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We are all taught to run our hands down a horse’s legs each morning, checking for filling or injury. But what does finding a filled leg mean — and is it always serious?

There is often confusion over this, because for a start the term “filled leg” isn’t very precise. Any tendon or ligament injury will result in swelling, as will a cut or a bang. Sometimes, however, there is more generalised filling of one or more legs without an underlying injury.

Typically, this kind of filling starts at the pastern or fetlock and extends up to the knee or hock. It also goes right around the leg, making it difficult to see or feel the normal structures such as the tendons.

To understand why legs fill with fluid in this way, it is helpful to know how the circulatory system works.

Blood is pumped away from the heart under pressure. It travels down the arteries and smaller arterioles and finally into a dense network of tiny capillaries that spread through all the tissues.

However, there is no similar pump powering the return trip. To get back to the heart, the blood is dependent on the squeezing action of muscles and tendons to push it back along the thin-walled veins. These have valves, which make them effectively one-way routes, preventing the blood from leaking back.

There is a second return method called the lymphatics, another system of multiple tiny channels that carry fluid from the tissues. These, too, depend mainly on muscular action to push the fluid along.

There is an obvious consequence of this system. If a horse is standing still there is very little helping the blood to return from the legs, particularly as these are far away from the heart. In addition, the fluid has to go uphill — against gravity.

So it is no surprise that horses, particularly larger ones, can develop filling in their back legs while they are stabled overnight.

Oedema explained

Filling that develops this way is called oedema, and is due to the collection of excess fluid outside the blood vessels. The capillaries have tiny holes in them and this allows fluid to leak out and surround the individual cells of the muscles and other organs.

When the horse stands still for long periods there is not much muscle activity, so more fluid than normal collects in the tissues. Such mild filling is not significant; a few minutes of exercise the next morning is usually enough to drive excess fluid out and return the legs to their normal size.

Overnight filling tends to be worse if the horse has worked hard the day before. It is also more common in older horses because their circulation deteriorates with age. Scar tissue from old injuries can sometimes constrict the return circulation too and lead to more filling in a single leg.

Another possible cause of oedema is a viral infection, because some viruses cause inflammation of the small blood vessels and reduce their effectiveness. It also occurs if a horse’s blood protein levels fall.

Too much protein in the diet is still sometimes blamed for causing filled legs, but there is no scientific truth in this. In fact, it is low protein levels — usually as a result of starvation or a severe worm burden — that can cause oedema. This is because a normal level of protein molecules in the blood is required to retain water in the capillaries. If protein levels drop, more fluid leaks out.

Filled legs can also be the result of heart, liver or kidney disease. However, these are rare causes and would be accompanied by other signs indicating that the horse is seriously unwell.

Spot the difference

Oedema is one of the two most common causes of filled legs, the other being infection. It is helpful to be able to distinguish between them.

Legs that are filled due to oedema are cool to the touch. The horse will not resent the leg being handled, and if you press firmly on the filled area it will leave finger-shaped indents in the skin. This is called pitting oedema.

Normally, there is no unsoundness. Oedema often affects more than one leg, most commonly both hindlegs, but sometimes all four.

By contrast, a leg that is filled because of infection will feel warm to the touch and will be obviously painful when pressed. The horse will usually be lame.

Swelling under the skin due to infection is called cellulitis (pictured top). The filling results from bacteria — typically from a small wound or from mud rash scabs — getting under the skin and multiplying. This causes inflammation, which in turn causes leakage of extra fluid from the capillaries and lymphatics.

putting on exercise bandages

Movement is medicine

Mild filling of the legs due to oedema does not need any specific treatment except exercise. If injury has been ruled out, the horse is otherwise bright and well and the legs return to normal after work, then it is safe to say that there is not a significant problem.

Bandages can be used to help keep filling out of the legs overnight, but care must be taken to use adequate padding and not to tighten them excessively.

If the filling does not go down or the horse is showing any other signs, then the vet should be called. A careful clinical examination and a blood test may be necessary to identify the cause of the problem.

If cellulitis is diagnosed, this is usually treated with antibiotics and anti-inflammatories.

The division between oedema and infection is not absolute, because each can be a cause of the other.

Consistently filled legs mean that bacteria are not filtered out as effectively as usual, giving them the opportunity to increase. This is why a cold, non-painful filling of the leg can sometimes become sore and infected.

The reverse is also true: horses suffering from low-grade infection due to mud fever, for example, are also prone to more general oedema of the legs.

Ref: Horse & Hound; 31 March 2016