Snap judgments: Aggression problem or medical condition?

dvm360dvm360 January 2021
Volume 52
Issue 1

Understanding the intersection between common clinical issues and primary behavioral concerns in dogs.

"I don’t know what’s changed, but my dog growls a lot now.” “Bella is normally so loving, but she just bit my husband when he tried to pet her.” “My dog isn’t acting like himself. I’m worried he’s going to hurt me or someone else.” Although it can be tempting to assume behavioral or training problems are at the root of these issues, such snap judgments (pun intended) may be missing the real causes—and there- fore the real solutions, according to Christopher Pachel, DVM, DACVB, CABC, of Animal Behavior Clinic in Portland, Oregon.

During a recent Fetch dvm360® virtual conference session, Pachel said that, clinically, there’s often more to these patients than meets the eye or shows up on a routine physical exam. “Not every case of aggression has an underlying medical cause,” he said. “And just because we identify a medical cause doesn’t automatically mean that the behavior and medical cause are related.” However, he added, the veterinary community has an opportunity—if not an obligation—to seek to understand the intersection between the clinical issues common in everyday practice and primary behavioral concerns of clients.

Where there is pain, discomfort, distress, or irritability, there may be aggression

Because it would be impossible for Pachel to cover every medical issue that could result in aggression, he focused on what he sees more than anything else. “Starting out really, really broadly, I tend to first think about anything that could be causing any degree of pain, discomfort, irritability, or distress,” he said. In a dog with aggressive tendencies in certain contexts, these clinical triggers can lower the threshold at which the animal reacts, or they may cause a dog with no history of aggression to have a fundamental behavioral change.

Pachel placed these medical causes into 4 main categories, or systems— musculoskeletal, neurological, hormonal, and gastrointestinal (GI)— and discussed the various conditions and common signs associated with each.

Musculoskeletal causes

Pachel sees these musculoskeletal issues most commonly in practice:

  • Osteoarthritis: Pachel admitted he’s biased against young pups when it comes to this diagnosis because he tends to associate it with old age. This is a mistake, he said, because that 2-year-old dog may have had previous trauma (eg, cruciate ligament rupture) at 18 months of age that predisposed the pet to the condition.
  • Pinched nerve or neurogenic pain: You could see an overlap between musculoskeletal and neurogenic pain, said Pachel.
  • He also cited tumors or cancers affecting the musculoskeletal system.

June: A case for a thorough history (and open-mindedness)

June was a 6-year-old Great Dane whose owners expressed concerns about general anxiety and aggressive behavior. On her first visit to Christopher Pachel, DVM, her behavior during the consultation was relatively unremarkable (soft body posture, comfortable exploration of the physical and social environment, settled easily, etc.) and the initial cursory physical exam was unremarkable, too. That might have contributed to an assessment of “healthy” or “no medical factors contributing to behavior changes.” But on questioning her owners as well as looking a bit more closely at her exam findings and medical history, Pachel learned that June was deaf as well as blind in 1 eye, and had a cataract in the other. She also suffered from allergies and seizures, but there were no signs of either condition during the exam. Finally, June had urinary incontinence issues that had not been responsive to medication therapies. Her owners had tried diethylstilbestrol and phenylpropanolamine. The latter had triggered an episode of hypertension, which cascaded into cardiac therapies and diagnostics. “Physically, she was a mess,” Pachel said. But with just a physical exam, he said he could have given her a relatively clean bill of health and started going in the direction of a behavioral treatment plan. Thankfully, June’s owners were able to provide Pachel with a thorough history, but what if she had just been adopted from the shelter a week earlier and then growled at someone? In such cases, Pachel tells owners that although he may not immediately see anything, that doesn’t automatically mean there’s not a medical cause. “I don’t want the client to
walk away from a routine exam thinking we’ve completely ruled out underlying medical issues as a cause,” he said. So instead of saying, “Clean bill of health. No medical issues causing aggression,” Pachel suggests keeping things open: “We haven’t identified anything yet, but we’ll continue to monitor her.”

Pachel is especially suspicious of musculoskeletal causes when he sees an acute behavior change and some degree of localizable signs (such as an overt limp, lameness, or obvious hesitance to move). However, other cases might be much more subtle or general, such as a dog with decreased muscle tone in its hind end. These must be watched carefully. “Are they weight shifting to be able to remove discomfort from that area?” he said. “They may not be limping or lame. Weight shifting and muscle asymmetry may be what we need to look for.”

Other common signs of musculoskeletal issues include reluctance to jump, difficulty getting into the car or aggression toward the person helping them into the car, increased likeliness of aggression after exercise or first thing in the morning (before they’ve had a chance to loosen up), decreased play or activity, and an inability to settle.

Regarding the last sign, Pachel referred to a patient named Abby whose owner had described the dog as restless and sometimes aggressive. Upon observation, Pachel noticed some subtle mobility issues. For example, Abby never settled enough to lie down during the appointment, and when she got herself into a corner, she looked like a bus trying to turn around in a narrow street. Abby eventually was diagnosed with an immune-mediated connective tissue disease that was not particularly well differentiated, Pachel said. Once the patient was treated appropriately, her behavior and mobility improved.

