Meet the teachers who can cure the creatures

But she is already prepared for the worst, as there is no doubt he is seriously ill. The star attraction at Gorgie City Farm, Red has suffered brain damage, is losing his eyesight and is obviously in pain. Volunteer Donna has already asked the vets to make sure she is there if – or when – they had to put him to sleep.

Fortunately, the 13-year-old horse is in the hands of some of the world's leading experts at the Royal Dick Veterinary School in Edinburgh. And for the first time, the school has opened its doors to camera crews – allowing viewers to watch their journey as vets battle to save Red's life.

This is just one of the emotional stories – mainly happy, but some sad – that are told in STV's new documentary series, starting tonight. In Vet School, viewers will see a beloved pet cat undergo surgery for a tumour near its heart and a puppy rushed in with liver failure. The more unusual patients include a pet skunk and a chinchilla which has lost its appetite, while wild cats and swans also undergo treatment.

Donna, a volunteer at Gorgie and a mother of four, first noticed Red seemed "a little out of sorts" one night. When she returned the next morning, she found him hanging his head and staggering as he tried to walk. She called the vet straight away.

She says: "We knew there was something seriously wrong with him. They didn't think he was going to make it and were preparing me for the worst. I had to tell the children who visited the farm they might not see him again."

In tonight's episode, vets try to unravel the cause of Red's mystery illness. At first they think it is colic – which can be fatal in horses – then salmonella. But eventually they realise it is ammonia poisoning. As his condition worsens, he loses the ability to walk properly and suffers partial brain damage. He is taken to an isolation ward as he undergoes intensive treatment.

After caring for Red for several years, Donna says she felt almost as if one of her children was in intensive care.

She says: "I spent a long time with Red at the hospital. I thought he would just want to hear a reassuring voice. If they had to put him to sleep, I wanted to be there with him. It would be a very difficult decision, but I didn't want him to suffer."

But fortunately Red's story has a happy ending, and he is one of the first horses in Scotland to survive this illness. He is now back at the farm, delighting scores of young fans and giving regular rides. He had been nicknamed "the patchwork horse" when his hair was shaved during treatment, but it has now grown back.

The equine hospital is just one part of the internationally renowned facility. Viewers are also taken behind the scenes in the school's very own dairy farm, where they see a vet and a group of students struggle to turn a cow on her back for treatment.

We also see a pet snake, Marley, undergo an X-ray and surgery, after her "pregnancy" turns out to be a serious illness. Edgar, a chameleon with a swollen eye, is also treated at the Exotic Animals Unit.

And one of the most nail-biting parts of the documentary is when six-year-old cat Brodie undergoes an operation to remove a tumour four times the size of his heart. When his brain is starved of oxygen during surgery, his owner faces an anxious wait to see if he will make a full recovery.

Ronnie Soutar, the school's director of veterinary services, says their state-of-the-art facilities mean they deal with cases from all over Scotland, as well as training the next generation of vets. It now boasts a specialist cancer centre, including a room-size accelerator to give radiotherapy to animals.

He says: "We treat thousands of cases each year but people generally only see a tiny part of what we do. This series gives a real insight into what goes on behind-the-scenes.

"This could range from checking bearded dragons for dental disease to carrying out surgery on horses. Our vets and the students they teach see all the challenges and rewards of our chosen profession. This is a great opportunity for everyone else to see that too."

The vets certainly deal with "all creatures great and small" at the large and continually expanding campus at Easter Bush. They refer to each of them as "patients" and take care to explain everything to the owners and build up a relationship with them.

It is clearly an emotionally draining experience for those who see the sick animal as a member of their family. These include Jo Bussell and David Bruce, from Oxgangs, who were devastated to come back from their holiday to find their pet chinchilla, Colin, had gone off his food and lost weight.

By the time they take him to the clinic, he is very weak and vets have to act fast to save his life.

Jo, 36, a financial services manager, says: "He obviously wasn't his normal self. We discovered he had broken one of his teeth, and had severe dental disease. One of his teeth had overgrown and was causing him pain. He'd lost lots and lots of weight. He was in a bad way and it was quite touch-and-go.

"We don't have children, so he is really my substitute. I couldn't bear to think about anything happening to him. But I thought that even if I couldn't watch the programme, it might help someone else in a similar situation recognise the signs."

The tiny animal has had to undergo several dental operations since July, each time under a general anaesthetic.

But fortunately he has recovered well, although he will need to go back for more treatment once or twice a year. Jo and David hope he will still enjoy a long and healthy life of up to 20 years. Jo says: "We have to be very careful, because if he gets an infection that could be very serious. But the vets taught us a lot, and we know we need to keep on top of his diet now."

Michael McAvoy, executive producer, says the programme will give audiences a unique insight into the vets' work.

He says: "I think people will be astonished to see the amazing work that goes on at the school.

"In one stunning location you can get to see cats and dogs undergoing crucial operations, exotic birds getting their beaks checked, skunks getting neutered and in one episode we even see how the vets deal with a goat with a phantom pregnancy. The variety of work is breathtaking."

Vet School is on STV tonight at 8pm.