Recurrent Mastitis

We have a 25-year-old mare that gets a case of mastitis almost like clockwork every 35 days. The mastitis begins with swelling in a teat and varies from side to side. It is sore, sensitive, and produces very little fluid. It responds to antibiotics, but takes a full 10-14 days of treatment. She had her last foal four years ago, and we have no desire to breed her again. What could be the cause?    Ms. Petterson


This 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.

About the Author

Michelle LeBlanc, DVM, Dipl. ACT

Michelle LeBlanc, DVM, Dipl. ACT, is a theriogenologist (reproduction specialist) for Rood and Riddle Equine Hospital in Lexington, Ky. She was previously a professor in equine reproduction at the University of Florida. Her interests deal with mare infertility, embryo transfer, placental infections in mares, and acupuncture in infertile mares. She owns Thoroughbred/warmblood crosses and she competes in dressage.

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