A Cold Wet Nose

By Andrea DeBrish

It’s a great feeling, you come home from work and there at the door waiting for you is your dog. They jump around and push their cold nose against your hand, vying for affection. For dogs their nose is of great significance. As a species they rely heavily on smell for identification and socializing. They can detect scents far beyond what we can.

How do they do it?

At first glance from the outside a dogs nose does not seem that different from ours. They have two nostrils and a nasal cavity. Their nasal cavity extends farther than ours though and has turbinate bones on the upper side increasing surface area for olfactory epithelium. (Olfactory= relating to the sense of smell, epithelium=a membranous tissue). Olfactory receptor cells are spread throughout this epithelium. Of these receptors dogs have approximately 300 million in comparison to our 6 million (Olfactory receptor=receive an odorant). These olfactory receptors are on the surface of olfactory neurons which initiate the movement of the signal to the brain, specifically the olfactory lobe. (Dogs have a much larger olfactory bulb than humans). The cold wet nature of a dogs nose helps to dissolve different molecules in the air, bringing them to the specialized olfactory epithelium.

Sniffing in dogs is the action of quick short inhalations and exhalations in series. The upper surface of the dogs nose creates a pocket,under the bony subethmoidal shelf (just a bony shelf) where odor molecules will accumulate between sniffs to increase the intensity and the likelhihood of detection. They also split the flow of air as it comes in between the lungs and the olfactory sensors. A majority of the air passes down towards their lungs for respiration but about 12 percent is kept behind in the pocket. Here it is filtered through bony structures called turbinates which separate out the odor molecules based on chemical properties. Olfactory tissues lining the turbinates will eventually bind the odorants and pass on the signal.

As air exits the nose of a dog it travels via flaps to the side of the nose to increase the rate of smelling for new scents by ushering in the new scents. These improvements over our own nasal sensory system have increased their detection rate to around 100,000 times greater than our own. An analogy for their greatly increased sense of smell-If a person can see around three miles on a clear day, a dog would be able to see 3000 miles if their eye sight were better than ours at the same relative rate for which their sense of smell is better.

Dogs have a second area for sensing smells, the vomeronasal organ which also has olfactory epithelium. It is still unclear whether the vomeronasal organ is responsible solely for pheromone detection in dogs. This organ contains two fluid filled sacs and also sends it impulses to the hypothalamus, a region associated with sexual and social behaviours. 

The receptor neurons of the nasal cavity and the vomeronasal are different from one another. In the nasal cavity, each receptor neuron ends with a dendrite with several thin mucus covered cilia (hair like projections). The receptors for the vomeronasal organ usually do not have cilia but do have microvilli (microscopic cellular membrane protrusions). 

So Where did your dog get this great sense of smell?

The ancestor of your pet dog is the wolf. In the wild wolves follow their noses when on the trail of their prey until it is within sight. They will then switch to sight to finish the chase.

There is some research currently suggesting the acuity of a dogs sense of smell may come from their olfactory receptors, and not simply because they have more of them. The olfactory receptors are members of the G protein-coupled receptor superfamily. This means they have 7 transmembrane domains (where the protein crosses the membrane). At these loops there is significant polymorphism (varied within the population). This is believed to be related to how frequently an odorant and receptor bind.

Ok, so your dog has a great sense of smell. Why is that important?

Dogs’ sense of smell has been used by people for a long time. They are used to recover people from disaster sites, track crime suspects, detect drugs and bombs in public areas and more recently to aid in detecting illness in humans. There have been multiple studies looking at the rate of success for dogs sniffing out cancer in people. They have tested dogs smelling breath, urine and blood samples.

 They may also be able to alert a person right before they have a seizure or if they are hypoglycemic.

Dogs can be trained relatively quickly (2-3 weeks) to detect cancerous breath samples from non cancerous for breast and lung cancer patients. In that particular study they found canines were accurate for both types. Another study found there was no greater likelihood of the dog detecting cancer than the dog just randomly guessing each time and eventually getting it right. A third study found dogs were able to detect colon cancer from urine samples at a significant rate. There is not yet a definite consensus on the effectiveness of dogs sniffing out different types of cancers.

If dogs are sniffing out cancer it is likely due to patterns of biochemical markers which are exhaled by people with cancer. Their sensitivity has been confirmed at least in part by using gas chromatography/mass spectroscopy to identify some of the volatile chemicals exhaled by cancer patients. (These are chemicals from the cancer, not from cancer treatment) This could allow us to detect cancer a lot sooner and increase chances of survival, finding it in the early stages.

 Even if dogs do not prove reliable, the structure of the dog nose can help to teach us a lot about smelling things. There are engineers working to build machines which can sniff with the accuracy of a dog. These are especially important for dangerous scouting missions including looking for landmines. An e-nose could also have applications in diagnosis for more than just cancer.

So even if you aren’t a person who likes to go home and be greeted by that cold wet nose everyday, that nose is working overtime for our safety as well as for some treats. 

References

 Cornu, J.N., G.C. Tassin, V. Ondet, C. Girardet, and O. Cussenot. 2010. Olfactory detection of prostate cancer by dogs sniffing urine: a step forward in early diagnosis. European Urology 59:197-201.

Correa, J.E. 2011. The dogs sense of smell. Alabama Cooperative Extension System. http://www.aces.edu/pubs/docs/U/UNP-0066/UNP-0066.pdf

Derr, M. 2001. With training, a dog’s nose almost always knows. New York Times. http://www-psych.stanford.edu/~bigopp/Behaviorism.html

Gordon, R.T., C.B. Schatz, L.J. Myers, M. Kosty, C. Gonczy, J. Kroener, M. Tran, P. Kurtzhals, S. Heath, J.A. Koziol, N. Arthur, M. Gabriel, J. Hemping, G. Hemping, S. Nesbitt, L. Tucker-Clark and J. Zaayer. 2008. The use of canines in the detection of human cancers. The Journal of Alternative and Complementary Medicine 14:61-67.

Lesniak, A., M. Walczak, T. Jezierski, M. Sacharczuk, M. Gawkowski and K. Jaszczak. 2008. Canine olfactory receptor gene polymorphism and its relation to odor detection performance by sniffer dogs. Journal of Heredity. 99(5):518-527.

McCulloch, M., T. Jezierski, M. Broffman, A. Hubbard, K. Turner, and T. Janecki. 2006. Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers. Integrative Cancer Therapies 5(1):30-39.

Tyson, P. 2012. Dogs’ Dazzling Sense of Smell. NOVA scienceNOW. http://www.pbs.org/wgbh/nova/nature/dogs-sense-of-smell.html