Our chow mix has lived with her best friend, a border collie for 10 yrs. They love each other. Chow (angel) a female has been spayed, and border collie (bandit) a male has been neutered. They hate to be separated so they never have been. A couple weeks ago, for no reason Angel started attacking Bandit. We yelled at her and tried to get her off of him but she would not stop. We noticed Bandit was bleeding so my husband pulled Angel off and I routed Bandit out of pen into the house. Angel barked all night. Last week, due to cold weather we brought them into the garage. Angel started again, just aggressively attacking Bandit. Bandit started yelping and this made her madder. We actually had to pull her off him and put a leash around her neck really tight to get her out of the garage. Now Bandit is really hurt and doesn't want to go arounf Angel, yet she will bark and cry all night long because he isn't with her. Has she lost her mind or what?
A sudden behavior change like that in an a11 year old suggests a medical reason. It's not normal behavior for a dog to suddenly attack another dog it has lived with for a considerable period of time. - dogs just don't normally do that either without some eliciting event that caused their relationship to change or because the dog has a medical problem.
Get her to your regular veterinarian to start, and s/he may even recommend a neurologist.
And if you aren't already, the dogs now have to be separated until you can get help. Your border collie's safety should be your primary concern.
Your chow can still be attached to the other dog - hence the whining - but still be motivated to attack him as well.
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