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Hello, this is Dr. Bruce and I would like to help you with Polly.
I'm sorry to hear that she is having this issue.
I would be very concerned that Polly has some sort of physical issue that has led to this behavior change. It is NOT normal for a 5 year old dog to start being aggressive when she has been sweet all along.
Common causes of behavior changes include painful conditions, loss of senses (sight, hearing, etc), and internal organ dysfunction.
I would strongly urge you to get Polly to your vet for a complete nose to tail evaluation. Make sure your vet is aware of the behavior changes, so that he or she can be very focused on checking for any painful conditions, changes in vision or hearing, or abnormal palpation of internal organs.
I would also make sure that your vet runs some tests to check for these conditions. Consider running some bloodwork and/or some x-rays.
If all the tests come back normal, then consider getting a trainer to your home to evaluate for environmental cues that are triggering this behavior.
I hope this helps and that Polly is back to her normal sweet self very soon. If you would like to continue this chat, please reply to this post and I will get back to you as soon as possible.