Mastitis is not common in mares. Most cases in mares occur at weaning time, rather than during lactation. You should follow your veterinarian's advice as to the type and course of antibiotics to give the mare. Also follow the veterinarian's advice about milking out the mare (if necessary) and allowing the foal to nurse. A weakened immune system due to stress, illness or injury may predispose the mare to mastitis and other infections. So, prevention and treatment (in addition to antibiotics) for the problem are similar. Keep the mare well-fed and in a clean, dry environment (clean, grassy pasture or clean stall or paddock). Control insects around the horses. Avoid exposure to stressful situations and provide good nursing care until she recovers from the mastitis.
