Over the four days of the Melbourne Cup Carnival, hundreds of horses will take part, each trying to win a race at Flemington.

There will be millions of dollars in prize money at stake, and millions of dollars more in elite thoroughbred horses in action.

But what is the process a racehorse goes through before being passed fit to race in a big event?

What are the checks and balances in place to reduce the chances of a horse breaking down?

The focus on equine safety has intensified in recent years, with a series of high-profile deaths related to the Melbourne Cup.

Last year saw the latest, when the Aidan O'Brien-trained Irish galloper Anthony van Dyck broke down during the Melbourne Cup with a fetlock fracture and had to be euthanased.

An inquiry produced a report with 44 recommendations, of which Racing Victoria adopted 41.

The new safety measures focus on greater veterinary oversight and screening of international horses who travel to Australia for the Melbourne Spring Carnival, and of all horses — local or from overseas — in the Cup itself.

The use of scintigraphy — bone scans — and CT/MRI scans of their distal, or lower, limbs is now mandated for all horses within two to six weeks of travel, while a Racing Victoria-appointed veterinarian must conduct an inspection in pre-travel quarantine.

Racing Victoria had bought a standing CT scanner in 2019 and, as a result of the reforms, the use of the scanner is now mandatory for Cup entrants.

Unfortunately for RV, the CT scanning machine had mechanical issues this week in the middle of scans of the various Melbourne Cup acceptors — 16 of the 35 horses still in with a chance of making the field at the time had not yet been scanned.

RV set an alternative for the remaining horses of having comprehensive high-quality X-rays of the distal limbs, to be reviewed by the same expert panel which was there to check the CT scans.    

Pre-race veterinary inspections are undertaken on all final acceptors for the big three Spring Racing Carnival races: the Caulfield Cup, the Cox Plate and the Melbourne Cup.

On Thursday and Friday, RV vets inspected the by-then 29 horses remaining to ensure they were suitable to accept at Saturday's deadline for the final 24-horse Melbourne Cup field.

The one outlier was Spanish Mission, who had repeat inspections due to leg swelling, which had died down: the diagnosis was of cellulitis, an infection under the skin.

Spanish Mission was passed fit on Saturday in his final inspection and took his place in the field.

Great House, which won the Hotham Handicap to claim the last automatic place in the field, was also inspected and passed fit before being included in the final field.

As a result of the safety recommendations, an extra pre-race vet's inspection will take place the day before the Melbourne Cup.

"Racing Victoria has a statewide team of 60 veterinarians who work on-course to support the health, welfare and safety of every horse competing in the 550 race meetings conducted in Victoria each year," a Racing Victoria spokesman said in a statement.

Other tools available to check on horses include cortisol testing, where blood serum, saliva or urine is used to test the concentration of cortisol to gauge stress levels after exercise.

Identifying lameness the aim of inspections

Dr Ian Fulton, a specialist equine surgeon and former president of Equine Veterinarians Australia, said regulatory pre-race veterinary inspections for racehorses needed a space, either indoors or outdoors, to examine horses at a walk and trot.

"The trot is the gait, the action we check for possible lameness," Dr Fulton said.

"We need 15m or so on a straight, firm surface for horses to trot up and back to you, to check the action of the front and hind legs.

"The easiest way to explain it is to imagine a horse has got a nail in its right front foot," he said.

"When it's left front foot is on the ground, it brings weight on to the left foot and the head goes down.

"When the right front foot is on the ground, it rears its head up, to take weight off [the injured foot]."

This is referred to as the "head drop". In hind legs, the warning sign for lameness is often known as the "hip drop".

The other form of test is trotting the horse in a circle of 10-15m in diameter. Horse's inside legs tend to bear more weight, such as the left front leg when moving to the left — so vets keep an eye out for horses trying to transfer weight off the inside leg when circling.  

For trot-up exams, vets are not looking to identify the exact area of an injury or problem beyond saying the left or right front leg or left or right hind leg.

The aim of the exam is to find whether a horse is fit to race, and whether it is lame or not.

"The report will say something like, 'The horse is grade two of five lame in the left front leg' and 'The left front leg is an area of concern'."

This week, one of the certain starters for the Melbourne Cup, Young Werther, was withdrawn from the race after CT scans showed an "area of concern".

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Trainer Danny O'Brien and connections were disappointed but expressed thanks that a problem had been found before the horse had sustained an injury.

Lameness is often caused by pain. However, the horse's leg runs from the hoof to the shoulder — so identifying the precise source of the lameness requires further examination and possible scans undertaken by private veterinarians on behalf of the connections or trainers of the horse.

A Racing Victoria spokesman said inspection procedures might also include "a brief clinical examination which may include palpation, flexion of joints and an assessment of hoof sensitivity".

The inspection can also involve an examination of the horse's treatment record, and "consideration of any occurrences, conditions and/or treatments which may affect or impact on the horse's performance in the race".