How Dogs Smell Cancer

Cancer sniffing dogs aren’t as well-known as drug detection dogs, but dogs can smell cancer in humans – both dead and alive (the humans, not the dogs). Here’s how dogs smell cancer, plus a few things you need to know about cancer sniffing dogs.

“A dog can detect scents such as the smell of bladder cancer in urine or a corpse buried deep in the group that even the most advanced scientific instrument can’t detect,” writes veterinarian Bruce Fogle in Dog: The Definitive Guide for Dog Owners.

Dr Fogle says some breeds are better sniffers than others. For instance, Beagles take less than a minute to find a mouse in a field. Fox Terriers took 15 minutes, and Scottish Terriers were still looking when the researchers lasted checked :-)

“Some breeds, such as BloodHounds, are better at detecting and following ground scent,” writes Dr Fogle. “Others, like Border Collies, are superior at following air scent. Medical conditions such as epilepsy, diabetes, and upper respiratory tract infections interfere with scenting ability.”

How Cancer Sniffing Dogs Work

Cancer sniffing dogs use a “sniff-sniff-sniff behavior” to maximize detection of odours. Repeated sniffing concentrates odour molecules in a nasal pocket that is not washed out by normal breathing. So, the smell of different types of cancer stays in dog’s nose for some time. According to Fogle in Dog, the odour molecules build up until there are enough of them for the dog’s brain to work on and recognize.

Dogs aren’t born to sniff cancer – they’re trained. The smell of cancer gets into a dog’s nose, and triggers a response of scent memories that is built up over time. So, to “create” a cancer sniffing dog, a trainer or scientist would need to teach him what specific types of cancer smell like. Then the scientist would teach the dog how to communicate the presence of that type of cancer. This is a very basic explanation of how to train dogs to smell cancer.

can dogs smell cancer

I’m Not a Cancer Sniffing Dog – I’m Just Cute!

A cancer sniffing dog can detect cancerous cells from urine or breath samples. “What I envisaged were dogs in satellite units, across the country,” says Dr Amanda Claire, an animal behavioural psychologist in Daisy, the dog who’s sniffed out over 500 cases of cancer. “They don’t need direct contact with individuals, because samples of breath and urine can be brought to the dogs to record their reaction.”

Following is a summary of two different research studies that assessed the ability of cancer sniffing dogs to detect two different types of cancer (lung and bladder).

How Dogs Smell Lung Cancer

Lung cancer is the most common cause of death from cancer worldwide. Cancer sniffing dogs could be used for the early detection of this type of cancer, according to research published in the European Respiratory Journal. The study was done by scientists from Schillerhoehe Hospital in Germany, and is the first to find that sniffer dogs can reliably detect lung cancer.

Lung cancer is not strongly associated with any symptoms. Finding lung cancer early is often by chance. Current methods of detecting lung cancer are unreliable, and scientists have been working on using exhaled breath specimens from lung cancer patients for future screening tests.

Using sniffer dogs to detect lung cancer relies on identifying (VOCs) that are linked to the presence of cancer. This method of detecting cancer is difficult to apply in a clinical setting for three reasons: 1) patients aren’t allowed to smoke or eat before the test; 2) sample analyses can take a long time; and 3) there is also a high risk of interference. Because of these reasons, no lung cancer-specific VOCs have yet been identified. Enter the cancer sniffing dogs!

This study assessed whether cancer sniffing dogs could be used to identify a volatile organic compounds (VOC) in the breath of patients with lung cancer. The researchers worked with 220 volunteers, including lung cancer patients, chronic obstructive pulmonary disease (COPD) patients and healthy volunteers. They used cancer sniffing dogs that had been specifically trained.

Several tests were carried out to see if the cancer sniffing dogs were able to reliably identify lung cancer compared with healthy volunteers, volunteers with COPD and whether the results were still found with the presence of tobacco. The dogs successfully identified 71 samples with lung cancer out of a possible 100. They also correctly detected 372 samples that did not have lung cancer out of a possible 400.
Dogs can detect a stable “marker” for lung cancer. The cancer sniffing dogs could also detect lung cancer independently from COPD and tobacco smoke. These results confirm the presence of a stable marker for lung cancer that is independent of COPD and also detectable in the presence of tobacco smoke, food odours and drugs.

“In the breath of patients with lung cancer, there are likely to be different chemicals to normal breath samples,” said the lead author of the study, Thorsten Walles. “The dogs’ keen sense of smell can detect this difference at an early stage of the disease.” He adds that this is a big step forward in the diagnosis of lung cancer, but researchers still need to precisely identify the volatile organic compounds observed in the exhaled breath of patients.

How Dogs Smell Bowel Cancer

Cancer sniffing dogs can smell bowel cancer in breath and stool samples with a very high degree of accuracy – even in the early stages of the disease – reveals research published in Gut.

In this study, a specially trained Labrador retriever completed 74 sniff tests, each comprising five breath (100 to 200 ml) or stool samples (50 ml) at a time, only one of which was cancerous, over a period of several months. The samples came from 48 people with confirmed bowel cancer and 258 volunteers with no bowel cancer or who had had cancer in the past.

Some volunteers in this study had bowel polyps, which are benign but considered to be a precursor of bowel cancer. Six per cent of the breath samples and one in 10 of the stool samples from this group came from those with other gut problems, such as inflammatory bowel disease, ulcers, diverticulitis, and appendicitis. The bowel cancer samples came from patients with varying stages of disease, including early stage.

This cancer sniffing dog successfully identified which samples were cancerous, and which were not, in 33 out of 36 breath tests and in 37 out of 38 stool tests, with the highest detection rates among those samples taken from people with early stage of bowel cancer disease. This equates to 95% accuracy, overall, for the breath test and 98% accuracy for the stool test, compared with conventional colonoscopy – a procedure involving a tube with a camera on the end inserted through the back passage.

This research shows that there are specific discernible odours given off by cancer cells which circulate around the body. These results are backed by other research and anecdotal evidence indicating that dogs can sniff out bladder, skin, lung, breast and ovarian cancers.

Using cancer sniffing dogs to screen for different types of cancer is likely to be impractical and expensive, say these researchers, but a non-canine sensor could be developed to detect the specific compounds.


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