Dogs Can Smell Cancer

It is well known that dogs can be trained to sniff out drugs, explosives, and lost, injured or deceased humans. Dogs can also detect cancer in people. It is a well-known fact that a dog's sense of smell is far superior to ours.

Dog and human noses have scent receptor cells that recognize odor molecules by their shape and send nerve signals to the brain to identify the smell. There are about 300 million scent receptors in a dog's nose that allow him to smell in parts per trillion. For example, a dog can detect one cc of blood (that's about a drop), diluted in the water of 20 Olympic sized swimming pools. Compare a dog's sense of smell to yours. We have about 5 million scent receptors that can smell a spritz of perfume in a room. A dog can detect the same whiff of perfume in an enclosed stadium and distinguish the ingredients.

With a nose like that, is it surprising that dogs are being taught to sniff out cancer? Dogs are trained to smell cancer in the same way they are taught to detect bombs or narcotics. Detecting cancer is a bit trickier. The cancer scent is one of thousands in a human's breath. The dog must be exposed to the breath from patients with cancer, patients with other diseases, and healthy individuals.

Pine Street Foundation's Michael McCulloch and coworkers recently used a food-based reward system to train five ordinary household dogs with only basic behavioral puppy training skills to sniff for cancer. The dogs were taught to identify breath samples of lung and breast cancer patients and healthy individuals. It took only a matter of weeks for the dogs to learn to distinguish by scent the breath samples of 55 lung and 31 breast cancer patients from those of 83 healthy people. Sitting or lying in front of a cancer sample got the dog a food reward. Ignoring a non-cancer sample was a correct response.

Once trained, the dogs' sniff detection was tested using breath samples from subjects that had not been present during their training. The dogs identified the breath of lung and breast cancer patients and healthy people with a high degree of accuracy. Their results were confirmed by conventional biopsy diagnoses.

The Pine Street Foundation is not the only group that trained and tested dogs to sniff cancer. Ted Gansler, MD, MBA, director of medical content for the American Cancer Society noted that research has shown that malignant tissues release chemicals that are different from normal tissues. A study published in 2011 in the journal Gut reported that a Labrador retriever trained in cancer scent detection correctly identified 91% of breath samples and 97% of stool samples from patients with colon cancer. Gansler pointed out that it's not surprising that dogs can quite accurately recognize the chemicals contained in cancer samples.

Lucy, a cross between a Labrador retriever and an Irish water spaniel was basically kicked out of guide dog school, because she was easily distracted by random scents. She went on to be trained to sniff out bladder, kidney and prostate cancer with great accuracy. Over the years, she has been able to detect cancer correctly more than 95% of the time. That's better than some lab tests used to diagnose cancer.

Dogs not only identify patients with advanced stages of cancer, they can detect cancer in very early stages. They have even identified patients who are at Stage 0 cancer. There are no clinical tests that have accomplished that. More research and testing is required, but there is hope that dogs may play an important future role in predicting health problems including cancer.

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