Canine lymphoma develops first in white blood cells known as lymphocytes and most commonly in the lymph nodes. Enlargement of the lymph nodes is often the first sign of disease. This form of the condition is known as multicentric lymphoma and accounts for 80 to 85 percent of canine lymphoma cases.
The next most common form, alimentary lymphoma, affects only 10 percent of dogs with lymphoma. The condition affects the intestines and prompts the development of symptoms such as diarrhea, vomiting, and lack of appetite.
Rarer are mediastinal and extranodal lymphomas. Extranodal lymphoma develops in a particular organ, most often the skin, where it is known as cutaneous lymphoma and presents with the growth of lumps.
Mediastinal lymphoma, by contrast, arises in the thymus, the mediastinal lymph nodes, or both. Because these structures are located in the animal's chest, symptoms include muffled cardiac sounds and shortness of breath.