Fear Aggression

Stress can cause high cortisol levels that can result in aggressive behavior. Did your vet check for medical issues as the cause? I see your additional description now. We had a puppy mill dog that was afraid of everything. It took a year before she was comfortable enough with us to even make a sound, a whimper,bark, anything. We were so overjoyed when she became vocal and then her personality started peeking out. She never totally lost her fear of anything other than us and as a result of her stressful life had Cushings disease when she passed, a disease that can be caused by constant stress. We never regretted giving her our love.