Hundreds of volatile organic compounds are emitted from the human body, and the components of these compounds usually reflect the metabolic condition of an individual.

Therefore, contracting an infectious disease often results in a change in body odour. A dog interprets the world predominantly by smell and, while a dog’s brain is only one-tenth the size of a human brain, the part that controls smell is 40 times larger than in humans and they can sense the change in scent diseases can cause. The first publication of a dog detecting cancer was the case of a young female from the UK, who remarked to a dermatologist examining a suspicious mole on her leg, which her dog had been licking, nipping and barking energetically and persistently. There have been several studies now that have concluded that dogs can smell cancer in people, but why would a dog want to smell cancer?

Dogs have lived side by side with man, co-evolving for well over 100,000 years. Man provided shelter and food for the dog, and in turn, the dog warned man against impeding danger, pointed towards prey and food, and eventually guarded his livestock and crops. Man provided for the dog, and the dog used his nose to help provide for, and protect, man. Smelling disease is part of this protection. If something is wrong with us, this will affect the dog. If we are sick, this could directly affect our ability to provide nourishment and shelter for the dog. The dog ‘notices’ small changes that could signify that we are in trouble. For example: if someone has a cut on his or her leg, under the jeans or socks, why does almost every dog find and sniff the cut? They are noticing a change, or something different. This small change can mean that we may become incapacitated in some way, which means the dog may suffer. Most prey that get caught and killed by predators are sick or injured in some way. The slowest, or sickest are easier to catch. Dogs have evolved learning to notice a ‘sick’ scent and this has been positively reinforced by the reward to eating the prey that was sick. Smelling disease helps the dog catch his own prey, as well as larger prey for man, and so the sick scent is very important for a dog’s survival.

Dogs play an astonishing range of roles in human society. Many individuals are saved by rescue dogs when stranded in the wilderness or capsized in cold water. Others rely on guide dogs to get them safely to multiple destinations on a daily basis. Drug dogs, de-mining dogs and police dogs are trained and utilised as substance detectors, even in the face of competition from the latest technology. Now, the role of dogs smelling disease has arrived.

One interesting part about this is that cancer actually has a smell. Most oncologists will tell you that humans can actually smell cancer in latter stages through a patient’s breath. If humans can smell it at stage 3 or 4, then of course a dog would be able to detect the scent much earlier. Dogs can differentiate smells much better than humans and, while a human will smell something like spaghetti sauce as one smell, a dog smells each individual ingredient.

Specialist doctors have diverse reactions to the notion of dogs detecting cancer. While some are extremely enthusiastic, others are more guarded. Dr Miranda Tompkins explains her reservations. “Although the olfactory test appears to be a promising tool for the detection of cancer, the main challenge is to determine whether the test can sufficiently discriminate between patients at risk, patients with benign disease, and patients with malignant disease.

We need to gain a deeper understanding before applying dogs as an assessment tool for cancer in clinical settings.”

Training dogs to smell cancer is done in the same way that bomb and narcotics dogs are trained: pairing the target odour with a high value reward. With breath, however, things can get a little tricky. The odours of drugs and gunpowder can be isolated, but ’cancer scent’ is one of the thousands of organic compounds within a human’s breath. In order for the dogs to generalise the cancer scent, many samples with the common odour must be used. Also, the dogs must be trained to ignore healthy breath, and all other breath with diseases other than cancer. This means that very large numbers of samples have to be used for a dog’s training. Cancer samples, disease controls and healthy controls are needed, and the order and specifics of the introduction of cancer through latter stage training is extremely specific in order for the dog to generalise the cancer scent.

Work toward the development of an ‘electronic nose’ for cancer detection has been underway for several decades. However, nothing has achieved the high sensitivity and specificity seen with dogs. Whether or not sniffer dogs actually make it into the continuum of diagnostic evaluation has yet to be seen, but if their image could be employed in public health screening, it may encourage people with worrisome symptoms to take earlier action. This on its own would be the definitive benefit for some sick people.