The lymphatic system plays an important regulatory and functional role in all our bodies and in those of our companion and livestock animals, birds, fish and even reptiles.
Recent cases examples in humans, involved: different court verdicts that sided with two plaintiffs who both developed non-Hodgkin’s lymphoma after using glyphosate (Roundup TM), a popular weed killer. Actor Kathy Bates – diagnosed with lymphedema – testified before Congress advocating for research funding at the National Institutes of Health into lymphatic diseases.
When most of us think of the lymph system in the body, however, we recall doctors touching our necks and throats to see if we had swollen lymph nodes…and that’s about it.
So, what is the lymphatic system all about and, in particular, what are some of the diseases termed “lymphatic” that affect dogs?
The Purpose of the Lymphatic System
Lymph nodes (glands) are located in regions throughout the body and make up the lymphatic system, which is a member of the immune system.
The lymphatic system consists of lymphatic vessels that filter blood, collect lymph (fluid), and drain excess fluid from tissue. It serves more as a storage unit and filtering cleanser of the body.
Lymph primarily consists of white blood cells called lymphocytes.
Lymphocytes are stored in the lymph nodes, and circulate in the blood and lymph.
The lymphatic system also helps the body fight infections, so it is usually one of the first indicators of disease, as are fevers.
Lymphadenopathy is often used to describe an abnormal enlargement of the lymph glands. However, a technically more correct word for enlargement of the lymph nodes is “lymphadenomegaly”, as the lymph nodes could be smaller than normal or abnormal in texture or shape.
Lymphadenitis refers to an inflammation of the lymph nodes typically due to infection from bacteria, viruses, fungi or parasites, but also from toxic and chemical exposures.
Lymphoma or lymphosarcoma refers to a diverse group of cancers. These cancers are considered cancers of the blood system – not of the lymphatic system – even though the definition is, “malignant tumor of lymphoid tissue.”
This makes sense as the disease starts in lymphocytes that change and multiply out of their regular growth control. Lymphocytes are not only found in the lymph nodes, but also the spleen, thymus, and other parts of the body. In fact, they are produced by lymphoid stem cells in the bone marrow and Peyer’s patches in the bowel, known aa the gut-associated lymphoid tissue (GALT.)
Dogs afflicted with lymphoma usually have multicentric lymphoma, where it first becomes bilaterally apparent in the lymph nodes. The lymph nodes, especially in the submandibular region under both sides of the jaw, in front of both shoulders (pre-scapular area) and in the popliteal area of both hind legs above the hocks, become firm and enlarged. They are not painful.
Similar to lymphomas, lymphocytic leukemia also starts in the cells that become lymphocytes. In leukemia, the cancer cells are mainly in the bone marrow and blood, while in lymphoma they tend to be in lymph nodes and other tissues.
Acute lymphocytic leukemia produces immature lymphocytes (lymphoblasts, blasts) that encroach upon and eventually replace healthy bone marrow tissue. It is an aggressive cancer that typically affects middle aged dogs.
Chronic lymphocytic leukemia is less aggressive, whereby mature and normal-looking lymphocytes accumulate in the body. It is a slower cancer than its acute counterpart, so it generally afflicts older dogs.
These conditions are fairly rare in dogs.
Lymphangiosarcoma is a cancerous neoplasia (tissue) that originates in the lymphatic endothelial cells. This is very rare in dogs.
Lymphedema & Lymphangiectasia
Lymphedema refers to a build-up of fluid in soft body tissues when the lymphatic system is damaged or blocked, and so cannot drain properly. Lymphedema in dogs is either congenital or acquired.
Limb lymphedema as a first manifestation of primary intestinal lymphangiectasia. The most common cause of lymphangiectasia is a congenital malformation of the lymphatics, i.e. present at birth. Secondary lymphangiectasia may be caused by granulomas or cancer causing lymphatic obstruction, or abnormal lymph drainage.
The acquired condition can be caused by congestive heart failure, hypertension, trauma, inflammation, or infection. Some of these problems result in excessive fluid production that overwhelms the lymphatic drainage system. If damage occurs to the lymphatic system from trauma, surgery, radiation therapy, infection, or cancer, the ability to effectively transport fluid is also affected.
Congenital lymphedema and lymphangiectasia are caused by an inherited malformation of the lymphatic system. Normal lymph vessels or tissues may be completely absent or dramatically reduced. It is most often seen in Yorkshire Terriers, Soft-Coated Wheaten Terriers and Basenjis although cases have been reported in other breeds. Affected dogs and their close relatives should not be used for breeding, as the inheritance pattern remains unknown.
Brooks, Wendy. “Lymphocytic Leukemia in Dogs”. Veterinary Information Network, 24 July 2018, https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4952613.
“Canine Lymphomas”. Purdue University College of Veterinary Medicine, http://www.vet.purdue.edu/pcop/canine-lymphoma-research.php.
Day, Michael. “Differential Diagnosis of Lymphadenopathy – WSAVA2004”. Veterinary Information Network, Oct. 2004, http://www.vin.com/apputil/content/defaultadv1.aspx?pId=11181&id=3852145.
Lively, Kristi. “Lymphedema”. Saint Francis Veterinary Center, 2011, http://www.saintfrancis.org/wp-content/uploads/Lymphedema.pdf.