Since summer started, I have not been around the house much, so almost all of these incidents I am being made aware of after the fact. But I honestly don't know what to do. I am going to be purchasing a muzzle asap (anybody recommend a good brand?), but if anyone has any advice for behavior modification work that would be wonderful. The backstory for those of you that don't know her well: Chloe is a five year old spayed Aussie/farm collie mix. I adopted around nine weeks old; prior she was living in a barn with zero outside world exposure. I know other litter mates of hers and they all have a similar anxious disposition, but Chloe also has aggression issues. When she was a pup she had no tolerance for being handled/restrained, had severe resource aggression, reactivity towards other dogs, I couldn't groom her without her seriously biting, etc., etc. Most of those issues have been worked through now. She went through a phase of serious place guarding for awhile, but that diminished to just not liking being messed with when she was laying down and sleeping which is easy to avoid. Recent issues: Over Christmas she bit my brother in the face. She was greeting him and he started roughing up on her like he does his roommates dogs and he wrapped her in a hug - she bit him in the face and left two puncture wounds. Typically if someone is stupid enough to restrain her she is either okay with it or will just growl/air snap, so the physical contact was new. We've had an incident or two in the past few months of her growling at people when they bump in to her while she is on the sofa resting. She does the same thing when she is just laying down resting, so I assumed it was the same issue, just in a different place. New rule was that Chloe was not allowed on the furniture when guests were present just in case. It appears now she is escalating. In the past two weeks she has attacked three visitors. Now, "attack" is subjective. She has growled/snarled/snapped/lunged at three visitors - she made contact with one, didn't break skin (left two red marks on her face). All "attacks" were sudden and without warning (knowing Chloe, she probably tensed and got hard eyed a split second before). All "attacks" were also brief and probably only lasted literally a second. Kind of like when you're trimming a dog's nails and he screams and whips around at you - he doesn't continue to assault you when you GTF out of the way. First instance: My grandparents were up visiting over Memorial Day. As they were leaving, Chloe was laying down in the entry way. Grandpa reached down to pet her and she snarled and lunged for his face. No contact was made. Second instance: On of my sister's friends was sitting on the couch, watching a movie or something. Chloe was on the floor. Chloe lunged after the friend for no apparent reason. Again, no contact was made. Last night Mike, Sam, and I all went out to the house and were shooting guns most of the afternoon. When we had packed up I was letting Chloe run around in the yard on her drag line with us and she was playing with the other three dogs and having a good ol' time. I didn't notice any abnormal behavior on her part. She was interacting really well with everybody, including Sam who she had only met once before. Mike had a headache and wasn't feeling well, so we left early and I put Chloe up in the house so my family didn't have to keep an eye on her outside. Later, my brother and his girlfriend showed up for the cookout. Chloe was excited to see them and went up to greet them. Bre was just petting her (no bear hugging involved) and Chloe lunged up and bit her in the face. Contact was made, no puncture wounds, just two red marks on her cheeks. This is unacceptable. She is fine 99% of the time, but for the love of God I cannot figure out what is going through her brain when she is aggressing at people. These are people she would normally not have an issue with in situations she would normally be okay in. I absolutely cannot trust her anymore and I don't know what to do. She will now either be kept in my room when visitors are over or she will be muzzled, end of story. She absolutely HATES being kept penned up away from everybody, so I think the muzzle scenario is going to be more likely as she'll turn into an anxious mess otherwise. But I am worried she is going to keep escalating and one day someone will screw up and she is going to seriously bite someone. I do not want that happening. And although Mike and I won't be living together for another year, when I move out in August she is going to start being around him almost all the time and he is going to be helping me out with potty breaks and such for her. Although she has never shown any issue towards him at all, he's not me or my sister and we are the only two people she has ever not aggressed randomly towards. I'm worried that Mike's going to trigger her in some way, especially since he is used to his dogs that let him do anything to them, and she is going to bite him. She's put her teeth on him once before (he went to pet her, I think, while she was resting - I was upstairs and not around), but he wasn't concerned about it because she didn't do anything to him. However, if she's escalating, the next time it will be a bite and Mike's initial response is going to be a Come to Jesus meeting....which is not an effective method with Chloe, me having done so before. My sister and I had talked briefly about me actually giving her Chloe (she takes care of her when I'm gone and Chloe is just as bonded to my sister as she is to me, if not more so), but I don't know if that is such a good idea now. JeanAnne is responsible with her and would manage her properly, but I don't think it would be fair or wise to have a 17 year old be the sole owner of an aggressive dog. But, then again, Chloe would also be kept in the environment she grew up in with people she knows and who know her, which might be beneficial versus moving into an apartment with me and suddenly being in a new, small area and constantly being on the go, adapting to three other dogs and a new second "owner", with other moves in the near future being a high probability. Gah, I just don't know. Thoughts, comments, advice, behavioral modification plan ideas? I love this dog and she is brilliant when she's around my sister or I, but she is making it very difficult to own her right now.