Mastitis develops when a pathogen that typically originates in the nursing infant’s nose or pharynx invades breast tissue through a fissured or cracked nipple and disrupts normal lactation. The most common pathogen is Staphylococcus aureus; less frequently, it’s S. epidermidis or beta-hemolytic streptococci. Rarely, mastitis may result from disseminated tuberculosis or the mumps virus.

Predisposing factors include a fissure or abrasion of the nipple, blocked milk ducts, abrupt weaning or cessation of breast-feeding, and an incomplete let-down reflex, usually due to emotional trauma. Blocked milk ducts can result from a tight bra or prolonged intervals between breast-feedings.

An untreated breast infection can lead to abscess.

Usually, the patient reports a fever of 101°F (38.3° C) or higher, malaise, and flulike symptoms that develop 2 to 4 weeks postpartum (although these findings may develop at any time during lactation). Inspection and palpation typically uncover such classic signs as redness, swelling, warmth, hardness, tenderness, nipple cracks or fissures, and enlarged axillary lymph nodes.

Cultures of expressed milk confirm generalized mastitis; cultures of breast skin surface confirm localized mastitis. Such cultures also determine appropriate antibiotic treatment.

Antibiotic therapy, the primary treatment, usually consists of penicillin G to combat staphylococci; erythromycin or kanamycin is used for penicillin-resistant strains. Cephalosporin or dicloxacillin are also used. Symptoms usually subside in 2 to 3 days, but antibiotic therapy should continue for 10 days.