How Chronic Illness Affects Your Dog’s Mood and Behavior

"A 2023 study of 238 dog owners found something quietly devastating: the worse a dog’s physical health was, the more likely their person was to report depression, anxiety, and a lower quality of life—and those owners also reported more behavior problems in their dogs.[1] The researchers didn’t just find “a bit of a link.” They found a tight knot connecting three things: chronic illness in the dog, changes in behavior and mood, and the emotional state of the human trying to care for them.

If your dog has a long-term condition and doesn’t seem like themselves anymore—more clingy, more irritable, more shut down—that knot is where you’re living.

This article is about what’s happening inside that knot:how chronic illness can change a dog’s behavior and mood, how your own emotions tie in, and how to think more clearly (and more kindly toward yourself) about what you’re seeing.

Chronic illness doesn’t just live in the body

“Chronic illness” in dogs covers a lot of territory: arthritis, cancer, heart disease, kidney disease, diabetes, endocrine disorders, chronic skin or gut problems, neurologic disease, and more. What they share is that they:

• last for months or years

• often require ongoing medication, monitoring, and vet visits

• may be progressive or incurable

We tend to think of these as “physical” problems—joints, organs, tumors. But biology doesn’t respect that neat divide between body and mind. Chronic disease acts as a long-term stressor inside the dog’s system, and stress changes behavior.

Research consistently shows that dogs with chronic illness are more likely to show:[1][3]

• increased fearfulness

• anxiety (restlessness, hypervigilance, trouble settling)

• irritability or aggression

• reduced responsiveness to cues (poor recall, pulling on the lead)

• withdrawal or “depressive-like” behavior

None of this means your dog has suddenly become “bad” or “stubborn.” It means their nervous system is working under new rules.

What chronic illness can look like on the outside

Different illnesses can produce different patterns, but some themes are common.

1. Anxiety and restlessness

You might notice:

• pacing at night or wandering the house

• panting when it’s not hot

• difficulty settling in their usual resting spots

• startle responses to noises they previously ignored

• seeming “on edge” on walks

Why this happens:

• Pain and discomfort make it hard to relax.

• Hormonal changes (for example, from endocrine disease) can alter arousal and stress responses.

• Chronic stress can disrupt sleep–wake cycles.

2. Clinginess or withdrawal

Some dogs with chronic illness become velcro dogs—following their person from room to room, demanding contact, distressed when left alone. Others go the opposite way: they retreat to quiet corners, avoid interaction, and seem “shut down.”

Both patterns can be signs of:

• trying to self-protect from pain (avoiding play, avoiding touch)

• seeking safety and comfort from their attachment figure

• reduced energy or “emotional fatigue” from constant discomfort

3. Irritability and aggression

Even the gentlest dog can become snappy under the right (or wrong) combination of pain, fear, and confusion. Owners and vets report that chronically ill dogs may:[1][3]

• growl when touched in certain areas

• resist grooming, nail trims, or being picked up

• react aggressively when startled or handled at the vet

• guard resting places or food more intensely

This doesn’t mean they’ve “turned aggressive.” It often means:

“I don’t feel safe. Please back off.”

but in dog language, which has fewer words and more teeth.

4. “Depressive-like” behavior

We can’t diagnose depression in dogs the way we do in humans, and researchers are careful about that line. But chronic inflammatory and stress states in animals are associated with behaviors that look eerily similar:[3]

• less interest in play or toys

• reduced exploration on walks

• sleeping more, moving less

• slower responses, “flat” expression

• eating less—or, sometimes, comfort eating

These changes don’t prove your dog is clinically depressed. They do tell us that their motivation and mood systems are under strain.

What’s going on inside: mood chemistry, inflammation, and stress

Under the surface, chronic illness and chronic stress reshape the dog’s internal chemistry.

