The lymphatic system of the horse is a network of vessels which are present throughout the whole body. The network carries lymph fluid around the body. This fluid is carried by vessels of varying sizes becoming larger like arteries when they are closer to the chest cavity and heart, but decreasing in size like capillaries as they move further to the extremities of the body. E.g. the limbs.
Unlike the cardio vascular system, which is driven by the pump action of the heart, the lymphatic system drains towards the chest via the massaging effect of moving muscles, tendons and a valve network within its larger vessels. The areas between the valves are known as angions which have their own pacemaker cells capable of rhythmic impulses. These valves prevent any backflow of fluid and unlike the cardio vascular system which circulates around the body, these valves ensure the fluid flows in one direction only.
The fluid also moves through the vessels due to their walls consisting of smooth muscle and elastin.
The evolution of the horse as a continual grazer and flight animal has resulted in its lymphatic system containing a higher percentage of elastin which with high levels of movement stimulates the flow of fluid effectively. The results of domestication and more sedentary horses stabled for long periods can adversely affect the efficiency of the lymphatic system and result in a number of problems, particularly in the limbs.
Lymph capillaries or initial lymph vessels are the smallest vessels within the system and have very thin walls. These thin walls easily allow the transfer of water, plasma proteins, hormones and enzymes released by blood capillaries through the interstitial space. The interstitial space surrounds all cells within the body and enables the transfer of fluid from each cell to another. For example the removal of excess fluid from tissues such as muscles and the cardio vascular system to the lymphatic system. Once the fluid is transferred into the initial lymph vessel it is known as lymph fluid.
Lymph fluid normally consists of water, enzymes, blood plasma proteins, hormones and debris from cells. It can also contain bacteria, viruses, fungal and infectious pathogens during an outbreak within the body.
The skin and subcutaneous tissue drains into the superficial lymphatic system consisting of initial vessels or fine capillaries which are present above the body’s fascia. These drain into larger collector vessels and are connected by vessels which pass through the fascia into the deeper system. The deep lymphatic system drains the internal organs, muscle, joints, tendon sheaths, periosteum of bone and nervous system.
The system is not symmetrical with more nodes and vessels draining into the left venous junction from the hind limbs, head, neck, left fore limb, abdomen and left half of the thorax. Into the right venous junction is the head, neck, right fore limb and right half of the thorax.
The lymphatic system does have a limit in which it reaches maximum capacity. This can results in excess fluid build-up and oedema usually in the lower limbs first. Under normal circumstances and no infection or abnormal stimulus this can be resolved over time and with movement of the body. For example due to the inadequate amount of stimulation provided by movement, stabling the horse for long periods results in the lymph fluid collecting in the lower limbs of the horses causing ’filled legs’ or ‘stocking up’. This usually disappears when the horse is turned out or worked.
But filling and problems can also be caused by bacterial or viral infections, dietary issues (high protein/sugar), allergies or the introduction of drugs.
Lymphangitis is an infection of the lymphatic vessels resulting in excessive fluid retention within the limbs, often a hind limb, but can affect any or one or more legs at a time. It is a painful and debilitating condition, sometimes resulting in ulcerative eruptions on the surface of the skin due to the escape of excessive fluid build-up from within. As the leg becomes more painful the movement and weight baring from the horse become very limited resulting in further fluid retention due to the lack of movement. Blockages along the vessels around the hock areas are also thought to be another cause. Antibiotics, pain killers and steroid treatments are usually prescribed to treat affected horses.
Often horses will suffer repeated out breaks and possibly permanent swelling to the effected limbs.
Other oedema seen in horses is the development of fluid under the abdomen in the pregnant mare just before foaling. This usually disperses after foaling.
The duties of the lymphatic system are significant and important to the overall balance and equilibrium of the body. The transportation of fluid and proteins back to the circulatory system ensure the correct balance and blood pressure is maintained and that certain essential fatty acids from the digestive tract are correctly absorbed and used by the body.
Equally important to the body is the immune system and the role which the lymphatic system plays in detecting and fighting invasions from foreign agents or abnormal activity within the body.
As lymph fluid flows towards the chest there are stations or groups of filters known as nodes along the vessels. The nodes filter and check the fluid for infections, viruses, pathogens, fungus and other foreign bodies.
Lymphocytes are present in the nodes and are responsible for checking and filtering the fluid and identifying or removing hazards to the bodies systems.
Lymphocytes are produced within the reticular connective tissue of lymphoid organs such as the spleen, thymus and bone marrow. There are three types of lymphocyte, T cells, B cells and Natural killer cells. These cells are the body’s first line of defence.
B cells are matured in bone marrow and secrete antibodies and activate the immune system to pathogens and infections. While T cells originate in bone marrow but migrate to the Thymus to mature and are responsible for attacking and neutralizing invading cells and producing cytokines which stimulate the innate immune response sending out natural killer cells which are able to kill invading cells or compromised/damaged/abnormal cells within the body.
To conclude, the lymphatic system is a vital filtering system within the body contributing to the overall equilibrium of many systems but playing vital roles in the immune and cardio vascular systems regulating blood pressure and the transfer of vital fluids and the protection of the entire body through the immune system.