This disease is recognised by the swelling of the legs, usually hind, that pits when pressed with a finger. There are 2 causes with very different implications for the horse.
The first reason for this type of swelling is known as Sporadic Lymphangitis. The swelling usually affects both hind limbs but can affect all four legs and can be considerable.
The cause is simple and it is brought on by an unusually lengthy period of box rest in a horse that doesn’t usually rest for long periods. If your horse is stabled for most of the time anyway then swelling of the lower limb is likely to be due to a genuine injury rather than this disease. This sporadic form can easily be differentiated from the more sinister form as it resolves to a completely normal looking leg with a short period of exercise. There is no underlying disease process and there are no long-term effects to the affected horse. As the colloquial term “Monday morning disease” suggests, it is commonly seen in horses that work hard through the week and are stabled over the weekend, presenting with swollen back legs on Monday morning when they are brought out to work again.
The other form of the disease – Ulcerative Lymphangitis – is a completely different disease. This presents similarly to Sporadic Lymphangitis with massive swelling of, usually, 1 hind limb but it is much more painful and you will often see discharge through the skin of the affected leg. It is caused by infection with bacteria either through an existing wound or in a limb affected by mud rash. The swelling is caused by a combination of inflammation and blocked drainage of the leg by small clots wedged in the vessels that would normally carry the oedematous fluid away. The affected horse will also often be non weight bearing, show a high temperature, reduced or absent appetite, sweating, increased respiratory rate and sometimes, mild colic symptoms due to the significant pain. Treatment is always necessary and should be rapid and aggressive as these cases are difficult and challenging to treat successfully and unfortunately often results in a permanently swollen leg.