Mastitis can be treated within a single day using Castor Oil alone and I recommend that a stop is put to all Veterinary treatments that result in damage to the mare and the foal and extreme difficulty with re-establishing normal feeding.
All you need do is to massage Castor Oil onto the mares teat and leave the foal locked away from her for 2 or 3 hours before washing the teat to wash off the oil and allowing the foal to suckle. After suckling repeat the procedure two or three more times and the mastitis will be cleared.
Note there is no danger to the foal from ingesting the curd even if it has become infected, the foals stomach acid deals with this without any danger to itself.
Castor Oil is the most effective drawing agent and the above program works every time.
When it is cleared up tell your vet and hopefully we can educate the profession not to use interventionist treatment.