Sometimes called stagnation edema, it is a harmless condition but may be a symptom of other problemsYou go out to the barn or paddock one morning and find your horses hind legs are enlarged, swollen from hooves to hocks. It is not uncommon for a horses lower legs to swell, especially if the horse has been confined (in a stall or small pen) after a period of activity or strenuous exercise. If you give a horse a hard workout and then put him in an area where he does not have room to move around much, his legs may swell. Some horses legs swell just from inactivity; if you bring them into a stall or pen from a pasture, their legs stock up overnight.
The swelling may only involve the pastern and fetlock joint areas, or it may extend up the leg to the knee or fetlock. Sometimes called stagnation edema, this type of swelling is painless and generally not damaging to the horse, though it may inhibit freedom of movement of the fetlock joint.

Causes of stocking up
Edema in the lower legs can occur because of the way equine feet and legs are constructed. The horse, by nature, is a very mobile and restless animal. Under natural conditions he is almost always on the move, traveling slowly even while he grazes. His normal pattern of activity is eat, rest, then eat again. Even at night he is active, grazing off and on, resting only for short intervals. He grazes on the move, seeking out the best grasses, traveling as he munches.
Circulation in the legs depends on muscle activity and the expansion and contraction of the feet as the horse travels. It is easy for a horses circulatory system to supply blood to the lower leg, but getting the blood back up the leg is harder. The horses body depends on movement for proper function; most of the veins are located between masses of muscles. As the muscles contract during activity, they squeeze the veins, aiding the return of blood to the heart.
There are no muscles in the lower leg, just bones and tendons. In the foot, the frog acts like a little pump at the end of the line. Each time the horse puts weight on a foot and the hoof walls expand, the sole flattens and the frog comes into contact with the ground. When the foot is lifted, the walls spring back and contract, the sole returns to normal position, and the frog pushes the blood on its way. Movement keeps the circulatory system efficient, which, in turn, maintains the health of feet and legs.
But the domesticated horse is often confined in a stall or small pen eating hay, grain, or pellets that he can consume while standing still. Thus, he may be immobile for hours at a time while his heart is pumping blood down to his feet and getting very little help in its return. Inactivity can produce temporary edema in the lower legsswelling results from the leaking of fluid into the space between the cells due to increased capillary pressure (from the blood being backed up in the lower legs).
Often, just the hind legs are affected, since circulation tends to be poorer in them to begin with. But front legs bear more weight and are subject to more concussion, and if the horse has been worked hard and long on hard ground, or subjected to extra stress on the front legs, you may find all four legs stocked up.
Some horses stock up more readily than others, and their legs may swell just from spending time in a stall overnight when they are normally accustomed to moving around in a pasture. Others stock up only if confined after strenuous exercise. Usually if these individuals are allowed to move around in a larger area after a hard workout, they will not stock up. Confining a horse whose system is accustomed to freedom of movement is probably the most common cause of stocking up.
What you feed a horse may also contribute to stocked-up legs. Horses that are fed an excessive amount of protein may stock up more readily than horses on a more moderate diet. An overabundance of protein can overtax the horses system. The excess protein must be excreted by the kidneys, which, when stressed and overburdened, may not do their job thoroughly, causing swelling in the lower legs.

More serious problems
Several things can cause swelling in the lower leg and you must diagnose the problem correctly. If the horse is lame, or if the legs are still swollen after exercise, you are dealing with a more serious problem than routine stocking up. The stocked-up leg is not sore, and usually not great cause for concern. You must be able to differentiate this type of swelling from that of injury, infection, or a serious health problem.
Is just one leg swollen, both fronts, both hinds, or all four? If just one leg is swollen, suspect injury or local infection, especially if the enlargement is confined to one area and does not involve the whole lower leg. An infected area, or swelling from a kick or a blow, will often have more abrupt borders, whereas the stocked-up leg will resemble a tear-drop (largest at the bottom) that gradually blends into the upper part of the leg. The stocked-up leg will also be cool. Heat in the swollen leg is indicative of infection or inflammation from strain or injury.
There are other clues to help you tell the difference between simple stocking up and swelling due to injury. Filling in the stocked-up leg is somewhat soft, with some give to it. If you press the area with your thumb or finger for a few seconds, it will leave a dent when the pressure is released. It will take a few seconds for the dent to disappear. This is called pitting edema. By contrast, swelling from infection or inflammation is much firmer. Also, there may be tenderness or pain, which should be cause for concern. The horse should not flinch when you press gently but firmly to check for pitting on a stocked-up leg. An indication of pain usually means injury or infection. In this case, the horse will also be lame, favoring the leg as he walks or trots.

