Dogs are famous for their sense of smell. With about 220 million scent receptors (compared to our 5 million), dogs can smell things that seem unfathomable to us. They can detect some odors in parts per trillion, and they can detect countless subtleties in scents.

As PBS points out, "Experts have reported incredible true stories about the acuteness of dogs' sense of smell. There's the drug-sniffing dog that 'found' a plastic container packed with 35 pounds of marijuana submerged in gasoline within a gas tank. There's the black lab stray from the streets of Seattle that can detect floating orca scat from up to a mile away across the choppy waters of Puget Sound."

And yes, there are the dogs who have sniffed out medical issues that even doctors weren't aware of. Dogs can pick up on tiny changes in the human body, from a tiny shift in our hormones to the release of volatile organic compounds, or VOCs, released by cancer cells. Researchers and dog trainers are just beginning to understand how dogs do this and how we might put them to work in being our helpers in health care. Here are six medical conditions that dogs are able to smell.

woman hugging dogDog are naturally tuned into their owner's emotions, but what about signs of ill health? (Photo: sherwood/Shutterstock)

Cancer

Perhaps the condition dogs are currently most famous for detecting is cancer. Dogs have been able to sniff out a variety of types including skin cancer, breast cancer and bladder cancer.

There are quite a few stories of a pet dog obsessing about an owner's mole or some part of their body, only to discover in a doctor's appointment that the dog was actually sensing cancer. For example, Canada Free Press writes of a 1989 instance when a woman's "dog kept sniffing at a mole on her thigh, but ignored other moles. In fact, the dog had actually tried to bite off the mole when she was wearing shorts. The woman consulted her doctor, the mole was excised and the diagnosis confirmed a malignant melanoma."

In the last couple decades, researchers have looked seriously into dogs' sniffing abilities when it comes to cancers. In studies, dogs have successfully been trained to detect the disease using samples from known cancer patients and people without cancer.

In a 2006 study, five dogs were trained to detect cancer based on breath samples. Once trained, the dogs were able to detect breast cancer with 88 percent accuracy, and lung cancer with 99 percent accuracy. They could do this across all four stages of the diseases.

Sometimes the dogs can do an even better job than the humans in these studies. According to Penn State News, Nancy Dreschel, a veterinarian in Dairy and Animal Science at Penn State University, tells of an illuminating example: "A scientist was training dogs to detect bladder cancer in humans by smelling their urine. She said a dog alerted them to a sample from a supposedly healthy person who was being used as a control. On reexamination that person was found to have bladder cancer, so the dog caught it before anyone else did."

Study after study has shown that dogs can detect cancer in people, but it may a while before your doctor employs a hound for your annual checkup. Researchers still don't know exactly what chemical compounds for different types of cancers the dogs are sensing in these samples to alert to the presence of the disease, and this remains a hurdle both for better training of cancer-sniffing dogs and for creating machines that can more accurately detect cancer in the early stages.

Narcolepsy

Narcolepsy is a brain disorder that affects the ability to control sleep-wake cycles. This can mean a person suddenly falls asleep, even in the middle of a task. It's a dangerous condition, as someone who has an attack could be injured falling to the ground or could have a car accident if it happens while driving.

Mary McNeight, Service Dog Academy director of training and behavior, has been working with narcolepsy service dogs since 2010, and she notes that there's a scent the dogs pick up on when an attack is coming on. “It’s a biochemical change in the body. We do not know what the particular odor smells like due to the difference between human scent perception and dog scent perception,” she tells Sleep Review.

In a study published in 2013, Luis Dominguez-Ortega, M.D., Ph.D., found that two trained dogs detected 11 of 12 narcolepsy patients using sweat samples, demonstrating that dogs can detect a distinct scent for the disorder.

Service dogs help people with narcolepsy by performing several different types of tasks. They can stand over the person's lap when an attack comes on, which prevents them from sliding out of a chair onto the floor; they can also stand over the person to protect them if they are out in public, or they can go get help. And most importantly, they can provide a warning up to 5 minutes before an attack comes on, giving their handler a chance to get to a safe place or a safe position.

While large dogs can be helpful in giving a narcoleptic sufferer extra support in balance and mobility after an attack, these dogs don't have to be big. According to Petful, even medium-sized dogs can do the job. "Theo, a 2-year-old cocker spaniel, has virtually put an end to [Kelly] Sears’ suffering. He can sense when she is about to have a narcoleptic episode and warn her so that she can sit or lie down, waking her after a few seconds with a kiss on her chin. Since arriving two months ago from Medical Detection Dogs, the only organization in Europe that trains narcolepsy service dogs, Theo has proven himself indispensable."

Dogs can smell all sorts of chemical changes in our bodies, including a drop in blood sugar or even the onset of a migraine. Dogs can smell all sorts of chemical changes in our bodies, including a drop in blood sugar or the onset of a migraine. (Photo: WilleeCole Photography/Shutterstock)

Migraines

For those who suffer migraines, having a warning before one comes on can mean the difference between managing the problem or succumbing to hours or days of intense pain. Fortunately, some dogs have a talent for sniffing out the signs that a migraine is on the way.

Psychology Today reports on a recent study that asked migraine sufferers with dogs if they noticed a change in their dogs' behavior before or during a migraine. The results show that "54 percent of the 1027 participants indicated they had noticed changes in the behavior of their pets during or preceding migraines. Nearly 60 percent of these subjects indicated that their dog had alerted them to the onset of a headache — usually an hour or two in advance." The results are fascinating, though it's important to point out that the study was conducted with self-reports rather than observation by researchers. Even so, the study shows evidence that many dogs seem to detect and point out a change in their human companion's health.

