In August 2009 Dr Claire Guest, a scientist and animal behaviour expert, was training dogs to use their sense of smell for medical purposes. Struck by the intelligence of animals, after years of intensive research, she had become convinced that dogs could be used to detect human conditions – from low blood-sugar levels in people with diabetes to cancerous cells in the otherwise healthy – using nothing but their noses.

In this edited extract from her book, Daisy's Gift, Guest describes how that summer her labrador Daisy discovered a lump on her own breast:

My operation was a success, and I was told afterwards that my cancer was so deep that by the time I had been able to feel it, it would almost certainly have been too late.

“If you hadn’t come in for a check-up . . .”

As the consultant said the words, I remembered Daisy nudging me, I could see the pleading look in her eyes, and I knew that, without her, I was unlikely to have ever noticed the small benign lump that led to the much deeper cancer being caught. She had saved my life.

I had joined the large pool of dog owners who had been alerted to serious conditions by their pets. And it's not only dogs that do it: there are stories of cats and other animals. John Church, who collects all this anecdotal evidence, even knows of a parrot who alerted his owner that there was something wrong.

The sentinel lymph nodes that were removed from under my arm were clear, which meant the cancer had not spread, and I did not need chemotherapy, which is like winning the breast-cancer lottery. I had a numb arm for some years afterwards, caused by the removal of the nodes, which affected me riding a horse and even walking a dog on a lead, but that is a small price to pay.

I decided against having reconstruction surgery. At the time, my self-esteem was so low it didn’t seem important. Rob [Guest’s new partner] told me immediately that it made no difference to him, which boosted my confidence, because it is a disfiguring operation.

I’m sure many men, if they’d been going out with someone for only a couple of months, would have run for the hills with everything I loaded on to him, but he was very straightforward and has never seen it as an issue.

I call myself “one and a half boobs”, and the only time I’ve been self-conscious about it, and regretted not having the reconstruction, was when I was at a pool party surrounded by women who had clearly had breast enlargements.

After the operation I was referred to Alan Makepeace, an oncologist at Mount Vernon Hospital in Hertfordshire, who organised my radiotherapy.

He admits he was sceptical about my story of Daisy finding my cancer, although he had heard other stories about the ability of animals to pick up on disease. But I talked with such conviction about my work that he agreed to come to see the dogs, and now both he and my surgeon are great supporters of Medical Detection Dogs (MDD).

Like everyone who sees the dogs working, Alan was moved as he watched them unerringly identifying the samples from patients with cancer. But he stresses that it is the solid scientific basis of our work that completely convinced him, the fact that everything is tested robustly.

He says he is immensely impressed by my dedication to scientific methods, and the fact that I am not simply an enthusiast with a collection of anecdotal evidence.

“When I first saw the dogs working I was won over, as most people are.”

So, having cancer brought me two more supporters, and from the medical profession, too. But it was a high price to pay. I found the radiotherapy gruelling.

The routine of going to Mount Vernon five times a week for four weeks really took it out of me, and I found the drug I was supposed to take for five years caused so many side effects that after two years I stopped taking it.

My divorce from Andy came through just as my radiotherapy treatment was finally over, although it did not bring the closure everyone predicted, and I was still very raw and emotionally battered by it all.

I believe the theory that stress can cause or exacerbate cancer. In 2012, research scientists at UCLA [University of California, Los Angeles] published a study that showed toxic relationships can lead to illnesses ranging from high blood pressure to heart disease and cancer.

My body was reacting to my deep depression, and the hassle I was having with the house renovations. It was all too much, and cancer was the outcome.

The Koreans have long accepted that trauma causes physical ill health, and it is now being accepted much more in the Western world.

There are many stories of people – and dogs – dying from a broken heart, and I accept these are true, too. One day there will be a scientific explanation of how unhappiness and trauma can lead to physical illness and death.

Thankfully, when the radiotherapy treatment was over, I recovered pretty well, apart from the side effects of the drug regime.

There was a residual tiredness, but before too long I was back to working long hours, especially as we were now getting into high gear with our diabetes-alert dogs. I had very little social life, filling my diary with speaking engagements and demonstrations, mainly to bring in more funding, but also to stop me thinking about my failed marriage.

Daisy's Gift: The remarkable cancer-detecting dog who saved my life by Claire Guest (Virgin Books, £12.99)