My GSD mix, Lucy, has taken to grabbing my mini Brussels Griffin, Pippa, by the neck and shaking her. It's hard to get her to let go. This started back in the summer when there was a rat on the back porch. They had great fun trying to find it. Something startled Pippa and she ran out from under a chair. I guess Lucy thought Pippa was what they were hunting because she grabbed Pippa and shook her. He has done this several times since then, resulting in vet visits for Pippa. I think Lucy needs to be rehomed but my daughter doesn't want her to go. (My vet agrees.) Can this prey drive be trained out of a dog? I don't want Lucy to go, but this absolutely cannot continue. Thanks.