Every so often we'll have someting similar on one of our dairy cows....maybe once every 2-3 years and we have ~250 head milking and dry. All we have ever done is make a hole and let the pus drain out. Stinks like hell, I will try to massage it to work all the bad stuff out, maybe even again the next day or two. Have never had one turn into anything serious, even a frail dainty dairy cow isn't bothered. I can't remember ever even having to give antibiotics, I would if the cow had a fever.