancers in dogs.
Since dogs share our environment, we know that they are exposed to many of the same environmental factors that may cause cancer in humans, including phenoxyacetic acid, toxic substances produced during the breakdown of chemicals, including pesticides, fungicides, medications, and chemical dyes. Researchers suspect that viruses, infections, age, and UV damage are possible contributors to cancer, with the most likely cause being a combination of genetic and environmental factors.
One study took a look at 78 breeds, and out of these, there was an increased risk of lymphoma in 30 dog breeds and a reduced risk in 26 breeds. Results for the remaining breeds were normal or not strong enough to reach a conclusion, meaning they were not statistically significant. Results suggested that male dogs, whom researchers noted were represented more than female dogs in the study, were more likely to develop lymphoma. Spayed or neutered dogs of both sexes were at higher risk for lymphoma when compared to intact dogs in this study.
There is hope that advanced genetic studies may help identify any underlying genetic and chromosomal causes and predispositions toward lymphoma. Genes can also help to explain why certain breeds are more predisposed to developing lymphoma, including the Golden Retriever, Beagle, Saint Bernard, and Rottweiler.
Diagnosing Lymphoma in Dogs
Once cancer is suspected, typically after observing an enlarged lymph node, the veterinarian will obtain a tissue sample from the affected organ. The most common diagnostic tool is a fine-needle aspiration. This extracts a sample of a dog’s lymph nodes or organs to be evaluated through a cytology exam (examining a single type of cell) or by a biopsy (examining a section of tissue).
Some veterinarians recommend “staging tests” or will refer you to an oncologist following a lymphoma diagnosis to determine how far the disease has progressed. These tests help veterinarians understand your dog’s overall condition (as well as the type of lymphoma) and may include blood tests, urinalysis, X-rays, abdominal sonograms, and bone marrow aspiration (removing a bit of fluid bone marrow).
There are five stages of lymphoma, depending on which sites are affected and the degree to which the cancer is localized or spread throughout the body:
- Stage I: Involves a single lymph node
- Stage II: Affects multiple lymph nodes in one region
- Stage III: All lymph nodes affected (generalized)
- Stage IV: Involves lymph nodes and organs such as the liver, spleen, or chest
- Stage V: Involves the bone marrow
Treating Lymphoma in Dogs
Chemotherapy is the treatment proven most effective for canine lymphoma. The type of chemotherapy your vet recommends will vary depending on the type of cancer. In some cases, the vet may also suggest radiation therapy or surgery.
For instance, dogs with multicentric lymphoma are often given a chemotherapy protocol known as UW-25. UW-25 is based on the CHOP protocol used in humans, which involves using four different drugs that are administered together. When combined, they have a better chance of stopping the growth of cancer. Administering the drug lomustine (also known as CCNU) is thought to be the chemotherapy protocol that’s most effective for treating cutaneous lymphoma.
Canine Lymphoma Prognosis
Lymphomas vary widely in their aggressiveness, and the prognosis will depend on the stage of your dog’s cancer at the time of treatment and the treatment choice itself. According to a study, most dogs who receive chemotherapy experience remission, which could be complete (undetectable with screening tests) or partial (reducing cancer burden by 50%).
Unfortunately, many dogs with lymphoma will have a relapse at some point. When the cancer returns, it is more resistant to treatment, although a second remission can be achieved in some cases. Unfortunately, lymphoma will eventually likely be fatal.
Living With Canine Lymphoma
Talk to your vet about available treatment options and about what to expect as the disease progresses. Discuss the possibility of hospice care for your dog and what you can do to keep them comfortable. Being proactive about your dog’s cancer can improve their quality of life and help you accept your role in their treatment program.
Lymphoma is a complex disease with many different factors that can impact your dog’s health. If you are worried your dog might have lymphoma, or if you have more questions about your dog’s cancer, call your vet.