Swollen or filled legs in horses can be worrying but usually there is no pain involved and can be easily prevented.
The swelling usually affects pairs of legs e.g. both forelimbs and doesn't usually cause lameness. Swelling in the hind legs is more common and the lower legs is usually affected, below the knee.
Filled legs can be pressed without a pain reaction and the swollen area will feel dough like. Swelling caused by infection or injury is usually painful to touch and often causes lameness.
Swelling occurs when fluid accumulates in the legs, usually due to the horse standing still for long periods, such as in the stable. When a horse's feet touch the ground it acts like a pump and sends blood and lymphatic fluid from his legs back up into his body. When a horse stands still for a while the circulation reduces and blood and waste pool causing the filling.
An imbalance between hard feed and exercise can also cause swollen legs. For example a horse that's eating lots of carbohydrates while standing in would be more likely to develop it than one in the field 24/7 grazing. It appears to be more common in older horses and finer breeds like Thoroughbreds than in native ponies or cob/chunky types.
Gentle exercise such as walking in hand or on a horse walker can reduce the swelling and bandaging the legs can prevent the legs filling when standing in the stable. Turning the horse out will help too. If lameness is seen do contact the vet for further advice.
Cold hosing will reduce the swelling and stable boots or horse fleece boots will support the limbs whilst standing in the stable for longer periods.