Why do horses' legs swell up?

How can we tell the difference between harmless leg swelling and something more serious? Karen Coumbe MRCVS looks at some causes of swollen legs

THE phrase “my horse has got a leg” is well understood by horse owners, but can be a surprise to those not used to the equine propensity for legs to blow up like balloons. There will always be a reason for the swelling, but it may not be obvious.

To understand filled legs, one needs to think about a horse’s internal plumbing. Essentially, horses have long legs and especially in larger horses, the lower limbs are a long way from the heart. The heart is the pump designed to drive the body fluids around the body, but in a big horse this is not a simple task.

The biological design means that blood is pumped away from the heart under pressure. It pulses through the big arteries, then travels through the smaller blood vessels, ending up in a network of tiny blood channels, called capillaries, that are spread through the body’s tissues. The blood then has to return to the lungs to reoxygenate and back to the heart, but there is no pump to power the return trip.

Instead, the blood relies on the squeezing action of muscles and tendons to direct it along the thin-walled veins. These veins have valves to stop blood flowing backwards, but there are no big muscles in the horse’s lower limbs to push fluid in the right direction. The same is true for the lymphatic system, which provides additional drainage tubing to remove fluids from the different body tissues. This also requires muscular action and movement to send the fluid back to the centre of the body.