With a little training, a dog can learn to heel. But a new study adds to growing evidence that man's best friend can also learn to heal by detecting invisible signs of disease.
Marine, a specially-trained 8-year-old black Labrador retriever, detected colorectal cancer 91 percent of the time when sniffing patients' breath and 97 percent of the time when sniffing stool, according to a study published Jan. 31 in the British journal Gut.
She even beat the fecal occult blood test, the most economic and non-invasive screening for colorectal cancer, which positively predicts the presence of cancer only 10 percent of the time, according to lead author Dr. Hideto Sonoda, from Fukuoka Dental College Hospital in Japan, and colleagues.
This is not the first time dogs have successfully sniffed out malignancies. The phenomenon has already been reported in skin, bladder, lung, breast and ovarian cancers.
"We shouldn't be shocked by this," said Dr. Marty Becker, a veterinarian at the North Idaho Animal Hospital and author of "The Healing Power of Pets." "We know dogs can detect accelerants, explosives, mold, peanuts -- even counterfeit CDs."
It's estimated that a dog's sense of smell is up to 1 million times better than that of a human, depending on the breed.
"It's like having an NFL stadium filled with yellow balls and one red ball," Becker said. "That's the concentration of scent a trained dog could find."
But Becker cautioned that not all dogs are so gifted.
"You don't see bomb-sniffing pugs," Becker said. "You've got to have pretty robust olfactory capabilities."
Dogs have even been trained to smell chemicals signaling seizures in epileptics and elevated blood sugar levels in diabetics, according to Rebecca Johnson, associate professor at the University of Missouri's College of Veterinary Medicine and Sinclair School of Nursing, and director of the Research Center for Human-Animal Interactions.
In August 2010, a terrier named Kiko bit off his owner's big toe while he was passed out drunk, alerting the 48-year-old to his undiagnosed type 2 diabetes.
But dogs use more than just smell to guard humans' health.
"They become very accustomed to our behavior," Johnson said. "They're very intuitive and they detect even the subtlest behavioral changes."
A slight drag of the foot, a change in walking pace or a tremor imperceptible to the human eye can send a message to dogs that something's off.
"These dogs are guarding our physical and emotional health," Becker said, explaining that a nudge on the arm during a stressful session at the computer can be a warning to a take a much-needed break.
For people who have anxiety disorders -- particularly post-traumatic stress disorder -- dogs serve not only as companions, but also perform particular skills that help their owners to live fuller lives, Johnson said.
And nothing motivates a person to get out for a walk like the hopeful eyes of a dog, leash in mouth, tail wagging.
"We showed that people who walked with dogs had 72 percent adherence to a walking regimen," Johnson said, explaining that adherence was better with dogs than with a human walking buddy. "Dogs are always ready to go and always encouraging."
Among older adults, dogs boosted adherence to 100 percent, which led to a 28 percent improvement in walking speed, Johnson said. Walking speed was recently linked to lifespan.
"It improved their balance and their confidence in walking," Johnson said. "It helped them maintain their independence."
As for the prospect of using trained dogs for cancer screening, Johnson said it would be wonderful.
"We're always looking for effective and inexpensive screening devices," Johnson said. "Training dogs is not inexpensive, but the cost to the end user is going to be a lot less than many of the tests we have."
She cautioned that the sniff test reported, or others like it, should not be used in making a diagnosis. Rather, they could be used to supplement current screening systems.
But the study authors hope that future research will tease out the cancer-signaling chemicals that Marine could smell. Then a machine could be programmed to detect the chemical quickly and cheaply, they wrote.
But until a machine can mimic dogs' extraordinary ability to monitor human health, people should pay more attention to their pets and take every opportunity to reward them, Becker said.
"We've always known dogs made us feel good, we just didn't know they were good for us," Becker said. "I think in the coming years we're going to be in awe of just how strong the connection is."