NTTB: pet aggression advice?

[kirstinb]

Hello.  I have a 1 year old Pitbull/Terrier mix (Mom was rescued and gave birth after...had him since he, Emmitt, was 3 months old).  Ever since we adopted him, wnenever we take him for walks he barks uncontrollably and aggressively darts toward any dog or person we encounter.  We had a trainer and nothing really seemed to help.  He didn't subscribe to the "clicker" style of training, and wanted us to use some kind of different leash that you place over their head (sorry...forgot the name).  It didn't seem to "train" him.  Flash forward to yesterday.  Emmitt goes to a doggie daycare during the week because of the hours DH and I work.  Tons of dogs there...around 20-25 daily...in a very large space with lots of different rooms indoor and out.  Emmitt has always been one of their favorites.  He is the most loving, snuggly dog you will ever meet.  Always wants to be touching you or right beside you.  Can't help but fall in love with him.  But yesterday an incident happened at daycare.  Apparently a bunch of dogs were playing and sort of ganging up on one dog and Emmitt gladly joined them.  When the workers started yelling at the dogs, all backed off except for Emmitt.  He bit through the other dogs face and ear and refused to let go.  One of the young workers who adores him, had to keep hitting Emmitt to get him to let go and actually broke her wrist in the process.  They say they have seen a few small bursts of aggression but nothing major so were surprised by this.  
I realize I am overgeneralizing, but is this fixable or do we just have an "aggressive dog" and there's not much we can do about it.  What are usually the causes of this?  Is it something we have done by letting him snuggle too much?  He's the only pet at home.

[osulori]

I would suggest getting him checked out by your vet since yesterday's behavior is new (behavior changes can be indicative of a health issue), and contacting a behaviorist (not a trainer) ASAP to help you get to the root of his behavior (it sounds like he's leash reactive), why it's happening, and what you can do about it. Your vet should be able to give you some referrals.