How Your Dog Knows When You're Sick, Even Before You

Written by Dr. Karen Shaw Becker

can dogs detect sickness

Story at-a-glance -

  • Our dogs know when we’re sick by using their senses — especially their noses — to pick up the tiniest changes in our scent and other signals we give off
  • Your dog may even know you’re coming down with something before you do, thanks to his truly remarkable sense of smell, which is up to 100,000 times sharper than yours
  • The canine sense of smell is helping detect several types of human cancer; dogs are also being trained to detect medical emergencies in humans before they reach the crisis stage
  • Dogs can also sense human moods with their noses, and respond by mirroring their owner’s emotional state

If you’re convinced your doggy BFF knows when you’re feeling under the weather, you’re probably exactly right. Most dogs, whether they show it or not, can sense changes in their humans that indicate something’s not quite right.

For example, when you’re dealing with a cold or sinus issues or a stomach bug or even a toothache, your dog’s super sense of smell will pick up immediately on a change in your scent. Changes in the tone of your voice, your energy level or the sounds you make (e.g., coughing, sneezing, etc.) will not go unnoticed, either.

A friend of mine adopted a little dog a while back and then proceeded to have one annoying health issue after another during the dog’s first few years with her. Peanut is part Chihuahua, and true to his breed is very protective of his human. To this day, if my friend heads to her bed for any reason other than at bedtime — to nap or read or watch TV — little Peanut gets anxious and is right on her heels.

He gets up on the bed by whatever means necessary and stretches out on her chest with his face close to hers to get a read on the situation. After he studies her for a minute or two, he climbs to his “post” on a pillow above her head, and there he stays until she’s up and moving again.

Your Dog’s Nose Knows

Of all the canine senses, it’s your dog’s nose above all else that provides her with an encyclopedic amount of information throughout her life. Inside your own nose are about 6 million olfactory receptors that allow you to recognize thousands of different smells.1 It sounds like a lot, until you realize that inside your dog’s nose there are up to 300 million such receptors.

While you can detect certain odors in parts per billion, you dog can detect them in parts per trillion. Plus, she has a part of her brain devoted to analyzing smells that’s about 40 times larger, proportionally, than the same area in your brain.2 This explains why your dog’s sense of smell is anywhere from 10,000 to 100,000 times more acute than your own, which, let’s face it, is nothing short of amazing. As reported by NOVA:

“… [I]n her book ‘Inside of a Dog,’ Alexandra Horowitz, a dog-cognition researcher at Barnard College, writes that while we might notice if our coffee has had a teaspoon of sugar added to it, a dog could detect a teaspoon of sugar in a million gallons of water, or two Olympic-sized pools worth. Another dog scientist likened their ability to catching a whiff of one rotten apple in two million barrels.”3

Dogs can detect subtle differences in the breath, urine, skin, blood and feces of cancer patients, allowing them to detect certain cancers with up to 97% accuracy. In one study, dogs were able to detect or rule out lung and breast cancer, at all stages of the disease with about 90% accuracy, just by sniffing breath samples.

Even more remarkable, the dogs used for the study were “ordinary household dogs” who received only three weeks of training beforehand. In another study, a black lab named Marine was able to detect colon cancer with 25% greater accuracy than the routinely used fecal occult blood test.4

In addition, research indicates that dogs are able to detect prostate-cancer-specific volatile organic compounds (VOCs) in the urine of prostate cancer patients with 98% accuracy.5 Medical detection dogs and medical alert assistance dogs are also being trained to detect medical crises before they happen, alerting patients so they can seek help for changes in blood sugar levels, seizures, severe allergic responses, sleep walking and narcolepsy.

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Your Dog Can Even Sniff Out Changes in Your Mood

A 2017 study revealed that dogs also use their talented noses to sense our emotions.6 And once your canine companion sniffs out your mood, he adjusts his own accordingly.

It has been scientifically proven that dogs can see and hear human emotions, but until this study, no one knew for certain they also use their incredible sense of smell to inform them about how their humans are feeling. It actually makes perfect sense when you consider that unlike people who typically respond most often to what they see, dogs lead with their noses and respond most often to what they smell.

For the study, a team of researchers set out to answer the question, "Do human body odors (chemosignals) produced under emotional conditions of happiness and fear provide information that is detectable by pet dogs (Labrador and golden retrievers)?"

Eight human volunteers watched a 25-minute video designed to provoke emotional states of either fear or happiness. The volunteers' sweat was collected on pads as they watched the video, and then the samples were pooled to obtain composite "fear sweat" and "happiness sweat" samples. There was also an unscented control sample.

Next, 40 Labs and Goldens were fitted with heart rate monitors and each dog was placed in a small room with his owner and a stranger who had not provided a sweat sample. The two people were seated, reading magazines and not purposely interacting with the dog. The samples (either fear or happy sweat, or no scent) were diffused into the room from an open vial containing the sweat pads. The dogs were able to sniff the vial itself but couldn’t directly touch the pads.

Behind the scenes, for five-minute periods the researchers evaluated the dogs' heart rate, body language, movements toward and away from the owner and the stranger, and stress-related behaviors. To goal was to learn whether the dogs would show a consistent set of behaviors in response to the three conditions.

Study Says: The Smell of Human Fear Stresses Dogs Out and Makes Them Fearful as Well

The dogs exposed to the happy sweat sample had fewer and shorter interactions with their owners, and more interactions with the strangers in the room. This indicates they felt relaxed enough to check out strangers and didn't need to seek reassurance from their owners.

The dogs exposed to the fear sweat sample displayed more frequent and longer-lasting stress-related behaviors, in some cases, for the entire five-minute period. These dogs also sought out their owners rather than the strangers, indicating they were looking for reassurance because they felt stressed.

The dogs exposed to the fear sweat sample also had consistently higher heart rates than the dogs exposed to the happy sweat sample and the control sample.

"While the dogs were clearly responding emotionally to the scent of fear," writes dog expert Stanley Coren, Ph.D., "it seemed as though their response mirrored the emotion that they were detecting in that they were acting in a fearful manner themselves. There was no evidence of aggression toward either the owner, the stranger, or the scent dispensing apparatus."7

Researchers are well aware of the role stress plays in canine disease, so perhaps something we all should ask ourselves is how long-term exposure to human stress and emotional imbalances in the home (fear, anger, frustration, etc.) impact the health and happiness of our dogs without our knowledge.