Why do they shoot horses with broken legs?

Outsider Rule the World stole the headlines with a stunning win in this year's Grand National, but the meeting at Aintree was overshadowed by the deaths of five more entrants.

More than 45 horses have died at the course since 2000. Two of this year's casualties, Gullinbursti and Minella Reception, suffered neck injuries and were put down while the other three, Clonbanan Lad, Marasonnien and Kings Palace, died after being pulled up. It is thought they had suffered leg injuries.

Seven horses also died at last month's Cheltenham Festival.

The continued deaths have prompted more protests against the sport.

But why is a broken leg so often a death sentence for a horse?

Light bones

There can be no real comparison between a broken leg for a human and a broken leg for a horse because their bones are so different.

Racehorses have been selectively bred for centuries and are designed for speed, vet Jenny Hall tells The Guardian. "Their bones have become lighter. They're very strong, to carry their weight, yet they're light, for them to be able to go fast. So, unfortunately, sometimes, when they break, they just shatter."

To make matters worse, in the moment before the bone snaps it bends, resulting in 'plastic deformation'. "Even if it were possible to put the pieces back together, you would end up with a madly bent bone," says the Guardian.

The horse's lower limbs have little soft tissue and that means that bones often pierce the skin when broken. Not only does that make the wound much harder to treat, it also affects the already limited blood supply to the lower leg, thereby compromising its ability to heal.

Complex anatomy

The complex bone structure of a horse's leg adds to the problems. "Out of the 205 bones that make up a horse's entire body, 80 of them are located in its legs. The complex system of joints, bones, ligaments, tendons, cartilage, lubricant, laminate and hooves that contribute to a horse's amazing speed can also be the cause of its downfall," explains website How Stuff Works.

Rebuilding a horse's leg is an almost impossible task.

Prey animals

A horse is a prey animal, which means it has evolved to remain on its feet at all times. Treatment for a broken leg requires the weight to be taken off the damaged leg, not something a horse can handle.

"Keeping a horse from reinjuring itself is a big problem in recovery. They can step on themselves, get excited and try to move around, or simply get bored of being in a stall and try to get out," says the Ultimate Horse Site. "When the ex-racehorse Alydar broke his leg at the age of 15, he underwent surgery, but two days later he broke the leg again moving about on it and had to be euthanised."

What about slings?

Slings can take the weight off the feet but can cause sores and also end up compressing a horse's internal organs. Knocking a horse out so it is lying down can have the same effect and allows fluid to accumulate on the lungs causing pneumonia.

Sheer weight

If a dog, for example, breaks a leg it can support itself on its remaining three feet, not so a horse. The problem lies with the hooves, explains the Guardian. "These are strong enough to support the horse's weight when that burden of 500kg or so is shared across four legs. But when a horse breaks one of those legs and tries to support itself constantly on the other three, the increased burden creates serious problems for the laminae at the base of those legs."

Laminitis, or inflammation of the tissue within the hooves, causes the animal terrible pain. It can be treated with painkillers, but the only long-term solution is for the horse to bear its weight on four legs, something that is not always possible.

A quick diagnosis

What often shocks racegoers and the public is the speed with which the decision to euthanise a horse is taken. But vet Jenny Hall tells the Guardian that there are trained medics by the racetrack who get there fast and the decision is often an easy one to take.

"For the horse to have a full clinical assessment and second opinion can be done very quickly," she says. "The horses that have fatal injuries, they have very extreme injuries. There's no possible misunderstanding of those. The horse will not have a supporting limb. The limb that he's injured will no longer be able to take any weight."