Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
This is an abscess which is a pocket of infection due to either a tooth root or a foreign body that has injected bacteria into this area. These are common in cows and will cause lethargy and weight loss because of the pain. The first thing to do is to start her on Penicillin G once daily that can be purchased at the farm supply. The bottle will have dosing directions.
Then, if possible , apply hot compresses to this area every 4 hrs for 3 minutes...this will help to draw this infection to the outside. This will eventually drain pus from this area and when it does flush this with peroxide daily until the pus is gone.. This will not cause her any harm as long as the pus comes out.
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