Doc Nick's Vet Talk

SWELLING

Swelling is frequently associated with inflammation. Inflammation means an increase in heat, pain and loss of function in the affected area. Just as an interesting aside, historically, the Latin word for swelling is tumor. Nowadays the word tumor is more frequently related to a type of cancer such as a 'benign tumor' but we know that horse’s rarely get cancer compared to most other species so we won’t consider that as a cause for swelling. People frequently get infection and inflammation confused. When an area is infected it is always inflamed but an inflamed area is not necessarily infected. For example, the swelling one sees with a bowed tendon or tendinitis is an inflammation related to disruption of tendon fibers and is not associated with any infection.

Swelling in the legs that goes away with exercise and is not painful when squeezed is most likely related to circulatory congestion or a physiological imbalance. Horses are said to have five hearts, one big one and one in each of their feet. If they are not moving then they are not pumpin. So, as a result, horses that stand in a stall the whole day frequently have swollen legs.

In the same way that blood drains from the capillary beds of the legs by way of the veins, the drainage of tissue fluid that doesn’t get back into the veins depends on the lymphatics. The lymphatic circulation is rarely discussed since it is mainly a microscopic drainage system that eventually dumps its fluid into the veins. Similar to veins the lymphatics have one way valves in them, preventing fluid from backing up and keeping the flow going in one direction – towards the heart.

Horses that stock up frequently have stretched dysfunctional valves that stop providing unidirectional flow of excessive tissue fluid. Diseases in the horse such as lymphangitis can cause permanent stretching of these valves leaving horses permanently susceptible to future episodes. Exercise, bandaging and massage all promote lymphatic drainage. More infrequently diseases that cause severe loss of protein in the circulation can result in distal limb swelling. A loss of the heart’s pumping efficiency can also result in swelling. Many virus infections can cause generalized swelling as well.

Cellulitis is a severe inflammation of the tissues under the skin. It is usually caused by a bacterial infection and results in severe painful swelling of an affected leg. If the bacterium is rather aggressive (flesh eating disease) the infection can spread quickly and cause skin to die and “slough” off leaving tendons and joint surfaces exposed.

Lymphangitis usually occurs in the hind limbs and cellulitis in any limb but the two problems can occur simultaneously and require immediate antibacterial and anti-inflammatory therapy. In the acute stage most horses run a fever (always check the temperature) and do not want to use the affected limb.

Abscesses are areas of localized swelling and infection. They are almost always painful to the touch. Cellulitis, where an infection is fairly diffuse, can lead to the tissues trying to “wall off” the infection resulting in an abscess. Treating abscesses with antibiotics is frequently unrewarding since abscesses typically have no circulation entering them so the antibacterial can’t penetrate and do its job. Surgical drainage is generally more rewarding.

Race horses are always bandaged with an emphasis on keeping the legs tight. Why? What happens if you let a leg stay swollen? Well, we already know that lymphatics and veins can stretch and lose their function but other tissues can also stay permanently stretched. This is seen frequently with tendon sheaths. Mild swelling within the sheaths are called wind puffs. A principle to keep in mind is that mild inflammation may have caused a swelling to begin with but swelling that remains for any length of time leads to secondary fibrosis. Fibrosis means scar tissue which is less elastic than the original tissue. Joints and tendon sheaths that stay stretched and full of excessive fluid for a long time frequently don’t completely reduce in size even after drainage and injection. The stretchy balloon has turned into an inelastic sac which always tends to fill.

The cause of swelling in a horse’s limb is important to determine. Radiographs and ultrasound of the affected area supplement a thorough exam and help determine the cause of a swelling. Therapy for swelling is extremely variable depending on what type of swelling the horse has. We need to know if there is any coexistent infection, what tissue or area is affected and how clinically significant the swelling is. Is the area painful, is the horse lame and is the swelling changing over time and exercise? As always, an accurate diagnosis helps provide appropriate therapy.