Key players include:

• Cortisol – the primary stress hormone

• Serotonin – involved in mood, impulse control, and well-being

• Dopamine – tied to motivation, reward, and pleasure

• Noradrenaline – arousal and “fight-or-flight” responses

• Inflammatory molecules – part of the immune response to illness

Research on dogs with behavioral disorders and chronic stress has found:[3]

• lower serotonin and dopamine levels  

• higher cortisol and noradrenaline  

• improvements in behavior when these systems are supported (for example, via diet or nutraceuticals)

In one study of 69 dogs with behavioral disorders, targeted interventions led to:

• increased serotonin and dopamine

• decreased cortisol and noradrenaline

• measurable reductions in anxiety and stress behaviors[3]

That doesn’t mean a supplement will magically fix a chronically ill dog’s mood. But it does confirm something important:

Long-term illness doesn’t just “make them grumpy.” It changes the actual brain chemistry that regulates mood and behavior.

Chronic inflammation from disease can also alter how the brain processes pain and emotion, contributing to anxiety- and depression-like states.[3] This is a well-known pattern in humans with chronic illness and chronic pain, and the animal data points in the same direction.

The invisible half of the picture: you

Here’s the part that’s easy to underestimate when you’re sleep-deprived and counting pills: your own mental health is part of your dog’s environment.

Multiple studies show that:

• Owners of seriously or terminally ill pets report significantly higher stress, anxiety, depression, and lower quality of life than owners of healthy pets.[2][4][6]

• In that 238-owner study, poorer dog health and more behavior problems were tightly linked to higher levels of owner depression and anxiety.[1]

• Caregivers of chronically ill pets commonly experience:

  • isolation

  • guilt

  • exhaustion

  • anticipatory grief—mourning losses before they fully happen[4][6]

This isn’t just a sad side note. It feeds back into the dog’s world.

Emotional contagion: when your dog catches your feelings

Dogs are remarkably attuned to human emotional states. Studies on healthy dogs show that positive interactions with humans can lower human cortisol and improve mood.[5][7] Other research and clinical observations suggest dogs “mirror” our emotional tone—what psychologists call emotional contagion.

In practical terms:

• When you’re tense, rushed, or tearful, your dog may become more unsettled or vigilant.

• If you’re too drained to walk, play, or train, your dog loses outlets for stress and enrichment.

• If you avoid social situations because of your dog’s illness, both of you can become more isolated—another risk factor for anxiety and depressive patterns.

The research synthesis describes this as a bidirectional relationship:[1]

Dog’s illness → owner distress → changes in care and emotional climate → dog behavior and mood changes → more owner distress… and so on.

This is not a blame loop. It’s a feedback loop. And recognizing it can be quietly liberating: you’re not failing; you’re living inside a system that’s hard on everyone.

Quality of life: the ethical and emotional tightrope

Chronic illness doesn’t just bring medications and vet visits. It brings decisions.

Owners of chronically or terminally ill dogs often wrestle with:[4][6]

• How much treatment is enough—or too much?

• How many side effects are acceptable?

• Is my dog having “good days” or just “less bad” days?

• Am I keeping them alive for them, or for me?

• When do we talk about euthanasia?

These questions don’t stay neatly in the “ethical” box. They leak into daily interactions.

• You may feel guilty setting boundaries (“No, we can’t do another surgery”).

• You may overcompensate with food, leniency, or avoiding any stress at all.

• You may delay difficult conversations with your vet because you’re afraid of the answer.

All of that shapes your dog’s experience: how predictable their routine is, how consistent their care is, how calm or strained the household feels.

Veterinarians are deeply aware of this. Studies note:

• Vets see owner distress influence treatment decisions and adherence.[2]

• The emotional weight of managing chronically ill animals and distressed owners affects veterinarian well-being too.[2]

In other words, everyone in the room is carrying something heavy.

What we know for sure—and what we don’t

It can help to separate solid ground from educated guesswork.

Well-established

Research strongly supports that:[1–4,6]

• Chronic illness is linked to behavior change in dogs—especially anxiety, fearfulness, and irritability.

• Long-term disease and chronic stress can alter neurochemical systems (serotonin, dopamine, cortisol, noradrenaline) that regulate mood.

• Owner mental health often declines when caring for a chronically ill dog, with significant caregiver burden.

• Emotional states of dogs and owners are bidirectionally linked.

