The canine and feline lymphatic system

The lymphatic system is one of the lesser talked-about, but equally important systems in your pet’s body. It works alongside the circulatory function of the cardiovascular system, the elimination function of the digestive system and the defence function of the immune system. There are several important organs in your pet’s body that make up the lymphatic system and its network of vessels, ducts, lymphatic fluid, lymphocytes and lymph nodes.

In this article, we’ll explore what the lymphatic system does, the organs of the lymphatic system and where they are located, and what happens when there is a malfunction or disorder in the lymphatic system or any of the lymphatic organs.

The functions of the lymphatic system

The lymphatic system works as an absorption, filtration, drainage, and immune defence system. The main functions include:

  • the drainage of excess fluid from tissues, filtering it back into the bloodstream
  • the removal of cellular waste products and proteins, which are transported in lymphatic fluid and drained via the thoracic duct into the vena cava, where it enters the vascular (vein) system
  • the absorption of fat and fat-soluble vitamins in the gastrointestinal tract
  • the production of antibodies and the transport of white blood cells and proteins, as part of the body’s defence against pathogens

Organs of the lymphatic system

The lymphatic system comprises a number of organs, which include:

  • lymph vessels (lymphatics)
  • lymph nodes
  • lymphoid tissues located in the body’s various mucosa (MALT), and in the gut (GALT)
  • spleen
  • thymus
  • tonsils
  • bone marrow

How does the lymphatic system work?

The lymph vessels of the lymphatic system are located alongside the blood vessels of the circulatory system. The circulatory system functions under high pressure, which causes fluid to leak out of capillaries and into the interstitial spaces in bodily tissues. The lymphatic system reabsorbs this interstitial fluid and transports it at a relatively low pressure through lymph vessels back into the bloodstream. The circulation of the lymphatic system is stimulated by bodily movement, which is why it’s important to keep your pets active.

Lymph vessels move the lymphatic fluid or lymph towards lymph nodes, which are clusters of small glands that play a role in immune function. As the lymph is filtered through the lymph nodes, it is exposed to immune cells, which detect the presence of invaders or pathogens. The lymph nodes work as a detection and elimination system that uses white blood cells to fight off bacteria, viruses and other pathogens and toxins.

Tonsils are also a form of lymph node, known as lymphoid nodules. They form part of the collective mucosa-associated lymphoid tissue (MALT). They absorb and attack invading bacteria and viruses before these have a chance to enter the rest of the body. The thymus (located near the heart) also comprises lymphoid tissue and it plays a vital role in newborn cats’ and dogs’ early immune responses. Bone marrow – found inside the large bones – is the site where red and white blood cells are made and sent into the body.

The spleen is another site for the production and storage of blood cells, and it helps to filter the blood and remove dead, damaged and old red blood cells. It also plays a role in the detection of viruses and bacteria, triggering the release of white blood cells to fight off these pathogens.

Lymph fluid also works as a transport mechanism for fats, which are absorbed from the gut (via gut-associated lymphoid tissue (GALT)) and fed into the liver to be processed. The liver then feeds usable fats into the bloodstream, supplying energy and vital nutrients to various parts of the body. Waste products from the liver are sent to the kidneys for elimination. If bacteria or viruses are detected via the GALT or MALT at any point in these absorption, detection, processing and elimination functions, they are trapped and destroyed by immune response processes in the lymphatic system.

Problems in the canine and feline lymphatic system

The lymphatic systems of dogs and cats are made up of many different parts, each exposed to harmful substances and triggering responses and changes to cells and organs. There is a risk of infection, damage, inflammation, blockage and even cancer. Here are some of the most common problems or disorders that can occur in a pet’s lymphatic system.


When the lymph nodes react to infection, they can become swollen and painful. Usually this is just a temporary response as the lymph nodes enlarge with white blood cells to fight infection, or as a reaction to vaccination. It can also occur during an allergic reaction or an autoimmune response. Lymphadenopathy can also be a symptom of a more serious problem like lymphosarcoma.

Lymphoma or lymphosarcoma

This is cancer of the lymphatic system, which can occur in any part or organ of the lymphatic system when lymphocytes change and multiply uncontrollably. The presence of enlarged lymph nodes that are not painful are usually an indicator of possible lymphatic cancer.

