IT'S the story that overshadowed Australia's most glamorous race. But why did Melbourne Cup runner Verema have to be euthanised after snapping a bone in her leg at Flemington?

In short, horses are bad patients that just aren't built to recover from fractures, says Australian Veterinary Association equine surgeon Dr Barry Smyth.

It's a high impact and often "catastrophic" injury when a 500kg horse, racing at 60km/h, breaks its leg, sending huge amounts of energy through the bone.

"When things go wrong, they go very wrong," Dr Smyth said. "Horses are not constructed to withstand major trauma or injury."

As thousands of people around Australia drank champagne to celebrate Fiorente's Melbourne Cup victory, the champion mare lay on the ground with a shattered leg, about to be put to sleep.

Verema, who dropped out of the great race in distress at about the halfway mark, snapped her cannon bone, a large bone in her lower leg. On-track vets made a quick decision to euthanase Verema but the million-dollar mare's death was met with outrage from punters.

Evolution has certainly been unkind to horses. Their bones shatter into irreparable pieces in a high-impact injury.

Even a clean fracture usually means a fatal end because their lower limbs are made up of just skin and tendons with little muscle to stabilise a broken bone.

Inserting metal plates, commonly used to repair fractures in humans, dogs and cats, is not an option because they are only designed to hold 100kg of body weight, Dr Smyth said.

Then there's weeks and months of trying to keep a horse still.

Horses can't lay down because they are vulnerable to pneumonia and pressure sores - not that a horse would lay still anyway.

Standing on three legs is a no-go too because horses develop problems in the opposite leg holding the extra weight.

"You can occasionally get a horse that stands in a sling or suspended in a pool but temperamentally, they aren't built for long periods of nursing," Dr Smyth said.

"If you try and put a horse in a sling it will often try and jump out and end up doing more damage.

"Horses have not evolved to be good patients, anatomically they are not built to withstand fractures and temperamentally they will not respond to long-term nursing."

OPINION: Why they had to kill Verema

Dr Smyth said Verema's injury was the equivalent to a human breaking the hand bone between the wrist and finger.

It sounds minor enough but when the lower part of the limb is poorly supported, it's a death sentence, he said.

"When this happens, it happens before crowds and is broadcast on TV, which is very distressing for people but there are also plenty of horses that snap their legs galloping around a paddock," Dr Smyth said.

"These injuries are not unique to racing ... horses are very vulnerable to catastrophic injuries.

"There's been a huge amount of research done over the past 20 to 30 years into repairing broken bones in horses but unfortunately I don't think we'll ever find one.

"We can sometimes recognise a clue that a horse is developing a problem in the bone but often there is no warning. It is almost impossible to predict."

For long-time followers of racing, the incident has echoes of the aftermath of the 1979 Melbourne Cup when race favourite and cult hero Dulcify was euthanised after fracturing its pelvis.

For younger, recent racing fans and once-a-year-punters, the Verema tragedy is a distressing reminder of the fact that racing is a dangerous sport for both humans and equine participants.

Some have taken to Twitter to express sadness about the loss of a horse's life.

Can't Australia dedicate a day to a sport that does not fatally injure it's participants? #MelbourneCup #verema — Krista Mav (@Krista02) November 5, 2013

Can we stop using the term "euthanised" for sad situations like Verema's? The whole point of euthanasia is that it's voluntary! — Dominic Knight (@domknight) November 5, 2013

RIP #Verema #MelbourneCup Why must we put down the animal coz it has something Wrong. You own a Horse you must take good care of it #shame — Sonia M Khan (@Smk4Khan) November 5, 2013

Melbourne Cup broadcaster Channel Seven also attracted some criticism on social media yesterday afternoon, with many people saying they didn't so much as mention the incident.

Johnny Letts and Damien Oliver went straight past the green screens which covered up verema. At least acknowledge what happened 7 sport. — Benjamin (@BanZiNi) November 5, 2013

Seems fashions on the field and rock stars are more important than acknowledging that Verema was put down after the Cup. Poor form Ch 7. — Cheryl Critchley (@CherylCritchley) November 5, 2013

Others have taken to Twitter simply to air their sadness.

#verema came from Europe to compete in the #MelbourneCup & has been put down. Feel sorry for trainer, jockey & owner pic.twitter.com/noe5GUVww3 — Andrew Day (@andrew_day10) November 5, 2013

Saw Verema break down, rest in peace, sometimes horse racing is a cruel sport — Holly Fahey (@holly_fahey) November 5, 2013

The world lost another beautiful soul today RIP gorgeous Verema-they may have hid you behind that green screen but we saw you and we weep :( — MOP (@membersofthepub) November 5, 2013

The Australian Racing Factbook, an annual publication produced by the Australian Racing Board, says that there are almost 20,000 races in Australia each year, contested by about 30,000 horses. The Factbook makes no mention of the number of fatalities.

News.com.au contacted the Coalition for the Protection of Racehorses to find out how many horses die in races in Australia each year. They weren't able to answer that question but issued a statement which in part read:

"It is a sad day for racing when a horse is killed in the most competitive race of the country. It shows that sometimes these horses are pushed beyond their physical limits, and it is a high price to pay.

"We would like to ask that the racing industry announce the news to all at Flemington, and hold a minute's silence in memory of the fallen horse, Verema."

Verema was trained by Alain De Royer-Dupre, the French trainer who brought American on a successful Melbourne Cup raid in 2010. The five-year-old mare's last run was in the Dubai Gold Cup, in which it finished third.

Uproar following a very public death of a racehorse is not uncommon around the world.

The horrific break-down of Go for Wand in the 1990 Breeder's Cup Distaff shocked North Americans when the champion two-year-old got back up after breaking her ankle and crossed the finish line on three legs.

Then, in the 1992 Breeder's Cup sprint, five-year-old European horse Mr Brooks fractured his cannon bone and fell on top of his jockey. Mr Brooks' death prompted organisers to introduce more stringent safety checks into the welfare of horses.

A 2005 study by the United States Department of Agriculture found that injuries are the second leading cause of death in horses, second only to old age.

According to Australian-based The Coalition for the Protection of Racehorses, the chances of a thoroughbred being a successful racehorse are slim.

It is estimated that of the 18,000 thoroughbred foals born each year in Australia, an average of 12,600 will be ruthlessly discarded and mostly end up at "the doggers".