Dr Liesel van der Merwe is a small animal medicine specialist. Send her your questions: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Dr Liesel van der Merwe

Dogs are pets, working and support animals and now also medical detection tools. They have a unique ability to relate to their world and to us through their sense of smell. 

I recently came upon an article on how dogs can be used to detect the onset of a seizure in humans – giving people enough time to find a safe space. Dogs are routinely used to detect explosives, drugs, money and trafficked animals and people. Recently work has been published on using dogs in detecting medical conditions. These dogs’ training is complex and it is not yet completely understood how dogs can do this. It isn’t only dogs; rats are also being used to detect landmines and diagnose tuberculosis in Africa due to their excellent sense of smell.

Diseases dogs have been used to test for include some cancers (prostate, breast and colon), hypoglycaemia in people with diabetes, epilepsy and infectious disease such as helicobacter gastritis and now also Covid 19. Illness and infection change the body odour by alterations in the volatile organic compounds we produce. 

The detection ability limit for volatile organic acids by dogs is in the ranges of parts per million to parts per trillion, depending on the substance. This is the equivalent of a single drop of liquid in 20 Olympic-sized swimming pools. Instruments made to detect odours can only detect 100 – 400 parts per million.

So why are dogs so special? They have a better designed nasal cavity, more smell receptors and a relatively bigger olfactory portion in the brain than humans. They are also attracted to new and interesting smells. To dogs, their senses of smell and hearing are much more important than visual input.

All dogs are not equal when it comes to their sense of smell, although all dogs are about 10 000 times better at it than humans. Belgian Malinois, German Shepherds and Labradors as well as beagles are routinely selected for odour detection work.

Long-nosed breeds are more effective than short nosed breeds. The dog’s nasal cavity has three turbinates, which are intricate curls of cartilage, covered with a membrane called the respiratory epithelium. The nasal epithelium has numerous olfactory receptor cells.

These olfactory cells have cilia, tiny hair-like extensions, on their surface, which have olfactory receptors that are responsible to bind the odour molecules. Nerve impulses collected by these cilia pass through the receptor cell and along axons which collect and form the olfactory nerve. 

In the average German Shepherd the lining of the nasal passages would take up 95–
126 cm², have about  200–300 million olfactory receptor cells (nerves),  each which would have about 20–100 hairs with receptors on them for molecule detection. Molecules are detected by variations in shape, size and orientation, which allow them to “fit” with a specific olfactory receptor. Thus the more receptors and the more different types of receptors, the more smells you will pick up.

Information is no good without interpreting ability and the olfactory area of dogs’ brains is also proportionally 40 times larger than in humans. Dogs’ excellent scent memory is also a vital component to develop this working skill.

The breathed-in air needs to get to the right place in the nose for smelling and here dogs’ noses function quite differently than ours. When we inhale, we smell and breathe through the same airways within our nose. When dogs inhale, a fold of tissue just inside their nostril helps to separate these two functions. Sniffing and breathing cause the air to move differently in the dogs nasal passages. Breathing is more directional, straight through the middle and lower air passage and down towards the back of the throat. Sniffing causes more upward air movement, into the back of the upper turbinates to the scent receptors.

When we exhale through our nose, we send the used air out the same way it came in, forcing out any incoming odours. When dogs exhale, the used air exits through the slits in the sides of their noses. The manner in which the exhaled air swirls out actually helps pull new odours into the dog’s nose. It also allows dogs to sniff more or less continuously.

We can’t move our nostrils independently. Dogs can, which helps them to determine which nostril an odour arrived in, helping them locate the source.

The exact type of training a dog receives is important. Detection dogs aren’t trained to memorise a specific smell for disease detection, as that would be specific to an individual. The dog needs to be trained to recognise a specific odour pattern within other unknown background odour patterns. This is quite a complex process. Patience, repetition and “keeping fit” by constant practice make the dog effective.

So Boelie or Honey is actually a well-designed highly effective piece of equipment, an X-man of scent, and when you next go walking, let them sniff away as much as you can. It’s their way of reading the environment and gaining more information than we could ever imagine.