scars

Hey everyone, I'm new to the community and I've had a couple of guinea pigs for a while now. They've always been very relaxed and fun to hold and play with, but just recently (in the last week or so) my long haired one has begun to bully and harm the short haired one and it's now showing scabs...

One of my guinea pigs is 3 and 1/2 years old and has recently acquired a large wound on her side. It is still open and has been there for 3-4 weeks and she will not let it heal. Any advice?