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Cancer Stinks and the Dogs Know It

By Jacob Schor, ND

First, A Joke

A woman brings her toy poodle to the vet. "Doctor," she says, "I think little Alex died in his sleep last night."

"Let's check it out," says the vet, and proceeds to bring out both a Siamese cat, followed by a Labrador Retriever. Both sniff at the inert poodle without reaction. The doctor turns to the woman and sadly explains that she was right, Alex is in doggy heaven, offers his sympathy and presents a bill for payment.

"Twelve hundred and twenty five dollars, that can't be right! I thought your office visit was only twenty-five dollars," exclaims the grieving, and now shocked, woman.

"Still is. This bill includes a thousand for the CAT scan and two hundred for the lab work."

Laugh now, because this joke might soon be closer to the truth than we could have guessed. Dogs soon might be employed as early screening technicians to detect cancer. Recent research suggests dogs potentially detect cancer as well as any high-tech screening equipment.

Cancer Stinks

Cancer cells cause many metabolic changes in the body, some of which lead to the production of distinct odors that dogs can be trained to smell. Cancer increases the oxidative stress in the body, which in turn increases the liver's production of cytochrome P-450 oxidase enzymes to deal with stress. Cell membranes oxidize or, in simpler words, go rancid and the liver breaks down these rancid fats to excrete them faster. These chemicals or Volatile Organic Compounds (VOCs), diffuse through the lungs, evaporate and flow out in the breath. Using sensitive instruments, researchers have been able to measure and identify different patterns of VOCs in breath samples from cancer patients compared to those who are cancer-free.

Once this was known, attempts were and still are being made to build machines to measure VOCs to improve cancer screening. A 2003 study reported that an instrumental breath screening to test for breast cancer was slightly better than mammograms (99.93% versus 99.89%) at saying when someone didn't have cancer, but mammograms were still more accurate at saying when someone had cancer.1 This breath test could potentially be employed as a preliminary screen for breast cancer. Several machines also were tested to screen for lung cancer.2,3

Machines of this nature are expensive, fussy and constantly need maintenance to ensure their accuracy. Dogs on the other hand are pretty much the opposite of high-tech machines. Dogs have powerful noses, perhaps among the most sensitive in the natural world, able to detect certain chemicals in the part per trillion range. Dogs probably are more sensitive than scientific instruments. The idea that dogs could be trained to detect cancer's distinctive VOCs more accurately than an electronic instrument is not far fetched. Dogs certainly have proven their mettle at detecting hidden explosives and illicit drugs. No one even dreams of a machine that will track animals or fugitives cross country.

Sniffing Cancer in Urine

In a 2004 study published in the British Medical Journal, researchers trained dogs to detect bladder cancer by smelling urine samples. The group used urine samples from 36 patients with bladder cancer and 108 control samples from cancer-free individuals. Six dogs of varying ages and breeds underwent a seven-month training course in cancer detection, carried out by trainers from Hearing Dogs for the Deaf.

In the final, double-blind experiment, each dog underwent nine separate tests in which they were shown seven urine samples, one of which was cancerous, and told to lie down next to the cancerous one. The dogs correctly identified the cancer sample on 22 out of 54 occasions. This success rate of 41 percent is much higher than the 14 percent expected from chance alone.4 Multivariate analysis suggested that the dogs' ability to recognize an odor characteristic of bladder cancer was independent of other chemical aspects of the urine which were detectable by urinalysis. Researchers inferred that tumor-related volatile compounds are present in urine, imparting an odor signature distinct from those related to ancillary effects of a tumor, such as bleeding, inflammation and infection.4 Additionally, one of the the dogs continued to identify one particular sample, even though the participant had tested negative for bladder cancer. This led to further testing and that donor was found to have a kidney tumor. Interesting, but far from impressive results. The newest studies are different.

Sniffing Cancer in Breath

The newest cancer-sniffing study published in the March 2006 issue of the Journal of Integrative Cancer Therapies builds on these prior studies. This study is the first to test whether dogs can detect cancers only by sniffing the exhaled breath of cancer patients.

Five household dogs were trained over a three-week period to detect lung or breast cancer by sniffing the breath of participants known to have cancer. The experiment included 86 cancer patients (55 with lung cancer and 31 with breast cancer) and a control sample of 83 healthy patients. All cancer patients recently had been diagnosed with cancer through biopsy-confirmed conventional methods such as a mammogram or CAT scan, and had not yet undergone any chemotherapy treatment. The dogs were presented with breath samples from the cancer patients and the controls, captured in a special tube. The dogs were trained to give a positive identification of a cancer patient by sitting or lying down directly in front of a test station containing a cancer patient sample.

The dogs correctly detected 99 percent of the lung cancer samples, and made a mistake with only 1 percent of the healthy controls. With breast cancer, they correctly detected 88 percent of the positive samples, and made a mistake on only two percent of the controls. The study also confirmed that the dogs could detect the early stages of lung cancer and early breast cancer.5 The authors used a food reward-based method of training five ordinary household dogs to distinguish, by scent alone, exhaled breath samples of 55 lung and 31 breast cancer patients from those of 83 healthy controls.5 Note that it took only three weeks to train the dogs in this breath-sniffing experiment, while the urine sniffers were trained for seven months. This pilot work using canine scent detection demonstrated the legitimacy of using a biological system to examine exhaled breath in the identification of lung and breast cancers. Future work will closely examine the chemistry of breath to identify which chemical compounds can most accurately identify the presence of cancer.5

Although these studies and the possibilities they invoke are interesting, what I find fascinating is trying to imagine the human world as a dog must perceive it. I say perceive rather than see because it's apparent that a dog must know a great deal about us that isn't seen, but is smelled. The idea that upon introduction to a person, a dog may have a greater appreciation of that person's morbidity and impending mortality than the person's physician, or anyone else in their life, is a humbling thought. Could this be why therapy dogs can offer such a deep level of comfort to the ill and dying? Dogs might be the only companions that truly perceive and understand what is happening.


  1. Phillips, M. et al. Volatile markers of breast cancer in the breath. Breast J. Jul-Aug;9(4):345.2003.
  2. Phillips, M. et al. Volatile organic compounds in breath as markers of lung cancer: a cross-sectional study. Lancet. Jun 5;353(9168):1897-81;1999.
  3. Phillips, M. Detection of lung cancer with volatile markers in the breath. Chest. Jun;123(6);1788-92: 2003.
  4. Willis CM. et al. Olfactory detection of human bladder cancer by dogs: proof of principle study. BMJ. Sep 25;329(7468):715;2004.
  5. McCulloch M. et al. Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers. Integr Cancer Ther. Mar;5(1):30-9;2006.

About the Author: Jacob Schor graduated with a bachelor of science degree from Cornell University and received his naturopathic training at National College of Naturopathic Medicine. He currently practices at the Denver Naturopathic Clinic. E-mail Dr. Schor at .


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Date Last Modified - Friday, 17-Oct-2008 12:10:42 PDT