A dog’s lymph system is a network of lymph nodes and lymphatic vessels that are used to process and circulate disease organisms and foreign substances so that they may be presented to and destroyed by the immune system. Lymphocytes, which are a type of white blood cells, are the main cells of the lymphatic system. They may be B cells, T cells, or NK (Natural Killer) cells, and they circulate within the lymph system.

Lymphoma, also called lymphosarcoma, is a cancer of these lymphatic cells. Usually, cancerous cells of this sort form into a solid lump or tumour after its regulatory cells stop working properly. This causes the cell to divide uncontrollably. Cancerous cells may separate from the main tumour and travel through the dog’s body, where they may lodge and start forming other tumours. This is called metastasis.

In dogs, lymphoma may form in the bone marrow, liver, spleen, gastrointestinal tract, eyes, skin, or lymph nodes. This is one of the most common forms of malignant cancer found in dogs.
The disease is divided into low- and high-grade types, and further divided depending upon the location of the tumours: extranodal, mediastinal, multicentric, and gastrointestinal. Extranodal means that the skin, heart, eye, kidney, or central nervous system is involved; mediastinal means the cancer is in the lymph nodes in the thorax and sometimes they thymus; multicentric lymphoma (the most common) means the cancer is in the lymph nodes, and may or may not be also found in the bone marrow, spleen, or liver; and gastrointestinal means the cancer may present as one tumour in the GI tract, a diffuse invasion of the intestines or stomach, and might also involve the liver, spleen, and/or surrounding lymph nodes. Further to the location, the cancer is also classed depending on the type of cells involved: the B type or the T type. Most cancers involve the B-cells.

Lymphoma is genetic, but it is also possible that environmental factors can contribute to a dog’s likelihood of developing it. One study, done in Italy in 2001, suggests that lymphoma in dogs may be linked to exposure of solvents, paints, and/or pesticides.

Although the link between chemicals and lymphoma has not been proven in subsequent studies, it would be prudent for dog owners to reduce their dogs’ exposure. Lawns that dogs walk and play on should not be sprayed with chemicals.
Since there is a genetic link, dogs that develop lymphoma should not be bred.

The signs of lymphoma are very general and may vary from dog to dog. The type of lymphoma also dictates the symptoms the dog will experience. Dogs with multicentric tumours will usually develop enlarged submandibular lymph nodes (behind and beneath the chin), in the armpits, behind the knees, or in the groin area. They may show no other symptoms – but if they do, they may display any of the following: depression, fever, exercise intolerance, lethargy, diarrhea, vomiting, loss of appetite, dehydration, skin masses or nodules, weight loss, polyuria, cough, distended abdomen, constipation, bruises or skin ulcerations, itchiness, hair loss, drooling, difficulty swallowing, polydipsia, abdominal discomfort, dark tarry stool, difficulty breathing, and neurological signs like seizures, vision abnormalities, circling, behaviour changes, and disorientation.

Dogs with cutaneous lymphoma, while uncommon, wills how hair loss, visible skin tumours, or itchy skin.
Dogs with the gastrointestinal form may show symptoms of diarrhea, vomiting, depression, lethargy, and/or weight loss. This is the second-most common form of lymphoma in canines.
A dog with mediastinal lymphoma can suffer from coughing and difficulty breathing, since the tumour can cause fluid build-up around its lungs.
Central nervous system lymphoma is rare, but when it occurs, it causes behaviour changes, incoordination, circling, seizures, and other neurological signs.

Dogs at Risk
This form of cancer is most commonly found in middle-aged or older dogs, but younger dogs can be affected. Both males and females can develop lymphoma. Some breeds that are more at risk are the Basset Hound, Golden Retriever, German Shepherd, Scottish Terrier, Boxer, Bulldog, Poodle, Airedale Terrier, and Saint Bernard. Some lines of Otter Hounds, Bull Mastiffs, and Rottweilers show an increased risk.
Breeds that do not tend to develop lymphoma are Dachshunds and Pomeranians.

Testing and Diagnosis
If there is suspicion that lymphoma is present, or if a dog displays general signs of illness, its vet will first perform a history and thorough exam. Blood work and urinalysis may show elevated white blood cells, anemia, and other telltale signs. If a lump in the lymph nodes is present, a needle biopsy of the tumour will assist the diagnosis. 
Sometimes, larger biopsies are required to make a conclusive diagnosis. This involves the surgical removal of a sample from the organ or node which is affected. In some cases, the entire node is removed.
Bone marrow biopsy or aspiration can determine if the cancer has spread to the marrow. Other tests that may be performed include abdominal and chest x-rays and/or ultrasounds, and a cerebrospinal fluid tap for dogs that show neurological symptoms

Treatment and Prognosis
Most cases of lymphoma are treated with chemotherapy. Treatment goals include remission of the cancer, and to improve and extend the dog’s quality of life. Chemotherapy can cause severe – and sometimes fatal – side effects. Radiation therapy may be used in combination with chemo. Sometimes, if the cancer is restricted to one spot, like focal gastrointestinal lymphoma, it may be surgically removed and the dog goes into remission with no further treatment. Stem cell transplantation is also a treatment option, like in humans.
Lymphoma – particularly the multicentric form – tends to respond well to chemotherapy. Dogs with Stage I lymphoma (where the cancer is confined to one lymph node or organ) have a much better prognosis than those with Stage V lymphoma (where the cancer is spread through the body, and involves the blood, bone marrow, or non-lymphoid organs). Unfortunately, treatment does not cure the disease, it only helps the dog to live longer, feel a bit better, and improve its quality of life if the goal of remission is achieved.