It’s hard to imagine how dogs experience the world, because so much of it is accessed through their snouts. Dogs live in an olfactory world, full of smells that tell complex stories. They detect odors direct from the source as well as residual odors that persist in an area long after the source has left.
Dogs are born sniffers. Humans have roughly five million olfactory cells in their noses. It sounds impressive, until you compare it to the 200 million cells in a typical dog’s nose. Canines’ sense of smell is generally 10,000-100,000 times superior to that of humans. Much more of their brains are devoted to processing smell, and they also possess more genes that code for olfactory ability and many more olfactory neurons than humans.
People have known about and taken advantage of dogs’ sense of smell for centuries, even breeding some dogs to be scent hounds used in tracking and hunting. In recent years, dogs have been trained to sniff out explosives, drugs, bodies and other scents.
The first scientific test of canine cancer-detecting, to my knowledge, was in 2004. James C. Walker, of the Sensory Research Institute at Florida State University, and colleagues trained two dogs to detect melanoma tissue samples hidden on the skin of healthy volunteers. The dogs were trained and tested with methods normally used for forensic bomb- or drug-sniffing dogs.
One dog “confirmed” the presence of melanoma on five patients, and even detected cancer in a sample that was initially deemed negative, but subsequent histopathological examination revealed to contain melanoma in a fraction of the cells.
A 2006 study by the Pine Street Foundation, a cancer research organization in San Anselmo, Calif., used more dogs and samples for even more robust results. The researchers selected three Labrador retrievers and two Portuguese water dogs with no prior training. Lung and breast cancer patients breathed into tubes which captured samples of their breath. The dogs then underwent several weeks of training with the samples. For testing, the researchers used a new batch of breath samples. The dogs correctly detected 99 percent of the lung cancer samples, and made a mistake with only 1 percent of the healthy controls.
With breast cancer, the dogs identified positive samples 88 percent of the time with no false positives. The dogs performed as well as the most recent screening tests for the diseases. It is important to note that all the tests were double-blind, meaning neither the dog handlers nor the experimenters knew which samples were which. By the scent of breath samples alone, the dogs identified 55 lung and 31 breast cancer patients as well as 83 healthy people.
Scientists again trained dogs to sniff out lung cancer in a more recent study published in 2011. A group of German researchers wanted to know if dogs could discriminate between breath samples from lung cancer patients, chronic obstructive pulmonary disease (COPD) patients, and healthy volunteers, and whether the presence of tobacco in the samples made a difference. The dogs correctly identified 71 samples with lung cancer out of 100. They also successfully detected 372 samples that did not have lung cancer out of 400. It seemed the dogs were able to detect lung cancer independently from COPD and tobacco smoke.
A 2004 study in which dogs were trained to detect bladder cancer in humans by smelling their urine had a smaller success rate, but is notable for an unexpected result. Carolyn M. Willis of Amersham Hospital in Great Britain and colleagues trained six dogs. One dog failed completely, but two picked out the positive samples 60 percent of the time.
The surprise came when one of the non-cancerous control samples caught the interest of the dogs.
The medical staff assured the disappointed trainers that the sample was from a healthy person, but because the dogs consistently identified this sample as “positive,” it was sent back to the hospital for further tests. On re-examination the person was found to have cancer on his kidney and bladder cancer. The dogs caught it before anyone else.
In a 2011 study from Japan, a Labrador retriever trained to sniff out colorectal cancer was at least 95 percent as accurate as a colonoscopy when smelling breath samples and 98 percent correct with stool samples. The dog was especially effective at detecting early-stage cancer and could also discern polyps from malignancies, which a colonoscopy cannot do.
Studies like these are fascinating for what they tell us about dogs’ keen sense of smell, but medical professionals also see practical and technological implications. Dogs’ noses are inspiring a race between scientists to create an artificial sniffer with similar acuity for quick and easy use in hospital laboratories — this involves precisely identifying the compounds dogs are picking up on in the samples from cancer patients.
Early detection is paramount in many cancer treatments. For some diseases, like prostate cancer, the blood tests currently used are notoriously inaccurate. Could man’s best friend become a tool in early screening? Whether actual dogs will be making future diagnoses is uncertain, but it is clear they possess a pretty powerful tool we are only beginning to understand and appreciate.
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