Dogs can already sniff out bombs, drugs, and contraband foodstuffs. Now, researchers think they can train them to sniff out colorectal cancer.

Researchers in Japan say a Labrador retriever they trained to sniff out the earliest signs of colorectal cancer was able to detect the cancer with up to 98 per cent accuracy.

In a study published in the journal Gut, the researchers reveal how they first trained the female Lab, named Marine, to detect other forms of cancer, such as bladder, esophageal and lung cancer.

Marine was taught to sniff cups of exhaled breath samples and then to sit down in front of the cup that contained the sample from the patient with cancer. When Marine got it right, she was rewarded with a tennis ball.

For this study, Dr. Hideto Sonoda, from the department of surgery at the Postgraduate School of Medicine at Kyushu University in Fukuoka, collected stool and breath samples from 48 patients with colorectal cancer. They collected similar samples from 200 volunteers with no history of cancer and 55 samples from patients with a previous history of cancer.

The bowel cancer samples came from patients with varying stages of disease, while the healthy patients included those with benign bowel polyps. Benign polyps are not cancerous but are considered to be a precursor of bowel cancer.

Over the course of seven months, the dog learned to distinguish the cancerous samples from noncancerous samples in 33 of 36 breath tests and in 37 of 38 stool tests.

That works out to about 95 per cent accuracy for the breath test, and 98 per cent accuracy for the stool test, compared with conventional colonoscopy.

As for how Marine was able to find the cancer with such high accuracy, the study authors suspect there are specific odours given off by cancer cells that circulate in the body and are emitted in the breath and in the stool.

Interestingly, Marine showed the most accurate detection rates when the samples were taken from people with early stage disease.

Even among volunteers without cancer but with other intestinal diseases, such as ischemic colitis or ulcerative colitis, the dog's accuracy remained high.

The study authors note that one of the key colon cancer screening tools currently used, called a fecal occult blood test or FOBT, picks up early-stage disease in only one out of 10 cases.

Nevertheless, the authors concede that using dogs to screen for cancers is likely to be too expensive and impractical. But they say their study offers hope that a sensor could be developed to detect the specific compounds that the dog responds to.

"Early detection and early treatment are critical for the successful treatment of cancer and are excellent means for reducing both the economic burden and mortality [of bowel cancer]," comment the authors.