Can Dogs Sniff Out Disease?

Dog nose.

Source: Mark Watson, via Wikimedia Commons. Distributed under a CC BY 2.0 license.

Humans have long known that dogs’ noses are something special. We regularly train dogs to use their sense of smell—estimated to be 10,000 to 100,000 times as acute as ours—to sniff out bombs, drugs, firearms, and people. But what about detecting the smell of disease?

The first report of a dog alerting its owner of disease, published in 1989, described a dog that persistently sniffed and bit at a mole on its owner’s leg, which turned out to be a melanoma. That raised the idea that cancer, and other diseases, might be detectable by smell, and that dogs could be used as diagnostic tools.

Since then, many studies have examined dogs’ ability to sniff out sicknesses, including cancers, bacterial infections, seizures, COVID-19, and diseases affecting other animals and plants.

In a new review paper, Texas A&M University animal welfare scientist Courtney Daigle and colleagues evaluated existing studies of dog olfactory detection to determine the extent to which dogs could serve as reliable indicators of disease.

“This came about because one of our collaborators asked me if I thought we could use dogs to sniff out sick cows,” says Daigle. “I said I didn’t know, but I can speak dog and we can find out.”

Sniff Tests

Daigle, with her Ph.D. student Aiden Juge and colleague Margaret Foster, conducted a meta-analysis of 58 studies in which dogs were trained to detect diseases or health conditions in humans, other animals, or plants.

Overall, the researchers noted a high level of success in most studies, whether the metric used was sensitivity (the proportion of the time they can detect the disease sample), specificity (the proportion of the time they can correctly not select a healthy sample), or accuracy (the number of samples they can classify correctly overall).

“Generally, the dogs did really well,” says Juge. “For all three of the metrics, we found the median percentage was in the 90s.”

Dogs trained to sniff out COVID-19 in Chile.

Source: César Cortés , via Wikimedia Commons. Distributed under a CC BY-SA 2.0 license.

While the breed of the dog did not seem to make much of a difference, variations in study design were noticeable and may have had an impact on results. For instance, specificity was higher in studies if the testing was not double-blinded (double-blinded means that both the dog and its handler are unaware of which samples are positive for disease and which are negative). This suggests that dogs may read subtle cues from experimenters in tests that are not double-blinded, muddying results.

Prior studies also varied in disease type to be detected, with lung cancer and prostate cancer being the most frequently studied conditions. The researchers found that disease type had an effect on detection, with greater success for cancers and bacterial conditions than for chronic illnesses, such as seizures, sleep apnea, and diabetes.

Juge says that the few studies they found related to chronic health conditions had more inconsistent results. This could be due to the fact that the dogs in these studies were initially trained as medical alert dogs to focus on the specific scent of their handler. When testing their abilities on unfamiliar people, their accuracy can suffer.

“I would be interested in whether these dogs rely more on smell or familiarity with their handler’s body language,” says Juge.

Doctor’s Best Friend

Juge says dogs could be a useful tool for facilitating disease detection in environments that lack the resources or time for laboratory testing.

“Dogs are very good at screening a lot of samples quickly,” he says. “They could provide a good first-line screening test to identify individuals that may need more accurate laboratory testing. But we need more research seeing how they perform in situations that more closely simulate real-world scenarios.”

One situation where Daigle and Juge are interested in applying detection dogs is sniffing out sick cows. The collaborator who came to Daigle with that question wanted to know if dogs could be trained to detect bovine respiratory disease, a condition that can be common in feedlot cattle.

Daigle says that dogs have two characteristics that could make them perfect for this task. One is their undeniably acute sense of smell. The other is their cognitive and communication abilities and their desire to work with humans.

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Animal and Plant Health Inspection Service National Detector Dog Training Center detector dog trainee Harlee.

Source: U.S. Department of Agriculture, public domain.

“Since cattle are a prey species, they are good at hiding their vulnerabilities or weaknesses and that can make it difficult to identify the ones that need help,” says Daigle. “But you can’t hide your body chemistry.

“Dogs could be the bridge between humans and cattle: We can communicate with them and they can pick up on olfactory cues from the cattle that we cannot and tell us what smells right and what doesn’t.”

Daigle, Juge, and colleagues recently completed a pilot study in which they trained two dogs to detect bovine respiratory disease from cattle using nasal swabs. While the dogs showed some ability to discriminate between healthy and diseased samples during training, their performance during testing was only slightly better than chance. The researchers say that the complexity of the task, including the extra “noise” from samples collected in the field, suggests that more testing is needed to determine if dogs could be an effective screening method for this disease. They are currently planning follow-up research to refine their methodology.

Daigle and Juge’s pilot study results emphasize the conclusions they made from their meta-analysis. Dogs are promising as disease detectors in contexts in which more invasive screening is impractical or where large populations need to be screened rapidly, at low cost. But most of the evidence for their success is laboratory-based at the moment. Information about dogs’ potential in real-world disease screening applications is lacking. More research on when and how to best employ dogs’ noses is needed before disease detection dogs become a regular part of human or veterinary medicine.