©Carlos Jasso

Dog days: Billie-Jean at The Royal Hospital for Neuro-disability in Putney, south London

As she makes her way through the hospital wards, Billie-Jean keeps up an impressive pace. She has to if she is going to see all the patients who are waiting for her. Wearing her official uniform, she looks neat and trim, and despite how busy she is, she always has time to stop if someone wants to say hello or slip her a Bonio. You see, Billie-Jean isn't a ward sister doing the rounds or a doctor bringing vital medicine, she's an Irish terrier. But despite the fact she's a canine, not human, carer, her medical value is second-to-none because she is a Pets As Therapy dog.

Pets As Therapy is a charity that takes pet dogs and cats to hospitals, hospices, residential care homes, day centres and special-needs schools. It was formed in 1983, explains chief executive Maureen Hennis, by a group of pet owners who were convinced that their animals could help other people. "At that time, people were moving into residential accommodation and nursing homes, and they had to give up their own pets," she says. "This wasn't only making them sad and depressed, sometimes it was actually making them ill."

The importance of regular contact with domestic animals has been highlighted by recent research conducted by the University of Minnesota. According to the study, having a cat around the house can cut the risk of having a heart attack or a stroke by almost half. After studying nearly 4,500 adults aged between 30 and 75 for 10 years, it was found that cat owners had a 40 per cent lower risk of suffering a fatal heart attack.

"For years we have known that psychological stress and anxiety are related to cardiovascular events, particularly heart attacks," says Dr Adnan Qureshi, executive director of the Minnesota Stroke Institute at the University of Minnesota in Minneapolis. According to Qureshi, the research shows that "essentially there is a benefit in relieving those inciting factors from pets". And in a study published last year, Dr Deborah Wells of Queen's University Belfast found that dog owners tend to suffer less from ill health, have lower cholesterol, and lower blood pressure. "It is possible that dogs can directly promote our well-being by buffering us from stress," says Dr Wells.

Today, Billie-Jean, along with her owner, Emma Charlton, is on her weekly visit to the Royal Hospital for Neuro-disability in Putney, south London. The hospital is just one of the thousands of places that PAT animals visit every year. "When we first started, we used to have to look around for establishments for registered PAT dogs to visit. Now we have more than 900 establishments waiting for PAT dogs to visit in," says Hennis. "Currently, we have 3,600 dogs and 92 cats working in the community and they are benefiting more than 100,000 patients every single week in the UK." As Billie-Jean makes her way through the hospital, clad in a bright PAT vest and fresh from the grooming parlour, it's clear that she is a real favourite with the patients.

When Billie-Jean bumps into Scott Robertson in one of the hospital's corridors, his face is wreathed in smiles at the sight of her. Charlton slips him a dog biscuit, which he hands to Billie-Jean as she enthusiastically licks his hands.

Not all of the patients here can articulate their likes and dislikes as clearly, so Charlton always checks with the ward staff who will welcome some time with Billie-Jean. Tina Loughney, Charlton tells me, loves to see her. Loughney usually communicates through facial movements and her expressive hands but today she has a surprise for everyone. Loughney has dogs at home and Emma asks how they are. "All right," she says. Charlton is gobsmacked - and thrilled. "That's the first time I've heard Tina speak." The staff agree. "It's moments like this that make it worthwhile," says Charlton, who has been visiting the hospital with Billie-Jean for the past two years.

For over an hour, Billie-Jean pads through the hospital, shaking paws with and licking anyone who says hello. She is wonderfully behaved - she seems to know instinctively when to be gentle and when she can be a little more boisterous. This is no accident - every PAT animal is assessed to ensure that only the best-behaved pets come into contact with patients. Billie-Jean and her ilk are a hard-working part of the healthcare system. "PAT animals are being used for desensitisation in phobia patients, as part of stroke rehabilitation, helping to get people to use their limbs again and to talk," says Hennis. "Very often people go into a world of their own and they stop communicating with anyone. But if you go in with a dog, it's amazing how a dog can get through barriers that humans can't."

When speaking to a doctor friend about PAT, he waxes lyrical about his experiences of the animals and tells me about a patient on a stroke ward who had not spoken since she arrived in hospital. During an encounter with one of the charity's volunteer canines, she stroked the dog and uttered her first word - "soft".

Hennis, herself a regular visitor to a number of establishments, tells me about how she realised the importance of what the charity was doing. "There was a lady that I visited who used to sit by the door every Saturday morning, and when she saw my dog she always used to say, 'Here's my ray of sunshine, she's my reason for staying alive'." One thing's for sure - it's hard to imagine the average GP getting such a warm welcome.

Animal magic: how pets prevent illness

Dog owners tend to have lower cholesterol and lower blood pressure, as dogs can reduce the risk of spikes in blood pressure due to stress or tension.

In some cases, the emotional support offered by an animal is greater than that offered by a human. For older people in particular, an animal can fulfil "the need to be needed".

Owning a pet can improve a person's chances of survival after a life-threatening illness, by helping to lower blood pressure. Pets can also help speed up rehabilitation following a stroke.

Children with pets have higher levels of self-esteem and function better emotionally than those without, studies have shown. Some teachers have introduced pets into the classroom and children with learning disabilities and behavioural disorders such as autism, for example, show immediate benefits from animal-assisted therapy.

Children who live with a cat or dog in their first years have a lower incidence of hay fever and asthma and are less likely to develop animal-related allergies, or to suffer a bout of gastroenteritis.