Healing Invisible Wounds

In the two decades since leaving the service, Williams’ journey has been marked by a series of setbacks and victories. Recently, she found a new source of support: a service dog named Montana from Freedom Service Dogs in Englewood, Colorado. The specially trained black Labrador does more than offer comfort. He senses her anxiety before it spirals. He applies pressure when she dissociates. He wakes her from nightmares. In public, he creates space between her and strangers. In private, he reminds her she’s not alone.

Williams is one of thousands of veterans living with PTSD who have experienced the life-changing impact of service dogs. It’s so profound that researchers at the Institute for Human-Animal Connection (IHAC) at the University of Denver’s Graduate School of Social Work (GSSW) are working to uncover the science behind it.

To begin, they started with the basics: Do dogs affect how our bodies respond to stress in everyday situations?

The biology of stress

When we experience stress, our bodies turn on two important systems to help us respond and adapt to the situation. The sympathoadrenal medullary axis (SAM) is part of the sympathetic nervous system. This is the body’s fight-or-flight response. It reacts quickly to stress, releasing hormones that increase heart rate, blood pressure, and energy. The hypothalamic-pituitary-adrenal (HPA) axis responds more slowly. It releases cortisol, which helps the body sustain energy, control inflammation, and restore balance.

To understand the biologic effects of stress and how pets may influence those effects, researchers needed to measure how both the SAM and HPA systems respond during and after stressful events. This involves tracking physiological indicators like heart rate, cortisol levels, and salivary alpha amylase, which reflect the activity of the two systems.

Here’s how the stress test went: 44 people who had dogs were randomly selected to participate with or without their pet, going through a 15-minute stress inducer called the Trier Social Stress Test (TSST), the gold standard for triggering psychological stress in a lab setting. Researchers measured levels before, immediately after, and 45 minutes later to capture both the reaction and recovery.

For 30 minutes, the subjects rested. Then came the instructions.

“Write a speech about why you’re the best candidate for your dream job.”

Moments later, they had to deliver that speech—on camera—while being silently observed and recorded by a panel of supposed behavioral experts in white lab coats, clipboards in hand.

Next came the mental math under pressure.

“Start at 2023 and subtract 17. Keep going, out loud, for five minutes.”

After the test, they returned to a quiet room to recover.

While there was no significant difference in self-reported anxiety, there was a profound difference in heart rate responses between groups. Individuals without a pet present experienced an average heart rate increase of 28.7%, nearly double that of those whose dog was with them (14.6%). Their cortisol levels also rose more sharply—by 51.3%—compared to a 31.1% increase in the dog group.

“It’s widely accepted and appreciated that dogs reduce stress,” says Kevin Morris, director of IHAC. “But what we found was more nuanced than that. If you had a dog present, you still had a stress response, but it stayed within a narrower range. The dogs didn’t just reduce their owners’ stress response—they seemed to keep them in a healthy zone of stress response, not too high or too low.”

That balance is something Williams feels every day with Montana.

“When I’m walking into a crowded space, I can pause, take a moment, and prepare myself for the challenge of being in public. He doesn’t take away the anxiety or PTSD, but he helps me navigate it. He supports me through grounding techniques and provides pressure therapy. It takes my attention off the anxiety and brings it back to him.”

Chronic stress doesn’t just affect mental health. Over time, it’s been linked to serious physical conditions like heart disease, high blood pressure, and even cancer. While pets won’t prevent those conditions, they may help keep stress in a healthier, more manageable range.

When science meets lived experience

The IHAC team is now taking their research a step further: studying service dogs like Montana who are specifically trained for PTSD support. This is funded in part through grants from the Human Animal Bond Research Institute and the Morris Family Animal Foundation (no relation to Kevin Morris).

IHAC is in the process of enrolling 50 veterans, none of whom have had a service dog for PTSD before. To aid in recruitment, IHAC is working with 12 service dog organizations, including Freedom Service Dogs. The research team is tracking the veterans before they get their dogs and again at one-month, six-month, and 12-month check-ins, when both the veterans and the dogs will have their blood drawn. Researchers will work with SomaLogic, a Boulder biotech company, to explore the biological changes the veterans experience when interacting with dogs that are linked to symptom reduction.

While Williams is not a part of this study, her experience reflects the kind of transformation researchers hope to better understand.

“Montana doesn’t take away the anxiety or PTSD, but he helps me better myself in those situations,” she says.

The goal is to identify new biomarkers that reflect how individuals respond to PTSD interventions, specifically by tracking protein changes that correlate with symptom reduction. These findings could also uncover potential new drug targets for treatment.

“There is really only one class of drug that’s approved for PTSD, antidepressants like Zoloft,” Morris says. “Sometimes veterans are also prescribed sedatives, but neither approach treats the root of the problem.”

While the focus is on the veterans, the dogs are being examined, too. For the first time, the study will explore a long-standing question: Do dogs enjoy being service animals? And more importantly, how does this role affect their own health and well-being?

“There’s almost nothing known about how being a service dog affects the dogs,” Morris says. “The goal is to identify biomarkers that correlate with their health and behavior status so we can optimize the protocols the dogs work under.”

Williams’ deep relationship with Montana also underscores this mutual care. She stays in touch with his puppy raisers and now advocates nationally for service dog access. Their connection shows how healing can move in both directions.

For Morris, who used to work in the biotech and drug development space, this work is deeply personal.

“I’ve never seen an intervention as broadly effective as our relationships with other animals can be. If you’re developing a drug, it’s for a symptom of one disease. Pet dogs can affect four to five different health issues a person has. We see profound positive impacts of human-animal bonds at the individual, family, and community-wide level,” Morris says.

Next year, IHAC will celebrate its 20th anniversary. What began as a certification course at GSSW has transformed into a powerful research center dedicated to advancing an understanding of the human-animal bond. Today, it’s working to unlock healing that medicine alone can’t always reach, striving to improve lives on both ends of the leash.