Here are the facts of the case. The patient was in an oxygen tent; she'd had heart failure and would have to go through major surgery; the operation would be tough and would require a big team of surgeons; there was no guarantee of success. And the last fact: the patient was a dog.

So how do you feel about that? A dog. A Tibetan terrier called Cassie. All that medical attention and expense, plus time in surgery, and it's a dog.

There are generally two reactions, and Cassie's owners, Jacqui McNeil and Craig Watson, are familiar with both. Some people say they were trying to do the best for their pet; others say: "It's just a dog." That's a harsh word, "just", because for Jacqui, 39, and Craig, 30, there was no alternative: they would do what they had to do for Cassie.

The truth is their case isn't that unusual any more. This high level of care for animals is becoming more and more common. You might have read this month about the chicken called Eve whose owners paid for radiotherapy to treat a cancer in her leg; last month, you might have read about Electra the golden eagle, who had a cataract removed. More and more, animals are undergoing operations that would once have been considered extraordinary. Twenty-five years ago, animals such as Cassie would simply have been put to sleep.

As it happens, she's doing fine now. In fact, she is enthusiastically chewing on the corner of my notebook with her cocktail-stick teeth. She looks exactly like an illustration a Disney cartoonist would do - all flippy ears and fluffy face - although she does look a bit odd, having been shaved for the operation. There are little tufts of fur left on her front paws that look like Ugg boots. She's both cute and fashionable.

The condition Cassie had is called patent ductus arteriosus. It's caused by an abnormal blood vessel linking two major arteries and made her heart work like a piston on an old steam train; in fact, it ended up feeling like it would jump out of her chest. To keep her alive, the only option was to open her up and fix it.

Perched on the end of her sofa in her home in Hamilton, Jacqui is telling me the story. She obviously adores Cassie. She always has one eye on her, the anxious mum with the vaguely unruly child. She discovered there was a problem when she took Cassie to the vet for a check-up and he suggested more tests on her heart. "There were no outward signs of anything being wrong with her, but internally it was all going wrong," says Jacqui. "About a couple of hours after she went in for the check-up, I got a call to say she was in an oxygen tent. Then I got a call a little later telling me she was in heart failure. I was pretty upset because I was already really attached to her. They said she could keel over at any time and die."

The operation to repair her heart was a big risk because Cassie was so small. "I didn't have the option not to do it, though. If they didn't, she would have died," says Jacqui. "She could have died any day - her heart would have failed completely. There was no question of not getting it done, although they said there were a couple of moments during the operation when it was touch and go."

So how much has all this cost? About three grand, as a matter of fact. Some people would never consider spending that much on fixing an animal; others would never consider any other option. In fact, they would spend far more. "Some people will say it's just a dog," says Jacqui. "It's the attitude that animals are animals and people are people. Some people think that way, but I just did what I had to do."

The interesting question is: as these kinds of operation - this almost human-like way of treating animals - become more and more common, what's changed? Are we anthropomorphising more than we used to? Have we gone soft?

Ian Ramsey, the director of the Small Animal Hospital at Glasgow University's Veterinary School, where the operation on Cassie was performed, doesn't think so. In fact, he has little patience with the argument. He says the British attitude to animals is the same as it was 25 years ago: we love them and care about them. What has changed is what is possible: the technology, the expertise. "Things that were done rarely 25 years ago are now done commonly," he says. "People are becoming aware that vets can do just about anything that a human medical hospital can do."

So we aren't anthropomorphising more than we used to? "I think we always have with cats and dogs, but people now don't give up so quickly - and frankly that is good news for the dogs and cats because, rather than being put to sleep because they have a broken leg, within a matter of two months they can be back up and walking."

As for the expense, that's just a fact of life, says Ramsey. The old idea of James Herriot doing everything on his own with his sleeves rolled up is gone. If you're doing a total hip replacement, for instance, you need three vets, an anaesthetist, two surgeons and two or three nurses, plus you've got the cost of the operation. Even a simple broken leg will cost several hundred pounds; if it's more complicated, it's £1000 or maybe even £1500.

Ramsey believes clients who are willing to do this for their pets aren't going too far; they are just doing what they think is right. "Sometimes people will come in for criticism from their families for putting in too much effort to help their animal. You often have an owner phone up and say, My husband thinks I should just put the dog to sleep.' Usually I end up talking to the husband and saying, This is OK, this is normal.' "I think the question comes when you hear of a bill of £7000 for a dog, and then people ask: should you not just buy a new dog and give the money to charity? That, I have to say, is a non-starter of an argument. You then have to ask all sorts of questions about how people spend their money - should they spend it on big cars?"

Margery Osborne from Helensburgh could answer that question. She has spent about £2500 on her Weimaraner, McDuff, and has never questioned signing the cheques for a second. McDuff was six months old when he was diagnosed with meningitis. It meant he sometimes struggled to walk or even get up. "I thought he was going to die so many times," says Margery. "It was heartbreaking because there was nothing I could do - but when we got the diagnosis, we were on a better track. We knew what was going on then."

For Margery, there was no question that this unusual condition should be investigated by lumbar puncture and then treated. "A very odd person said, You spent how much?' and some people say, Just get him put down,' but that is not the option I would take, unless you had to."

Irene Forrester from East Kilbride would feel the same about her cat Pumpkin, who had open-heart surgery for a problem similar to Cassie's. It cost £1500 but she said at the time of the operation: "He deserved the chance to live a normal life, and that's what the vets have given him."

Ian Ramsey believes treatments like those on Pumpkin, McDuff and Cassie will become more common as expertise grows. "Total hip replacements used to be a once-in-a-lifetime experience for an orthopaedic surgeon; now we do one a week." What will shape how things develop will be trends in animal ownership: if snakes become massively popular, snake medicine will improve massively. "The one thing that is coming up quickly is rabbits, and we're seeing advanced surgery on them now as a result," says Ramsey.

The final question - should we do all this (rather than could we)? - is clear for Ramsey: owners have always wanted to do the best for their pets. They still do. And they always will.

Here's the proof. In the next few days, Cassie the Tibetan terrier will go through more tests to make sure she is still on the mend, and Jacqui and Craig will get the final bill. They could have bought a car with that money, or gone on a two-week break abroad, but they didn't do either of those things. And they are not regretting that for a second.