September 24, 2021

I, Science

The science magazine of Imperial College

Dogs are already used to detect diabetes sufferers and illegal drugs but their acme noses might also be able to sniff out cancer ...


As far as Claire Guest was concerned, Tuesday was set to be a normal day. It was only when she took her pet Labrador, Daisy, for a walk that everything changed. Daisy suddenly went mad, jumping up at Claire, pawing at her face and body. The dog didn’t normally act like this – so what was going on? Claire pushed her down and walked away, brushing off the muddy paw prints.

Then she felt a painful lump in her chest.

Three weeks later, Claire was diagnosed with breast cancer. Luckily, it was caught at an early stage and was treatable. Remarkably, Claire thinks it was Daisy who first detected her tumour and, in catching the disease so early, saved her life.

This is not as unbelievable as it sounds, because Daisy is a trained cancer detector. The theory, first proposed by surgeon Dr John Church, is that malignant cells produce a distinctive scent, which dogs pick up. Claire, herself a trained psychologist, heard Dr Church on the radio saying dogs could smell cancer and was hit with an idea: to combine her experience training dogs with Dr Church’s theory, and produce a canine group with noses tuned to cancer. Thus, Medical Detection Dogs (MDD) was born.

Using the principal of operant conditioning, where the target odour is paired with a high value reward, the team trained dogs to recognise the scent of cancerous cells. By 2008, MDD was a registered charity and, along with other research teams, had produced a flurry of scientific papers proving that dogs really can smell cancer. For instance, a paper published in 2006 in Integrative Cancer Therapies showed that dogs could differentiate between exhaled breath from healthy controls and lung-cancer patients with 99% specificity. Other studies have shown that dogs are capable of detecting the ‘odour signatures’ of prostate, breast, bladder and ovarian cancers in breath, blood, urine, body and sweat samples.

So what makes dogs such super sniffers? The process of smelling, known as ‘olfaction’, begins when air is drawn into the nasal cavity. This brings air-borne odorant molecules into contact with olfactory receptor neurons (ORNs) that detect specific odorants. If an ORN comes into contact with its corresponding odorant, it sends an impulse along a nerve to the olfactory bulb, the region of the brain dedicated to unravelling ‘smell information’. This process is largely the same in most mammals, but what sets dogs apart from humans is the sheer density of ORNs. In fact, dogs have 60 times more than humans and their olfactory bulb is thought to be proportionately 40 times larger. This means that dogs can detect an extraordinarily low density of odorant molecules – some scientists suggest 1 part per trillion. That is like being able to smell one drop of blood in 20 Olympic swimming pools worth of water.

The way dogs inhale and exhale is also optimised to smell. A small flap of flesh in the nasal cavity directs a portion of inhaled air directly to a special olfactory cavity, which is dense with receptors. This recessed area has a series of turbinates which funnel and filter the odorant molecules, depending on their chemical properties. The process differs from humans as the air we use to smell and breathe is mixed in the central nasal cavity and this decreases smell efficiency. Once all the odorant information has been extracted, the dog exhales. This exhaled air is pushed out via small slits along the side of the nostrils; this produces a sort of suction, which encourages new air, carrying new scents, into the nose. Each nostril can also wiggle independently, another feature to facilitate optimal air flow. Combined, these adaptations mean that our canine friends have a far greater sensitivity to odorant molecules than our own substandard noses. It makes sense then, that dogs can sniff out volatile substances produced only by cancer cells, whilst humans remain unaware.

Cancer detecting dogs, however, are not without their critics. The results across studies have been fairly heterogeneous and vary according to factors such as the type of cancer, the medium of detectable substance and even the breed of dog. As such, cancer detection dogs have not yet made it into clinical trials. It is important to remember, however, that none of the currently available diagnostic tests for cancer can offer 100% reliability. For example, the PSA blood-test used to detect prostate cancer has a 75% false-positive rate. With this in mind, it seems advisable to expand and improve the range of diagnostic tools available to us. A review by Bijland and colleagues, published in 2013 showed that across all studies, the use of sniffer dogs shows significant promise as an effective diagnostic test and agrees that it would merit further funding. Another exciting possibility is the design of electronic noses, based on canine physiology, to smell cancer.

Hijacking the highly evolved sensory system of canines is not a new idea. Dogs are already in service as diabetes sniffers, drug detectors and even bumble-bee sensors (used to investigate the decline of the British bee). Now, with malignant cancer on the rise and no cure in sight, we must turn our attention to improving early detection methods. And for that, our faithful friends may just be able to help.


IMAGE: Mother Nature Network