Pitbull Bites Chihuahua

Two days ago, on Friday, I was walking my two chihuahuas around my new neighborhood with my son, Dylan. It's a nice neighborhood; better than our last one. We were finishing up our walk and were about 50 yards from home when a pitbull jumped out of a moving car, ran up to Eddy, and started to bite him vigorously. I wasn't sure if the pitbull was just trying to play, or if it meant to harm Eddy. Eddy did not yike, so I figured he was okay. The pitbull's owner quickly stopped his car, ran over to his dog, and pulled him away from Eddy. The owner apologized to me and said that his dog had never done that before. I told him that it was okay because Eddy looked like he didn't get hurt. We finished walking home. Eddy was acting normal and wasn't limping. When we got into the house I inspected Eddy's body. I discovered a large and very deep gash in his body near one of his back legs. I was amazed that Eddy didn't act injured at the time, but the next day he was very sore and had to lay around resting. He'll have a big scar, I'm sure. Eddy is a funny dog because he doesn't respond as expected to physical punishment. He always interprets it as playful. I've tried to punish him by smacking his rear in the past, but he always just looks at you like you're playing. He acts the same way when attacked by other dogs. Each time that a dog has bit him (it has happened a few times, but never this bad) he behaves as though the dog is playing nicely with him - even if it results in injury!