Dogs can detect stress-related compounds in the breath of people experiencing early signs of trauma, including those with posttraumatic stress disorder (PTSD), a new proof-of-concept study suggested.

The research provides evidence that some service dogs with PTSD can be trained to detect episodes of pending distress through a person’s breath and perhaps prompt the individual to use coping skills to manage the episode.

“Ours is the first study to demonstrate that at least some dogs can detect putative stress-related volatile organic compounds in human breath that are associated with PTSD symptoms,” study author Laura Kiiroja, PhD candidate, Department of Psychology and Neuroscience, Faculty of Science, Dalhousie University, Halifax, Nova Scotia, Canada, told Medscape Medical News.

The study was published online on March 28, 2024, in Frontiers of Allergy.

Heightened Sense of Smell

The lifetime prevalence of PTSD is about 8% in the general population, but data show it can reach 23% in veterans. In addition, many more trauma-exposed individuals experience subthreshold symptoms.

Dogs’ sense of smell is up to 100,000 times more sensitive than that of humans. Research is investigating the application of dogs’ sense of smell — which is up to 100,000 times more sensitive than humans’ — to detect cancers, viruses, parasites, hypoglycemia, and seizures in humans.

There is also some evidence that dogs can detect putative stress-related volatile organic compounds (VOCs) such as isoprene and monoterpenes from the human body through urine, sweat, and breath, with the greatest success achieved with breath.

The new study included 26 mostly civilian “donors” (mean age, 31 years; 18 females) who had experienced various types of trauma but had no severe mental illness. More than 50% met the criteria for PTSD.

Participants were recruited from a study examining neurocognitive mechanisms underlying the potential links between trauma and cannabis use. However, participants in the dog study abstained from using cannabis for at least 12 hours prior to the study experiments.

Breath Donors

Breath samples were collected via disposable medical-grade face masks at baseline and during ensuing experiments. In total, 40 breath sample sets were collected.

Two female companion dogs — Ivy, a red Golden Retriever, and Callie, a German Shepherd/Belgian Malinois mix — were trained to identify target odors from the samples.

The animals were tested to determine whether they were able to discriminate between breath samples collected from these same “breath donors” during a relatively relaxed state and during induced stress testing which is known as the alternative forced choice discrimination test.

The dogs’ ability to discern trauma cues from breath samples of various individuals was tested by presenting one sample (baseline or trauma cue) at a time. The researchers used signal detection theory to evaluate the sensitivity and specificity of dogs in detecting human stress VOCs.

Investigators found the dogs had about a 90% accuracy rate across all sample sets in the discrimination experiment and 74% and 81% accuracy for Ivy and Callie, respectively, in the detection experiment.

“Our study contributed to the evidence showing that not only are dogs able to detect some physical health conditions in humans but also that some mental health conditions alter the released VOCs in a way that is detectable by dogs,” Kiiroja said.

Emotion Detectors

At baseline and during each cue exposure, donors reported their affect using the Positive and Negative Affect Schedule. Ivy’s performance correlated with the donors’ self-reported anxiety, and Callie’s performance correlated with the donors’ self-reported shame.

Based on these correlations, the researchers speculate Ivy detected VOCs that likely originated from the sympathetic-adrenomedullary axis, which involves adrenaline and noradrenaline.

VOCs detected by Callie likely originated in the hypothalamus-pituitary-adrenal axis, which involves cortisol and corticosterone. These two endocrine subsystems play a major role in reestablishing homeostasis in response to a stressor.

The results suggest some service dogs could alert to upcoming intrusion and hyperarousal symptoms even before physical signs manifest and before the person is even aware of the situation, said Kiiroja.

“This would enable earlier distraction and reminders to use skills learned in psychotherapy; this would have a better likelihood of increasing the efficacy of these skills and preventing further escalation of the arousal,” she said.

Most breath samples likely included both early and late stress VOCs, as the breath donors wore the trauma mask for a relatively long time, the authors noted. Future studies should test dogs’ olfactory acuity on samples collected minutes after the trauma cue, they added.

Another limitation is that all donors were regular cannabis users, so the results may not generalize to others. However, the fact the dogs demonstrated their detection ability even with cannabis users makes the proof-of-concept “more stringent,” Kiiroja said.

The goal of the study was to see if some dogs are capable of detecting stress VOCs from people with trauma histories in response to trauma cues, so the small number of dogs in the study isn’t a limitation, the authors noted.

‘Wonderful Work’

Commenting on the findings, Elspeth Ritchie, MD, chair of psychiatry, MedStar Washington Hospital Center, Washington, DC, described the research as “wonderful work.” Ritchie, who was not a part of this study, has also studied PTSD supports dogs.

The study is yet another illustration of the “amazing things dogs can do…not just for veterans but for people with mental illness.” They can be a source of comfort and help people manage their anxiety.

Training PTSD service dogs can be expensive, with some well-accredited organizations charging about $50,000 for an animal, Ritchie said. Training a dog to detect VOCs could also be costly, she added.

Although such research has increased in recent years, it’s unclear how it would be applied in practice. Identifying funding for this sort of study and designing trials would also be challenging, Ritchie added.

“The idea is good, but when you try to operationalize it, it gets tricky,” she said.

The fact that all donors in the study used cannabis is a confounding factor and raises the question of what else might confound the results, Ritchie added.

Ritchie emphasized that although ideally veterans would learn to recognize the onset of stress symptoms themselves, a dog could serve as a valuable companion in this process. “That’s precisely why this research should progress,” she said.

The authors and Ritchie reported no relevant disclosures.

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