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Mastitis in MaresBy Kentucky Equine Research Staff · August 2, 2011

Unlike cows, mares are generally not plagued with mastitis or inflammation of the mammary glands. When mastitis occurs in mares, it is a painful condition that should be treated quickly. Prompt treatment usually yields positive results and no aftereffects.

Mastitis has been diagnosed in both lactating and nonlactating mares, and while it is usually seen in mature horses, a two-month-old filly has been diagnosed. Lactating mares seem to be most susceptible when milk accumulates in the udder, such as during weaning. In one study, 44% of affected mares were lactating, 28% had weaned a foal within the previous eight weeks, and 28% had not lactated for a year or more.

Pain and swelling are the two most common signs of mastitis. The pain may be so severe that docile and well-behaved mares sometimes react violently to palpation. Mares might be so sensitive that they do not allow foals to nurse. Mastitic udders usually yield thick, discolored, or blood-tinged fluid.

A culture and antibiotic sensitivity testing of mammary secretions should be performed in all cases of mastitis. Most cases are caused by hemolytic gram-positive cocci, especially Streptococcus species, with Streptococcus zooepidemicus the most commonly reported pathogen. More than 20 different bacteria have been identified as causative agents in cases of mastitis.

Therapy for septic mastitis should include systemic and intramammary antibiotics. Many cases respond to standard doses of penicillin, due to the high probability of gram-positive pathogens. Other therapies include trimethoprim-sulfonamide combinations and aminoglycosides alone or combined with penicillin to broaden the antimicrobial spectrum against gram-negative bacteria, if needed. At least a five-day course of antibiotic therapy seems to work best.  A variety of bovine intramammary antibiotics have been used successfully in mares to increase the local concentration of antimicrobial in the udder.

Supportive therapy includes stripping (removing the milk) of the udder as often as possible if the mare will allow it, massage or hydrotherapy, and nonsteroidal anti-inflammatory drugs to decrease swelling and control fever, if present.

Response to therapy is often rapid, and unlike cows, most mares do not suffer permanent fibrosis or loss of mammary function.

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