Neurological causes

When it comes to neurological causes (though some overlap with the musculoskeletal system) of aggression, Pachel most often sees the following:

  • Intervertebral disk disease
  • Lumbosacral disease
  • Degenerative myelopathy
  • Tumors or cancers
  • Cognitive dysfunction
  • Metabolic issues (eg, liver shunt)
  • Seizure disorders

Regarding shunts, Pachel said, “The vast majority of dogs that are affected by a portosystemic shunt, unless we’re seeing them in that postprandial window or they’re more severely affected and we’re getting that level [of] obtunded mentation, we’re not likely to go, ‘Hmm. I think this is a shunt animal.’ We have to look at the history to be able to see that.”

Regarding seizure disorders, Pachel said that, similar to metabolic issues, the owner may not know about it unless the dog is actively seizing, or the owner has witnessed a tonic- clonic episode with loss of bowel or bladder function. “But it needs to be on that index of suspicion,” he added.

While the signs of neurological disorders vary widely, Pachel listed weakness, pain, postural change, and an altered mental state as common.

Hormonal influences

The hormone-related issues Pachel most regularly encounters include the following:

  • Intact vs altered: Aggression can be tied to the reproductive cycle in females, and competitive aggression among intact males can also be seen, Pachel said. “Or we may see testosterone exacerbating other aggression types due to increased emotional arousal,” he added. “I’m not saying aggression is automatically caused by testosterone or fluctuations in estrogen or that all animals should be spayed or neutered, but it’s a worthy thing to explore for an individual animal to determine whether it’s a factor.”
  • Steroid and cortisol problems (eg, Cushing disease, Addison disease) may occur.
  • Thyroid dysfunction (hypo vs hyper): “For many of our patients, it’s worth doing a screening test, not only because it may be a relevant medical issue but also because some of the medicines that we reach for [clomipramine (Clomicalm; Elanco), in particular] do have a documented suppressive effect on thyroid levels,” Pachel said. “So, if I’m putting
    a dog on long-term antidepressant therapy, I want (but don’t necessarily require) a baseline thyroid reading—ideally a thyroid profile—prior to initiation of therapy.”
  • Hypothalamic-pituitary-adrenal (HPA) axis dysregulation: Pachel described this as a “catch-all” category that encompasses anything that interferes with the animal’s ability to navigate physical, emotional, or environmental stress.

Sometimes, reaching a hormone- based diagnosis requires patience. Pachel said that, on many occasions, he has suspected a patient of having an endocrine-related concern during the initial visit without being able to obtain definitive evidence during the visit. Many patients required follow-up appointments to confirm a diagnosis. He said, “It’s not always as clear-cut as, ‘Are they hypothyroid?’ It may be a more subtle shift in the early stages for some animals.... We may see indications of behavior changes before we see the classic or pathognomonic clinical exam findings.”

For Pachel, this means that if a patient is a bit hungrier than usual or has put on a few pounds, endocrine problems should not be ruled out without the support of diagnostic testing. “I also want to recognize that not every owner will approve every single test, so we don’t always have the benefit of knowing the results of all these tests from the get-go,” he added. “However, we can still include it in our discussion and have it on our differential list. We can plant the seed of possibility with the owner and follow up, even if they don’t have the interest or resources to do the testing at the beginning.”

In addition to a shift in appetite and weight gain, Pachel cited weight loss, change in overall energy, agitation or anxiety, and changes in skin, hair, and coat as common signs in this category.

GI causes

The GI system is a tricky category, Pachel said, because these many issues can cause pain and discomfort:

  • Malabsorption, maldigestion, malnutrition
  • Hunger, nausea
  • Pancreatitis, food hypersensitivity, irritable bowel syndrome
  • Intestinal parasites, bacterial overgrowth
  • Esophageal reflux, gastric ulcers • Diarrhea, constipation
  • Dental disease

If the onset of aggression occurs after a dietary indiscretion or a change in food, GI issues could be the culprit. Other common signs include stool changes (quality, odor, texture, presence of blood), pica, picky or ravenous appetite, regurgitation, vomiting, gassiness (including burping), and lip licking. Excessive licking of surfaces (eg, carpeting, furniture, walls) should also raise suspicion, Pachel said. One study ran extensive diagnostics on 19 dogs with excessive licking behavior to examine whether it could be a sign of an underlying GI issue rather than a primary behavioral concern.1 The researchers found GI abnormalities in 14 of the 19 participants and, when treated, the vast majority of these dogs experienced either clinical improvement or complete resolution of the licking behavior.

The client may need training

Even with all these potential medical diagnoses in mind, Pachel still looks at behavioral causes first, and often finds that the client needs his attention before the dog does. When getting the history, look for possible knowledge deficits you can fill. Perhaps the dog has received insufficient training or the wrong training. Maybe the behavior is a normal for the dog’s age. The client may simply need more realistic expectations regarding what is normal.

Animal behavior, regardless of the underlying cause, is complicated. So, if you’re ever uncertain about the next steps you should take with a patient, don’t hesitate to reach out to your local boarded behaviorist. And if you don’t have anyone local, Pachel says you can contact him at

Sara Mouton Dowdy is a free- lance writer and editor in Kansas City, Missouri.


  1. Bécuwe-Bonnet V, Bélanger MC, Frank D, Parent J, Hélie P. Gastrointestinal disorders in dogs with excessive licking of surfaces. J Vet Behav. 2012;7(4):194-204. doi:10.1016/j.jveb.2011.07.003
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