Lymphoma in cats is relatively common compared to other forms of feline cancer. It makes up about 30% of all malignant tumour diagnoses in felines, with intestinal lymphomas being the most commonly diagnosed lymphoma.

Lymphocytic leukaemia

Leukaemia in the lymphatic system occurs when cancerous lymphocytes are found predominantly in the bone marrow and blood. Leukaemia is rare in dogs and cats, but if it does occur, it’s usually in middle-aged and older animals and is an aggressive form of lymphatic cancer.


The lymph nodes can become inflamed, which is a condition called lymphadenitis. This can occur when bacteria, fungi, viruses and other pathogens infect the lymph nodes directly and cause inflammation as an immune response.


One of the primary functions of the lymphatic system is to remove excess fluids from bodily tissues. When the lymph vessels and ducts are blocked or damaged, this creates a fluid build-up (oedema) called lymphoedema. This can be a genetic condition or an acquired one. Dogs and cats with congestive heart failure, inflammation and infection, as well as high blood pressure can develop acquired lymphoedema; when other systems in the body don’t function well, cause an overproduction of tissue fluids and overwhelm the lymphatic system.

Diagnosing and treating problems in the lymphatic system

The vet will physically examine a pet with a suspected lymphatic system problem. One of the most frequent ways to tell if the lymphatic system is abnormal is by the enlargement of the lymph nodes. The vet can run blood tests and urinalysis to determine the health of the internal organs and to pick up on the presence of infection or inflammation. If there is an abnormal white blood cell count in the body, the vet may run screening tests for bacterial, tick-borne, fungal or other systemic diseases.

The vet can also perform imaging tests – X-rays and ultrasound – to have a look at the internal organs or to see whether there is a fluid build-up in the chest or abdomen.

If the vet suspects cancer in the lymphatic system, they may perform a lymph node biopsy or a fine needle aspiration of the lymph node or other lymphatic tissues. These samples will undergo cytology (examination of the cells) in order to diagnose lymphoma, lymphadenitis or lymphadenopathy. In the case of suspected lymphocytic leukaemia, a bone marrow aspiration and cytology will help to confirm this diagnosis.

Many problems in the lymphatic system are an indicator of infection or diseases elsewhere in the body. Resolving these problems will usually involve treating the systemic issues. Very often dental diseases will cause swelling in the (submandibular) lymph nodes below the jaw. The vet will perform the necessary dental work, teeth cleaning, extractions and removal of infection. This will subsequently reverse or reduce the related lymphadenopathy.

Treatment for lymphomas will involve chemotherapy. Other types of cancer can be treated with radiation or chemo, or surgical removal of tumours. Infections can be treated with antibiotics, which ‘share the load’ with the lymphatic system of fighting infection. Therefore, the type of treatment will depend on the type of problem causing the disorder in lymphatic system.

How to support your pet’s healthy lymphatic function

There is a lot that pet owners can do to keep your pets’ lymphatic system healthy and their immune system strong:

  • High-quality nutrition will give your pet a healthy foundation and keep their gut balanced, their energy needs met, and their bones, brain, eyes, and heart in good condition.
  • Daily exercise will help to keep your pet’s lymphatic system functioning well. As mentioned, physical movement encourages your pet’s lymph to move throughout their body, so a vigorous bout of exercise, regular walks and an active lifestyle will ensure their lymph drainage system is working adequately.
  • Access to fresh water 24/7 will ensure there is enough fluid in your pet’s cells to support a healthy lymphatic system. This system relies on adequate hydration in order to function properly.
  • Preventative veterinary medicine like vaccination protocols and parasite treatments should be kept up to date. The less chance your pet has of contracting a bacterial, viral or parasitic infection, the less chance there is of their lymphatic system being overburdened.
  • Regular dental check-ups are part of your pet’s preventative care. The sooner the vet can detect and/or prevent any dental diseases, the less the likelihood of your pet suffering with gingivitis, periodontitis or other oral infection.


The lymphatic system of dogs and cats functions as a filtration, drainage and defence system that supports their circulatory and immune systems. The intricate role this system plays in our pets’ overall health shows the importance of a holistic approach to their veterinary care. Regular vet check-ups, teeth cleaning, and a daily regimen of healthy exercise, nutrition and hydration can work wonders for pets’ internal health environment.

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