Lymphoedema is broadly categorised into two types:

• 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.

  Pic By:Bob Langrish

  
  It is also likely that when some of these horses are confined to  boxes the venous system in the legs does not function adequately, forcing the already struggling lymphatic system to carry an extra and eventually permanently damaging amount of fluid.
  

  Currently a research programme is being undertaken by UC Davis, California and the University of Ghent, Belgium into a form of primary lymphoedema that has been identified in some breeds of heavy horses and may be inherited. The initial symptoms are consistent with pastern dermatitis (see Conditions), and because secondary bacterial and mite infections are common, are easily mistaken for this. 

• Secondary lymphoedema occurs when a previously functioning lymphatic system is damaged, for example by injury or infection.  An example of this is recurrent lymphangitis.  The initial infection clears up but damages or destroys local lymphatic vessels, reducing or obliterating their function.  This puts strain on other lymphatics and symptoms of lymphoedema such as thickening and hardening of the tissues, and reduced immune response and vulnerability to further infection can appear.
  
  The stages of ELC
  Left untreated ELC will inevitably worsen, and the stages that it goes through are again identical to those defined in lymphoedema in humans:

• In stage 1 the condition may be latent, that is showing no apparent symptoms, or a swelling develops which 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.



In stage 2 the fibrotic changes resulting from chronic inflammation cause the leg to gradually harden and it no longer pits or reduces much with exercise. It is now at greater risk of infection and skin damage which can lead to further deterioration. In stage 3 the affected leg is now hard, grossly swollen and misshapen, the skin is damaged and vulnerable and may be crusty or weep, and is extremely prone to bacterial and fungal infections. Because of its appearance in human patients, stage 3 is also known as elephantiasis.

Picture By :UC Davis