Finding that your horse is developing a rapidly swelling leg, with no apparent sign of injury, is a very alarming experience. One of the principle causes of this may be lymphangitis, which is inflammation of the lymphatic vessels.
Until something like this happens, many horse owners are happily unaware of their horse’s lymphatic system. If it is functioning properly it does not draw attention to itself, but it is a crucial part of the body – if the lymphatic system stops working, death occurs within 24 hours. So, it is important to know something about it, and what may go wrong, including lymphangitis.
The horse’s body is approximately 70% water, much of it bathing the cells of the tissues, and is the medium through which nutrients and some metabolic products are transported. Fluid is continually leaving blood vessels and entering the tissues, and also has to be continually removed, and this is part of the job of the lymphatic system. This consists of vessels, which absorb and transport fluid (becoming lymph) from the tissues, and lymphoid structures of the immune system. Through its function to propel lymph, the lymphatic system plays an important role in immune response by transporting antigens e.g. bacteria and viruses to lymphoid structures such as lymph nodes distributed along the lymphatic vessel network, during disease and in response to infections.
The lymphoid tissues can be divided into primary organs, where white blood cells crucial to the immune response are created, and secondary organs where these interact to generate immune responses to antigens.
In addition to its role in immune surveillance the lymphatic vascular system is essential for fat absorption, fluid balance, and protein transport, and plays important functional roles in obesity, inflammation and fat metabolism. The lymphatic vascular system is divided into the ‘superficial’ system which drains the skin and subcutaneous tissue, and the deep system which drains deeper tissues and organs, the two parts being interconnected.
Lymphatic vessels begin as ‘blind ended’ capillaries collecting fluid from the tissues. These drain into consistently larger vessels which connect with lymphatic trunks deep in the body, draining into the blood circulation in the region of the jugulo-subclavian vein junctions. Except for the capillaries, lymphatic vessels can contract, moving lymph forward.
If for any reason the lymphatic system cannot drain the tissues adequately, oedema or fluid swelling develops. Sometimes, as in the case of injury, there is simply too much fluid entering the tissues from the blood stream for the lymphatic system to keep up, which eventually resolves over time. But on other occasions, the lymphatic system itself can no longer function adequately, creating a different kind of oedema.
Both of these reactions happen with, and frequently following, lymphangitis. Lymphangitis is much more common than often thought, and is usually caused by an infection, but can also be caused by an allergic reaction. The inflamed walls of the lymphatic vessels can no longer contract to move fluid and become permanently scarred and unable to function. Vessels around the lymph nodes however close up, preventing fluid which may be carrying antigens from moving further into the body, and allowing the lymph nodes to launch a defensive immune response. Oedema forms partly because of this blockage, and also because fluid floods in as part of the inflammatory response.
So what should an owner do when faced with lymphangitis? At the very first suggestion of oedema, ice and bandaging may help localised swelling by reducing the blood supply and fluid filtering into the tissues, but usually it is discovered too late for this, and it won’t control infection. The most important response is to call the vet out as soon as possible, so that anti-inflammatories and treatment for pain and for the cause of the inflammation can be given. Cellulitis, an inflammation of the deeper layers of the skin, though not as serious as lymphangitis can often lead to it so also needs to be treated promptly. Lymphangitis requires aggressive treatment, with the correct antibiotic for an infection. If an infection is carried deeper into the body the horse can become very ill. There is nothing to be gained from bandaging the oedemic leg. It was once thought that tissue fluid is reabsorbed into venous capillaries, but this is now known not to be true, and that it is the lymphatic system which drains tissue fluid. And to protect the body from invasion, the lymphatic system in the affected region has tried to close down. Similarly, making a horse in pain walk may force bacteria through the lymph nodes further into the body and will only help to bring more blood and therefore more fluid to the affected area, potentially increasing the oedema. When it is ready the lymphatic system will open up again and the leg will start to drain. Icing below a certain temperature also stops the lymphatic system functioning but may provide pain relief.
The fluid which sometimes oozes out of skin cracks in the swollen limb is not serum, but lymph, rich in protein and very attractive to flies and bacteria. It is also caustic and should be gently cleaned away
After an initial bout of lymphangitis, the affected lymph vessels will have been obliterated, and no longer function. Their role may be taken up by other vessels, increasing their burden of lymph, and the leg may return to its original appearance. However, lymphatic activity is compromised, and if something happens in the affected area to cause further inflammation, which may be as small as an insect bite or thorn puncture, the body will be less able to deal with it, and lymphangitis may occur again. It is also possible for the remaining vessels to become exhausted due to their extra burden and eventually cease to function properly. Frequently a second bout of lymphangitis results in a permanent swelling known as lymphoedema, which increases the risk of further attacks. As the tissues swell, transport of nutrients, oxygen, immune system elements and antibiotics into, and waste products out of the tissues is less efficient, creating an unhealthy environment and making them more vulnerable.
Many nutrients are carried to the tissues on large protein molecules, and can only be removed by the lymphatic system. When lymphoedema occurs these molecules remain in the tissues where they change into a form of scar tissue, known as fibrosis. A low level inflammation is set up. Fibrosis is a serious complication of lymphoedema as it blocks the blood and lymphatic vessels from adequately reaching the tissues, and provides an opportunity for bacteria to hide away from the reach of the immune system and antibiotics, leading to further infections.
Fibrosis initially feels ‘rubbery’ and resistant to pressure, and the size of the swelling does not necessarily indicate its seriousness, as a fibrotic leg can be a lot less swollen than one with more fluid. A horse with lymphoedema probably won’t appear lame either, unless the swelling interferes with other structures, so it is a mistake to think that the horse has recovered or is now sound.
Owners are frequently told that when lymphoedema develops there is nothing more that can be done for the horse, but this is not true. ‘Combined decongestive therapy’ is a treatment developed to control lymphoedema in people and has been adapted for use with horses. It consists of manual lymph drainage, specialised compression bandaging, skin care and exercise, to reduce the oedema and break down fibrosis, followed by maintenance bandaging or the use of a specially designed compression stocking to maintain the volume loss. When CDT is used to control lymphoedema the horse has a much better chance of a healthy and productive life, but it is very important that this starts as early as possible once the cause of lymphangitis responds to treatment, so that the development of fibrosis can be controlled. When CDT is used at this stage it can be speedily effective, and although CDT can be used with more established lymphoedema, this will probably require more treatments and may be less successful. CDT has to be undertaken with veterinary supervision, but as there is no veterinary alternative for the treatment of lymphoedema most vets agree to this, and it will be covered by insurance.
There is another form of equine lymphoedema which presents differently to that following lymphangitis. This is ‘Chronic Progressive Lymphoedema’ and is found in some draft breeds, and some Gypsy Cob and Friesian horses. It was identified at the University of California, Davis, where a research trial into the use of CDT was undertaken and will soon be moving into a further stage.
More information about the horse’s lymphatic system and conditions affecting it and their treatment can be found at www.equinemld.com.
Author information: Heather Powell (www.equinemld.com) is the first qualified Equine Manual Lymph Drainage therapist to work in the UK, setting up her practice Equine MLD® in 2006. She is the author of published articles and of original scientific research featured in veterinary journals and was a speaker at the 2013 International Equine Conference on Laminitis and Diseases of the Foot.
© Heather Powell 2014