Humans have put dogs’ remarkable sense of smell to use by training them to sniff out explosives and narcotics. Their powerful noses can also detect viruses, bacteria, and signs of cancer in a person’s body or bodily fluids.
In this article, we look at the evidence behind dogs’ abilities to smell and identify different types of cancer, and how medical professionals can use dogs to help diagnose the condition.
Can Dogs Smell Cancer?
Research suggests that dogs can detect many types of cancers in humans. The possibility that dogs might be able to “smell” cancer, due to their incredible sense of smell, has garnered growing interest.
The hypothesis was first raised in 1989 when doctors described a case of a woman concerned about a mole that her dog would constantly sniff at, and had tried to bite, which turned out to be a malignant melanoma. There have been several other reports since then of dogs detecting cancers by constantly sniffing or nudging an area of their owner’s body.
Like many other diseases, cancers leave specific traces, or odor signatures, in a person’s body and bodily secretions. Tumors produce volatile organic compounds (VOCs) which are released into urine and sweat and exhaled in breath.
Even in small quantities these compounds are thought to have a distinct odor, particularly in the early stages of cancer when cells are dividing.
Cancer cells, or healthy cells affected by cancer, produce and release these odor signatures. They detect these odors in substances called volatile organic compounds (VOCs).
Depending on the type of cancer, dogs are able to detect VOCs in a person’s:
- skin
- breath
- urine
- feces
- sweat
Dogs can detect these odor signatures and, with training, alert people to their presence. People refer to dogs that undergo training to detect certain diseases as medical detection dogs.
Trained dogs can detect some substances in very low concentrations, as low as parts per trillion, which makes their noses sensitive enough to detect cancer markers in a person’s breath, urine, and blood.
Which Types of Cancer Can a Dog Smell?
Research has shown that animals may detect differences in body odors between healthy people and cancer patients, especially for lung, breast, and prostate cancers.
Studies over the past decade have shown trained dogs could identify the urine of patients with bladder cancer almost three times more often than would be expected by chance alone, detect lung cancer in exhaled breath samples with very high accuracy (in two separate studies) and identify ovarian and colorectal cancers by smelling breath samples.
Dogs can detect many types of cancer, such as:
- melanoma
- colorectal cancer
- lung cancer
- ovarian cancer
- prostate cancer
- breast cancer
For example, one case report describes how a 75-year-old man visited a doctor after his dog licked persistently at a lesion behind the man’s ear.
The doctor performed diagnostic tests and confirmed malignant melanoma.
Nobody had trained this person’s dog specifically to detect cancer. However, most research studies into canine cancer detection involve teaching individual dogs to sniff out specific cancers.
Scientists have found evidence that some dogs can detect colorectal cancer from people’s breath and watery stool with high levels of accuracy, even for early-stage cancers. The presence of gut inflammation or noncancerous colorectal disease does not seem to affect dogs’ ability to detect these cancers.
Dogs may also detect lung cancer from a person’s breath. One study found that a trained dog had a very high rate of accuracy in distinguishing between the breath of people with and without lung cancer.
A study by Liu et al. (2023) found that training dogs using exhaled air samples resulted in a lung cancer diagnosis rate of 71.3% to 97.6% (mean 83.9%), with a false positive rate in healthy individuals ranging from 0.5% to 27.6% (mean 7.6%).
This study also indicated that the sensitivity and specificity of using exhaled breath target training (91.7% and 85.1%) were higher than using lung cancer tissue training (50.4% and 50.1%).
In another study, two dogs received training for one year. After this, researchers presented the dogs with a number of urine samples. The dogs proved 45–73% accurate in detecting lung cancer through the samples. However, the study by Liu et al. (2023) found that dogs had a very low response rate to urine target samples.
Dogs have also detected ovarian cancer from blood samples and prostate cancer by sniffing a person’s urine.
In 2021, researchers reported that a dog trained to detect signs of breast cancer in urine was able to detect breast cancer with 100% accuracy among urine samples from 200 people. Of these, 40 had breast cancer, 182 had other cancers, and 18 had no cancer. This study has yet to be repeated with a larger population of dogs to see if the outcomes can be reproduced.
One study found that dogs trained only to detect breast cancer were also able to detect melanoma and lung cancer. This suggests there may be a common odor signature across different types of cancer.
The use of several species of animal to detect scents also seems promising, with success rates above 80% for most. A large variety of animals, including dogs, rodents, insects, and roundworms, have been used for this purpose and have all shown their capacity to detect cancer.
This research primarily involves animals being presented with different sources of stimuli, such as urine, to see if they can distinguish the odors of people affected by cancer.
Are Dogs Used in Cancer Research and Diagnosis?
The fact that trained dogs can detect cancer may have significant benefits for humans. Using dogs to support the detection and diagnosis of cancer is a low-risk, noninvasive method.
Medical detection dogs present few side effects and may offer advantages because they are mobile, can begin work quickly, and can trace an odor to its source.
They also have the potential for use in patient care settings or laboratories to identify cancer in tissue samples from people with suspected cancers.
Dogs’ abilities may also help with developing machines that can reliably detect odor signatures from cancer, such as electronic noses.
Scientists are also using chemical analysis and nanotechnology to try to identify cancer biomarkers in breath, sweat, and urine that could be used in blood tests or other tests to detect cancer. If they can identify the chemical changes responsible for the odor the dogs are picking up, it may be possible to develop computerized screening instruments with the same sensitivity dogs have.
The study by Liu et al. (2023) also found that sniffer dogs could diagnose lung cancer independent of staging, pathologic type, and tumor location. This suggests that using sniffer dogs to screen for early lung cancer may have good clinical and economic benefits.
The study concluded that training dogs using breathing target samples to recognize exhaled samples had a higher diagnostic rate than training using lung cancer tissue samples or urine samples.
Research is still underway and the effectiveness and reliability of canine cancer detection requires further investigation.
Other areas for research include which breeds of dogs are most suited to detection and which kind of training will be most effective.
Currently, the matter remains a subject of research rather than procedure.
Sources:
- Can dogs detect cancer?, Medical News Today, January 19, 2024
- Studies say dogs could be trained to sniff out cancer, Cancer Council, January 24, 2024
- Sniffer Dogs Diagnose Lung Cancer by Recognition of Exhaled Gases, National Library of Medicine, February 15, 2023
- Dogs team up with AI to sniff out cancer, Science News, December 23, 2024
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DISCLAIMER
This article provides general information and does not constitute medical advice. Consult your healthcare provider for personalized recommendations. If symptoms persist, consult your doctor.
