Being the lowest parts of the horse's body, the lower limbs often are the anatomic region that show fluid that has escaped from the blood vessels into the connective tissues (called edema) from any cause. This edema pools here simply as a result of gravity.

Many different conditions result in swelling of multiple limbs. Trauma is a common cause of damaged blood vessels and swelling in a single lower limb. Swelling of multiple limbs can occur because of trauma to multiple limbs, but this is uncommon. But trauma and inflammation can occur high on a limb or even on the body and the resulting edema can "drain" down to the lower limbs.

There are many other reasons for swelling of multiple lower limbs. If there is backup of the blood return from the legs (congestive heart failure), the limbs swell. If there is loss of albumin (blood protein) and the levels become low (colitis), the limbs swell. If vessels are injured (vasculitis from viral or allergic causes for instance), the limbs swell. Diseases that cause these problems are associated with multiple leg swelling, and the list of possible causes is very long.

Stocking-up (a/k/a stagnation edema) commonly occurs in stabled horses or horses placed in a stall after exercise. It is caused by reduced lymph circulation with lack of normal movement. It is commonly more obvious in the hind limbs.

A small amount of edema in the lower limbs of a stalled horse is usually not a cause for concern. However, it may be impossible for you to distinguish between "normal" stocking up and edema from other causes, indicating a problem.