Equine Lymphoedema Complex

Lymphoedema is a disease in which the flow of lymph is obstructed by mechanical failure of the lymphatic system.  Fluid containing protein and waste products cannot be effectively removed resulting in swelling in the skin and subcutaneous tissues.  The  excess protein and its degradation causes changes to the affected tissues and underlyng structures such as ligaments, tendons, joint capsules and synovial membranes can be affected.  The body's attempts to deal with the excess protein result in increased connective tissue which reacts to produce granulation tissue.  Initially this is soft but over time becomes fibrotic and hardens, limiting the effectiveness of lymph and blood flow, the lymphatic vessels undergo damaging changes, elastic fibres are destroyed and immune deficiency develops.  The skin becomes more fragile and may crack, weeping a caustic fluid called 'lymphorrhea', and vulnerability to infection increases.  A chronic low grade inflammation is present.

An early sign of lymphoedema is a soft swelling which 'pits' under fingertip pressure, this may not be noticable under the horse's coat.  Initially this swelling may reduce with exercise, returning again if this is prevented.  With time the swelling becomes firmer and lumps and folds of firm flesh may develop, initially usually in the pastern area.  The condition is broadly categorised into two types:

Pic:Bob Langrish

Primary lymphoedema occurs when there is a congenital problem with the lymphatic system itself, and it may be ‘latent’, causing no symptoms to appear for many years until the system becomes exhausted, or is further damaged by an infection or trauma compromising it to the point where swelling becomes discernible. Research indicates that horses that develop ‘swollen legs’ when kept in have latent primary lymphoedema.  The absence of movement prevents the lymphatic system from working adequately, and whilst some horses will be unaffected, others with ‘borderline’ lymphatic function will develop oedema.

Secondary lymphoedema occurs when a previously functioning lymphatic system is damaged, for example by injury or infection such as recurrent lymphangitis.  The initial infection clears up but damages or destroys local lymphatic vessels, reducing or obliterating their function.  This impairs lymph transport, and if these tissues become inflammed again, symptoms of lymphoedema such as thickening and hardening of the tissues, and reduced immune response and vulnerability to further infection can result.

In practice the distinction between primary and secondary lymphoedema is not always clear, and the two can be combined in a 'mixed' lymphoedema.
 

The stages of ELC
Left untreated ELC will inevitably worsen, and the stages that it goes through appear to be similar to those in lymphoedema in humans, although infection seems to play an important role in the development of equine lymphoedema:

• Stage 0. The condition is 'latent', showing no visible symptoms.

• Stage 1. A swelling develops which initially will ‘pit’ if the skin is pressed, and reduce or disappear with exercise – as happens when a horse which is kept in develops swollen legs. EMLD and regular exercise are recommended.

• Stage 2.  This is a lymphatic swelling caused by an infection such as cellulitis or lymphangitis.  It is usually treatable  with medication, e.g. antibiotics, but usually results in some damage to the lymphatics.  This may encourage further infections, with increasing damage, resulting in the start of fibrotic changes in the tissues. EMLD is not recommended because of the presence of infection but may be used after this is under control to support recovery.

• Stage 3a.  An 'intermediate' stage between 2 and 3b. A persistent swelling remains after acute infection, the start of the most severe form of lymphoedema. EMLD is recommended as soon as possible after infection is treated.