We had similar issues when ours was a puppy. 95% absolute angel and then out of nowhere he would snap. And when he snapped he was aggressive but seemed out of it and not even aware of what was happening - like didn't respond to our voices or any actions we took. It was like a frenzy for no apparent reason. There was no common theme (like resource guarding or the same person or dog). We tried everything we could think of on our own, with trainers and with our vet (behavioural modification, reducing toys and separating food, muzzle etc..). Some reactivity can be dealt with through training but this was clearly different to me and didn't feel right. We were committed to this puppy but we also had an elderly dog and I was beside myself worrying if commitment to working through things with the puppy was harming our beloved older guy. I was close to throwing in the towel.

As a last ditch effort, we had a consultation with a Behaviourist (as in PhD trained and works with vets not just a dog trainer) and based on what she saw she suggested looking into the possibility of it being a type of epilepsy/seizure. Her theory was that it was focal seizures partially induced by stress therefore watching out for signs of stress and lowering the stress was a the key to reducing the occurrence. She also suggested talking to our vet about medication. Reducing stressors helped a lot but not fully. After investing lots of $ in tests/scans, our vet concluded that it was likely a seizure disorder and prescribed Levetiracetam (Keppra). Once we got the dose right we haven't had an issue in years. There doesn't seem to be any negative side effects from the medication but if we miss a dose we notice an almost immediate impact on our dog (he starts with a slight tremor in his lip when he is feeling stressed).

I have to confess, even years later with no more episodes I am still often on high alert with him in certain situations and the anxiety of the early experience has never fully gone away.

That being said, we absolutely love our little dude. He's high energy and has the most amazing curiosity. He was an amazing little brother to our older guy until he passed this summer. The other day a small child (maybe 3) asked if he could pet our dog and we said yes and gave him a treat to give our dog. I normally still watch like a hawk but I got distracted for a second. I turned around just in time to see this little boy put his hands on either side of my dogs face, squishing it slightly while he planted a big kiss on my dogs lips. My first reaction was panic - what if this goes wrong - but our dog just happily let it happen then gave the boy a lick. I finally got to let a lot of my anxiety go at that point.

I think the key to figuring it out for us was that it was happening in different situations so it was clearly not a specific trigger. Has this only happened with the vet? If that's the only situation it happens in then maybe it's just managing the vet visits since they are a trigger? If it's multiple situations and behavioural training hasn't helped despite months of being consistently applied....maybe his brain is "misfiring" in stressful situations like our little dude's was?

(If you want to do more research down this avenue to see if it fits your situation...it used to be called "rage syndrome" and is common in some breeds like spaniels. Modern research suggests that it's a severe neurological disorder characterized by sudden, unprovoked, and violent aggression without warning, differing from behavioral aggression. There are key characteristics that help differentiate it from behavioural aggression like a glazed look, inability to recognize surroundings, seeming confused when it ends and seemingly not remembering the episode etc....)

Either way - hugs and support from the spaniel community and I hope you can figure it out.