So long story short, my dog is now a little over 2 years old and has exhibited resource guarding issues since she was a puppy. Only with other dogs, never people. When younger, she attacked 2 dogs over a toy leaving the other dogs with a small face puncture. So now I make it my duty to inform other owners not to have toys/food around her. Fair warning. Everything was fine, prevention proved to stop those incidents. Recently at a dog park another dog with guarding issues tried to bite her when she ran by, they fought, I broke it up because the other owner was missing in action (typical). No injuries and no more dog parks for my dog anymore. I have found the pack structure confusion among all the dogs with a combination of clueless owners makes it a dangerous place for my dog. I mean bringing in one toy for your dog with 30 other dogs with no toy, possession issues are bound to occur even without history of resource guarding. Morons. Anyways my current issue is walks. I believe my dog (maybe due to her scuffles in the past or a medical reason) is now leash reactive. For those that don't know leash reactivity is leash based aggression usually you will see a dog that lunges, pulls, barks, or growls on a leash and will attack another dog, when off leash they may be fine. Mine however is a silent aggressor. After a few recent normal greets with other dogs on leash I notice she displays a frustration when trying to sniff around the other dog/whatever it is the leash pisses her off during greetings and she now tries to attack other dogs. So being the responsible owner that I am, I am avoiding leashed greetings until I can master training her out of that behavior. I have a sneaking suspicion that she has a thyroid issue which I read can cause aggression so a vet visit is imminent. But, I was wondering if anyone else deals with this type of aggression? What are your methods or thoughts on curbing this behavior? E-collars when used properly...yay or nay? What about head collars like the halti head collar, good/bad for that? Also she is from a reputable breeder who has been breeding since 1960. I picked her up at 8.5 weeks. So any thoughts in general would be appreciated. Thanks guys.

I have curbed Snoopy's leash reactivity almost 100% (excluding yellow labs for god knows what reason) using purely positive reinforcement. Aggression, especially reactivity, is fear. Using a correction (discomfort/pain) can make the problem worse because you're adding discomfort and pain to fear. So, you start with a dog that is uncomfortable with the presence of another dog, so they lash out in a fight response, as their flight response is restricted. Add a correction and now you have a dog who understands that the presence of another dog is not only uncomfortable, but physically hurts! More aggression. Using a correction for this type of behavior modification is tricky. It's very very VERY easy to screw up. Timing is everything. A poorly timed reward is much more easily forgiven than a poorly timed correction.

Does your dog know a command that means to look to you, like "watch me"? This is key, this is the foundation for the rest of the training. After that, you work on increasing the dog's threshold.

Dogs have a stress/excitement threshold: at what point will the dog react, aggressively or excited? In this case we're looking at an aggressive reaction. You need to figure out that point. I took Snoopy to Petco (hungry and with his favorite treats) on a flat collar and we stood at the edge of the parking lot, furthest from the store. Dogs were going in and out, he could see them and hear them, but we were at a good enough distance that he didn't feel the need to react to them. This is called under threshold. He's completely aware of their presence but not reactive. This is basically what I did:

Have Snoopy do some tricks, especially "watch me" -> reward, each time -> wander around a little, sniffing the grass, with some heel work -> repeat

Snoopy sees dog -> watches dog intently ->
Now this can go two ways, either an aggressive response (hackles up, growl, lip raised) or not. If the dog is already lunging and forging it's already over threshold and the stress level is to high that learning is at it's most difficult.
1. aggressive response -> I make a weird noise to catch his attention and run backwards, heavily praising when he comes with me and reward as we continue to talk around
2. looks to me when do disappears/loses interest in dog-> mark and reward repeatedly

Increasing threshold means decreasing the distance at which a stress response occurs, this is basically how that happens.

After a while sight of unfamiliar dog = GOOD things! Why be afraid of something that means I get deliciousness?

I don't let people let their dogs walk straight up to Snoopy as, even though he's not reactive, it still makes him highly uncomfortable and it probably always will, and I want to minimize any potential problems or aggressive tendencies from surfacing.

I used to be a purely compulsion trainer but I hardly use compulsive techniques at all anymore. In my opinion, it's not necessary most of the time, especially when dealing with a fear response.

My dog is dog aggressive off leash, on leash, and just hates animals of all kinds in general so much so that I have sought the help of a professional trainer. We have been doing very similar to what Caitlin has suggested. We have been pounding obedience commands into Kenzi for weeks now. The better she follows the commands the easier she has been to handle with other dogs around. She still has adverse reactions at first but settles much quicker now. We started with very calm dogs and have been working with gradually more excitable dogs in the area while we go through tricks. It's become about distraction. She needs to focus on me and what I want from her and not worry about the dogs around. I would suggest making sure you work in a controlled environment if possible. We have had some issue at the park with off leash dogs lately and those set her back training wise every time.

The hardest part is getting the dog to make that "click" where they see that they have options: fight, flight, ignore. And then reinforcing the option that we want them to choose. It's a lot of work, but worth it when you can produce a reliable dogs. Snoopy isn't to a reliable point yet, but we're working on it.

When a new dog enters the area, Snoopy will hard stare and then look at me for reassurance, and then shake off if it stressed him out enough. He now knows that when something makes him uncomfortable, he can come find me, I'm his safe place; be it other dogs, strangers, or gunshots.