Years ago, when I was a senior veterinary student working as an extern with OSU’s clinical behavior service, I saw a case that stuck hard in my memory and has never faded.

The patient was a beautiful three-year-old female German shepherd I’ll call Heidi.  She was a lovely dog in most respects – friendly and gentle with people, very bright, and a quick learner.  In the consultation room, she was calm and well-mannered, approaching us readily with a wagging tail for petting and treats.

Heidi had one flaw, and unfortunately it was a serious one – she was intensely, violently aggressive towards other dogs.  She had to be brought in to the veterinary hospital through a rear entrance to avoid walking her through the lobby, and great care had to be taken to make sure she did not encounter any other dogs on her way in or out of the building.  Her owner was a petite middle-aged woman, who loved Heidi dearly and was deeply committed to her.  She had been managing Heidi’s dog aggression as best she could for a long time, but things were getting worse.

The incident that finally prompted her to seek out a veterinary behaviorist had occurred the week prior to her visit with us.  A smaller, off-leash dog belonging to a neighbor in her apartment complex had approached her and Heidi during their daily walk.  She tried to pull Heidi with her and hurry away in the opposite direction, but the other dog began running towards them and Heidi was frantic to get at it, lunging and snarling at the end of her leash.  In desperation, her owner tackled Heidi and lay on top of her, using her body weight to keep her thrashing 80 pound dog pinned to the ground until the other dog was retrieved safely by a neighbor and taken away.

The owner, in tears as she related the story to us, concluded by stating that she was certain Heidi would have killed the other dog if she had been able to reach it.

A sad story and a difficult situation, to be sure, but not unlike many other cases we saw on the behavior service.  Most aggression issues in dogs can be traced back to some combination of poor socialization, negative early experiences and/or a genetic predisposition to fearfulness, but Heidi’s case stands out for me because of something unusual in her history – her aggressive behavior had been triggered very abruptly, in response to a single traumatic incident at precisely the wrong time in her life.

According to her owner, Heidi had been friendly and playful since eight weeks of age with none of the early warning signs we so often see in fearful puppies.  Both parents had outstanding temperaments, and were friendly with both people and dogs.  She went for daily walks with her owner and regularly encountered strangers and other dogs with no problems.  So far, so good.

So what went wrong?

Heidi’s front yard had an underground electric fence paired with a shock collar, commonly known as an Invisible Fence type system.  She also had a particular dog friend that she enjoyed playing with, a young Labrador mix who belonged to a neighbor.  The dog often came into Heidi’s yard to play with her, and the two could be seen wrestling and chasing each other all around the house almost every day.

One afternoon when Heidi was around seven months old, something happened.

She and her doggy friend were playing and wrestling as usual when they inadvertently strayed too close to the boundary line.  Heidi received a shock from her collar, yelped in pain and confusion, and redirected aggressively towards the other dog – who, for his part, reacted defensively and fought back.  A full-blown dog fight erupted in the blink of an eye, and the dogs had to be physically separated to calm down.

Under most circumstances, this type of incident would be scary and unpleasant but quickly shaken off.  But for Heidi, it was life-changing.

From that point on, it was like a switch had flipped inside her head.  She lunged and snarled at the neighbor dog whenever he came near the yard, and never played with him again.  Over the next several weeks, she began reacting aggressively toward other dogs on walks, at the vet’s office, and outside the window at home.  Her owner was at a loss to understand why her previously gentle, friendly dog was suddenly behaving like Cujo.

The answer lies in a perfect storm of variables, coming together in the worst possible way.

I have discussed the socialization period for puppies in a previous post – this is a developmental phase in which puppies are learning about the world.  Things that they see and experience in a positive way during this period will be seen as normal later on.

But – and this is a big BUT – the flip side is also true.  Young dogs go through two separate “fear periods” as they grow, which are essentially times when the pup is extremely sensitive to bad experiences.

The first occurs fairly predictably at around 8-10 weeks of age.  The puppy is very young at this point and owners are (hopefully!) managing her environment carefully and exposing her to lots of great stuff for socialization purposes, so often times this first fear period passes without any obvious signs or behavior changes – many owners never notice that it has taken place.

The second is more variable, but for most dogs it occurs as a 2-3 week period in late adolescence, somewhere between 6 and 14 months of age.  This one is sneaky – it pops up when owners least expect it, long after their tiny pup has become an independent teenager.  By this point, most of us are giving our dogs more freedom and no longer micromanaging how they interact with the world.  It can be a shock, then, when something happens at this age that turns all of our assumptions upside down.

So what happens during the fear period, exactly?

You may notice that your previously friendly, confident adolescent dog becomes spooky about certain things that don’t normally bother her… perhaps she refuses to go near a new garden flag in the yard, or barks at a man with a beard who says hello to her on the street.  This sudden increase in suspicion and reactivity towards things in the environment is normal – as long as you are cheerful and don’t make a big deal of the problem, it will pass on its own and you’ll have your familiar, happy-go-lucky companion back in 2-3 weeks.

The dangerous part is this: during this particular developmental stage, your dog’s brain is on a hair trigger, exquisitely sensitive to anything “bad” that may happen.  A single frightening or painful experience during the fear period can have a lasting impact for the rest of your dog’s life.

This is a phenomenon called single event learning – meaning that it only takes one experience to result in an intense, permanent emotional reaction to the trigger that caused it.  It makes a lot of survival sense in the wild, when a young wolf needs to steer clear of a dangerous predator without requiring multiple near-death experiences to teach the lesson.  But in our pet dogs, this peculiar little “glitch” in the brain’s memory processing system can have devastating consequences.

An all-too-common example:

When my oldest boy Remy was a puppy, he had his first nail trim after coming home with me at the age of 9 weeks.  (Recall that the first fear period normally occurs from 8-10 weeks of age!)  Unfortunately, one of the nails was cut too short – he yelped and struggled, and it bled briefly.  Styptic powder was applied to stop the bleeding, the rest of the nails were trimmed without incident, and he seemed perfectly happy five minutes later.

No big deal, right?  For a well-adjusted adult dog, probably not.

However… over the next few months, nail trims with Remy became progressively more difficult.  I was careful to make sure that he wasn’t “quicked” again, and he got plenty of treats and praise during the procedure every time thereafter – but it didn’t seem to matter.  My otherwise laid-back, cooperative dog would struggle with something akin to panic as soon as the trimmers appeared.

Why?  Because his single, isolated bad experience at 9 weeks old still held more weight than all the others put together.  Single event learning is powerful stuff.

And thus, we have Heidi.  Sweet, easy-going and perfectly dog-friendly… until the day she wasn’t.

Her owner left OSU that day much encouraged, with a training plan for Heidi and a better understanding of the things that made her that way.  And Remy, as an adult, learned to stand comfortably for nail trims again after many months of careful work to overcome his fear.  Thankfully, these kinds of behavior setbacks are not insurmountable – with time and effort, adult dogs can be taught better coping skills and form new, more positive associations with things that frighten them.

So what can we do with this knowledge?

Unfortunately, we can’t keep our dogs in a bubble.  The world is an unpredictable place, despite our best efforts… we cannot always control everything that happens to them or how they may react.  But, we can try to ensure that scary experiences are few and far between during these sensitive periods of development.

And if bad things do happen – we can recognize them for what they are, and be proactive in responding and addressing the fear before things get worse.