According to Kendall Winship, a migraine sufferer with a service dog, "These [migraine alert service] dogs are highly valued because the ability to tell when a migraine is approaching is an innate talent; it can’t be taught. Similar to diabetic alert dogs that can smell when their handler has low blood sugar, migraine alert dogs can hone in on the scent of serotonin, a chemical that skyrockets when the body is about to have a migraine. By alerting to the danger long before their handlers might feel any symptoms, these dogs can warn them to take preventative medication. When Rally looks up at me and whines, I know I have about two hours before the migraine will strike, and if I can take my medication early enough, I might be able to avoid the stroke-like symptoms and incapacitating pain."

Low blood sugar

Increasingly, dogs are helping diabetics know when their blood sugar level is dropping or spiking. Dogs4Diabetics is one organization that trains and places service dogs with insulin-dependent diabetics. These dogs undergo extensive training to be able to detect and alert their handlers to changes in blood sugar levels.

Medical News Today reports, "Explaining how dogs can be trained to sniff out hypoglycemia in diabetic patients, [Ralph Hendrix, executive director of Dogs4Diabetics] said the dogs are trained on a scent collected from a diabetic's breath or sweat when they are experiencing hypoglycemia."

The dogs are trained to discriminate between the scent from a previous episode, which may linger on their clothes or in their home, and the scent of a current episode. They detect when their handler is having an issue and alert the person, giving them time to test their blood sugar and take the insulin they need.

A 2013 study published in PLOS One showed that having a diabetic alert dog seems to provide significant improvements in both the safety and quality of life of insulin-dependent diabetics. "Since obtaining their dog, all 17 clients studied reported positive effects including reduced paramedic call outs, decreased unconscious episodes and improved independence."

There's still some skepticism about whether or not dogs can accurately alert handlers to a blood sugar change at a level beyond chance, something that can be determined with more studies. Even so, for those diabetics living with alert dogs, the sniffing ability of their companions seems to be a big help.

Seizure

One of the more controversial areas where dogs are used to alert to a medical condition is with seizures. There is growing evidence that dogs can and do detect the onset of a seizure; however, the level of accuracy and, most importantly, our ability to train dogs to alert a handler to an oncoming seizure remains a bit questionable.

As is the case with some other conditions, dogs cannot be trained to predict seizures. We don't have a way to provide them with a scent or information that can be used for training. We can, however, train dogs how to respond to and assist a handler when a seizure occurs. Some service dogs that are placed with seizure patients do develop the ability to detect when a seizure is coming and can provide an alert if the handler pays close attention to the signals the dog provides. WebMD reports, "Jennifer Arnold, founder of Canine Assistants in Georgia, ... says about nine out of 10 of the service dogs her organization has placed develop the ability on their own within a year of placement."

However, we have to be careful about the specifics. How Stuff Works points out that in 2007, "two small studies in the journal Neurology reported that four out of seven seizure alert dogs studied turned out to be warning their masters of psychological, rather than epileptic seizures. This may not seem like a big deal, but the two disorders are different. A 2006 study revealed that up to 30 percent of patients who suffer from psychogenic nonepileptic seizures (PNES) are misdiagnosed with epilepsy. PNES, which results from emotional difficulties and can often be successfully treated with counseling, rather than unnecessary and harsh epilepsy drugs. The 2007 study also revealed one instance of seizures being triggered by the patient's dog's warning behaviors, indicating another flaw in the reliability of canine seizure prediction."

A 2003 study concluded that "findings suggest some dogs have innate ability to alert and/or respond to seizures" however, it notes that further research is needed to uncover which seizure patients would benefit from the help of a dog. We also need more research to learn how to train dogs to be as effective as possible. We have to first learn how dogs know when a seizure is coming — are they smelling a change in body chemistry, or are they picking up on behavior changes? — before we can reliably train them to alert handlers before a seizure happens.

Dogs can smell fear and stress, and can use that ability to help people with issues such as PTSD. Dogs can smell fear and stress, and they can use that ability to help people with issues such as PTSD. (Photo: Glynnis Jones/Shutterstock)

Fear and stress

The age-old notion that dogs can smell fear is an accurate one. Dogs can smell when we are feeling fear or are experiencing an increased level of stress, even if we aren't showing outward signs. What dogs are smelling is the surge of hormones our bodies release to respond to stressful situations, including adrenalin and cortisol.

Thankfully, this can be used to humans' benefit, as dogs can signal a handler that they (or someone else) needs to take a few deep breaths. Dogs that alert handlers of the change in their emotional state — a change that often people aren't even aware they're experiencing — can help prevent panic attacks and other possible episodes associated with post-traumatic stress disorder or other issues.

Cali is one such dog. DogTime writes:

"This 18-month Rhodesian ridgeback is the first cortisol-sniffing dog on staff at a school. Cortisol is known as the 'stress hormone,' and people on the autism spectrum tend to have higher levels of cortisol in their blood. Cali is employed to detect rising cortisol levels in the students. She waits outside of school each morning as the students file by. If she notices anyone with a high level of cortisol in their blood, she will signal to her handler, Casey Butler, a health teacher on staff at The Calais School who is a certified specialist in natural canine behavior rehabilitation and in animal adaptive therapy. When Cali stares at a child, Butler knows that is the signal. She then takes that child aside and works with him or her before a meltdown occurs."

We still have a long way to go to discover exactly what dogs are smelling about us, let alone how we can train them to be as accurate as possible about a change in our bodies. Even though many details are not yet known, it's clear that dogs have an uncanny ability to sniff out certain medical issues, and that's a skill that could be a real lifesaver.

Jaymi Heimbuch ( @jaymiheimbuch ) focuses on wildlife conservation and animal news from her home base in San Francisco.