AThis could be due to one of a few different problems. The rule-outs should include bacterial mastitis, an idiopathic (unknown cause) hormonally induced mastitis, or a hormonal or hormone-responsive tumor. Before the next course of antibiotics, have your veterinarian collect a sample of mammary fluid for cytology and bacterial culture. This will determine if there is inflammation or infection. At this time, a manual palpation of the udder and possibly an ultrasound scan might help determine the cause.

One of the difficulties in diagnosing a mammary tumor such as adenocarcinoma is the large amount of inflammatory reaction associated with it. A core biopsy might show up as necrotic inflammation until it is very advanced. At that stage, it has often metastasized to the lymph nodes.

An idiopathic hormone-responsive condition associated with the estrous cycle might respond to Regumate therapy. If bacterial cultures are negative, I would not recommend using antibiotics. A strictly inflammatory, non-infectious, condition might respond to a corticosteroid such as Azium