• Some nutraceutical and medical interventions can improve certain mood and behavior parameters in dogs with chronic stress or illness.

Still uncertain or emerging

Researchers are still working out:[3]

• The exact neurobiological pathways by which each specific disease affects mood in dogs.

• How to define and diagnose clinical depression in dogs, if that’s even the right framework.

• The best ways for vets to communicate about chronic illness and end-of-life decisions to reduce owner distress.

• How dog behavior changes over the entire course of a chronic illness—large, long-term studies are still limited.

Living in that uncertainty is uncomfortable. But knowing where the edges are can make it a bit less frightening.

Making sense of your dog’s behavior without turning on yourself

When your chronically ill dog snaps, refuses a walk, or stares blankly at the wall, the instinctive questions are often:

• “What am I doing wrong?”

• “Are they mad at me?”

• “Is this my fault because I’m stressed?”

A more accurate, kinder set of questions might be:

• “What could their body be feeling right now?”

• “Has anything in their medical condition or routine changed?”

• “What might their behavior be protecting them from?”

• “What emotional climate are we both living in right now?”

You can bring this mindset into conversations with your vet or behavior professional. Instead of “He’s being difficult,” you might say:

• “Since his diagnosis, he’s more jumpy and less tolerant of handling.”

• “Her recall has really declined, and she seems anxious on walks.”

• “He’s still eating, but he doesn’t want to play anymore and sleeps most of the day.”

These are data points, not confessions. They help your care team:

• adjust pain management

• check for disease progression

• consider behavior support or environmental adjustments

• discuss whether mood-related medications or supplements might be appropriate

The daily reality: small patterns that matter

There’s no universal “plan” for supporting a chronically ill dog’s mood, because every disease and every household is different. But there are patterns that often help, and they’re as much about mindset as about actions.

Think of it in three intertwined layers: body, behavior, and bond.

1. Body: understanding that comfort is foundational

Behavior is downstream of physical state. When a dog’s behavior changes, especially with known illness, it’s reasonable to wonder about:

• pain levels

• nausea or digestive discomfort

• medication side effects

• sensory changes (hearing, vision, smell)

• sleep disruption

This is where your vet is your primary partner. Your role is to observe and report patterns, not to solve them alone.

Useful observations to track:

• When during the day is your dog most unsettled or withdrawn?

• What types of touch or movement trigger resistance?

• Any changes in appetite, drinking, urination, or stool?

• How often do they choose to move vs. need to be coaxed?

These notes give your vet something to work with beyond “He just seems off.”

2. Behavior: noticing, not judging

Try to see your dog’s behavior as information about their internal state, not a moral statement.

For example:

What you see	Possible internal message
Growling when lifted	“That hurts. Please don’t move me like that.”
Refusing stairs or the car	“I’m worried this will hurt or I feel too weak.”
Restlessness and pacing at night	“I’m uncomfortable / anxious / can’t settle.”
Ignoring cues they used to know	“I’m tired / distracted by discomfort / foggy.”
Hiding or choosing solitude	“I need quiet and safety right now.”

This frame doesn’t remove the need to keep everyone safe, but it can reduce the sting of feeling rejected or “disobeyed” by a dog you love.

3. Bond: acknowledging the emotional loop you’re both in

Your dog’s world is small, and you are most of it. That doesn’t mean you must be endlessly cheerful (no one is). It means:

• Your presence and predictability matter more than perfection.

• It’s OK to protect small pieces of your own life—sleep, meals, a walk alone. Those are not betrayals; they’re what keep you able to care.

• You’re allowed to seek support—for yourself. Studies on caregiver burden in pet owners echo those in human caregiving: support can reduce distress and improve coping.[4][6][8]

Support might look like:

• talking openly with your vet about emotional strain, not just medical details

• leaning on trusted friends or family who understand your bond with your dog

• joining a support group for pet caregivers

• speaking with a therapist, especially one familiar with grief and caregiving

You’re not “making it about you.” You’re recognizing that your mental health is part of your dog’s environment, and tending to it is a form of care for them too.