Possible causes
There are several serious health conditions that may cause a horses legs to swell, such as heart or kidney problems, or anemia caused by a disease such as equine infectious anemia (EIA). Pleurisy or chronic heart failure can cause swelling that begins at the top of the forelegs, tapering down from the elbows, or beginning at the hoofs and moving upward, resembling simple edema. But a stocked-up leg is only swollen below the knee or hock. If all four legs are swollen and the swelling includes the upper part of the legs, contact your veterinarian immediately.
This type of all-inclusive swelling can be the result of an allergic or toxic reaction. Sometimes reactions to certain proteins (either eaten or injected, such as a strangles vaccination) can cause leg swelling. Purpura hemorrhagica is a type of allergic reaction to an infection in the body, such as strangles or a viral respiratory disease. Some horses develop extensive swelling of all four legs (and sometimes the lips and eyelids as well) after experiencing one of these diseases. A veterinarian should be consulted quickly if this ever happens, since this condition can be fatal.

Prevention
Simple stocking up is not dangerous to the horse as long as it is not prolonged for weeks or months. And there are ways to minimize and alleviate this problem. If a horse that is normally active must be temporarily confined (such as a long van or trailer trip) and he is prone to stocking up, the application of standing bandages will keep his legs from swelling. Be sure to get advice from someone who knows how to wrap legs if you do not have experience doing so, because incorrectly applied bandages can do more harm than good, inhibiting circulation and possibly damaging tendons.
As a general rule in most other situations, it is best to leave the horses legs alone. Do not wrap them unless absolutely necessary. A horse may stock up at first when brought into the barn from the pasture but, in most cases, his circulation will adjust and compensate for his decreased level of exercise after the first week in his new environment. Bandaging the legs should always be a last resort, since the horses circulatory system will become dependent upon them, causing loss of tissue tone. It is better to try to alleviate the cause of the problem in the horses environment or management.
If a horse stocks up after being confined, a vigorous rubbing of the lower legs with liniment before a workout can help stimulate circulation and reduce swelling, as can a gradual warm-up period before he must do strenuous work. If legs are swelled when you take a horse out of the barn, give him a long warm-up with gentle exercise, such as leading or walking under saddle, for 15-to-30 minutes to get his circulation functioning more efficiently. This will also help prevent injury to tendons, ligaments, and joints. Do not start with fast works before he has a chance to warm up or his fetlock joints may be stiff and clumsy due to swelling and more prone to injury. The swelling should go completely away with exercise, though it may take 30 minutes to two hours depending on the extent of swelling.
A proper cool-out after exercise is also helpful to prevent recurrence of the swelling, giving the horses heart, respiration, and circulation a chance to return to normal before putting him away. Give him slower work or mild exercise such as walking at the end of the workout until he is completely cooled down. You can also rub his lower legs with a brace or tightener to help his circulation. He may still stock up overnight, but not as severely.
Nature did not intend the horse to be idle, so stocking up will continue to be a problem for some horses kept in stalls or small paddocks. Stocking up can be considered an early warning signal that all is not exactly as it should be. Circulatory problems in a horses feet and legs should always be taken seriously and minimized as much as possible to prevent more critical problems. Keep in mind that the less total exercise you are able to give the horse, the more room he needs in which to provide it for himself. If he must be confined, make sure he is exercised regularly. Keep his diet balanced according to his needs. Take longer to warm him up and cool him down. If he is worked strenuously, take the necessary steps to keep his feet and legs healthy, with careful thought as to what they must withstand during and after that exercise.