Working with your veterinarian: a partnership, not a performance

Many owners of chronically ill dogs feel they must appear endlessly competent and positive in front of the vet. Research suggests vets are acutely aware of caregiver distress and that it affects clinical decisions and adherence.[2]

You’re allowed to bring your full, imperfect self into that room.

Things you might say that are completely legitimate:

• “I’m overwhelmed by the medication schedule—can we simplify it?”

• “I’m struggling to tell if she’s having more good days than bad. How do you think about that?”

• “His behavior has changed a lot. Can we talk about whether this is pain, progression, or something else?”

• “I need help thinking about when euthanasia becomes the kindest option. I don’t trust myself to know.”

Good vets understand that chronic illness is as much a relationship journey as a medical one. They may not have all the answers, but naming the questions out loud often reduces the fog.

When your dog doesn’t seem like themselves anymore

Perhaps the hardest part of chronic illness is the sense of losing the dog you knew, even while they’re still here.

• The frisbee fanatic who no longer runs.

• The social butterfly who now avoids the park.

• The confident dog who now startles at every sound.

It can feel like a betrayal, as if the illness has stolen their personality.

From a biological perspective, what’s happening is less like theft and more like adaptation under duress. The same nervous system that once powered joyful zoomies is now trying to:

• protect painful joints

• conserve energy

• manage altered hormones and inflammatory signals

• navigate a world that suddenly feels less predictable

From a relational perspective, the bond is still there—it’s just expressing itself through quieter, sometimes more fragile behaviors: the way they lean into your hand, the soft sigh when they settle near you, the trust in their eyes even when their body is failing them.

You don’t have to pretend this is easy or beautiful. It’s often not. But understanding the biology behind the behavior can soften one particular kind of pain: the fear that your dog has simply “checked out” or stopped loving you.

What the science and the lived experience both suggest is this:

• Their behavior is changing because their body and brain are under strain.

• Your emotions are changing because your heart is under strain.

• Both of you are doing your best with nervous systems that were never designed for endless uncertainty.

If there’s a quiet goal in all of this, it’s not to keep your dog endlessly happy or yourself endlessly strong. It’s to make the time you have—whether that’s months or years—as honest, comfortable, and connected as the situation allows.

That’s not failure. That’s caregiving, in its real, complicated form.

References

1. Parthasarathy V, et al. Dog owner mental health associated with dog behavioral problems in chronic illness. Scientific Reports. 2023. Available at: https://www.nature.com/articles/s41598-023-48731-z  

2. Spitznagel MB, et al. Caregiver burden in owners of a sick pet. BMJ (reported via ScienceDaily). 2017. Available at: https://www.sciencedaily.com/releases/2017/09/170918222241.htm  

3. Titulaer M, et al. Behavioral disorders and neurochemical changes in dogs with chronic stress. Frontiers in Veterinary Science / PubMed Central. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC5284471/  

4. PetMD. Caregiver burden in pet parents of chronically sick dogs and cats. Available at: https://www.petmd.com/dog/caregiver-burden-pet-parents-chronically-sick-dogs-and-cats  

5. Barker SB, et al. The effect of animal-assisted therapy on anxiety and fear in hospitalized psychiatric patients. Applied Psychophysiology and Biofeedback. Contextualized via PubMed Central: https://pmc.ncbi.nlm.nih.gov/articles/PMC12346317/  

6. Kent State University. When caring for a sick pet becomes too much. Available at: https://www.kent.edu/flash-feed/news/when-caring-sick-pet-becomes-too-much  

7. Powell L, et al. Dogs supporting human health and stress modulation. Frontiers in Veterinary Science. 2021. Available at: https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2021.630465/full  

8. Rodan I, et al. The roles of pets in managing chronic pain. Pain Medicine. 2019. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC6920602/  

9. University of New Mexico. The mood-boosting power of dogs. Available at: https://hr.unm.edu/docs/ehp/mood-boosting-power-of-dogs.pdf  

10. Herzog H. The surprising link between pet attachment and mental health. Psychology Today. Available at: https://www.psychologytoday.com/us/blog/animals-and-us/202511/the-surprising-link-between-pet-attachment-and-mental-health"