• Mastitis is an inflammation of the breast parenchyma and possibly associated tissues (areola, nipple, subcutaneous [SC] fat).
  • Usually associated with bacterial infection (and milk stasis in the postpartum mother)
  • Can be lactational or nonlactational
  • Usually an acute condition but can become chronic cystic mastitis


  • Predominantly affects females
  • Mostly in the puerperium; epidemic form rare in the age of reduced hospital stays for mothers and newborns
  • Neonatal form
  • Posttraumatic: ornamental nipple piercing increases risk of transmission of bacteria to deeper breast structures; Staphylococcus aureus is the predominant organism.


  • 3–20% of breastfeeding mothers develop nonepidemic mastitis, with greatest incidence among breastfeeding mothers 2 to 6 weeks postpartum.
  • Neonatal form occurs at 1 to 5 weeks of age, with equal gender risk and unilateral presentation.
  • Pediatric form occurs at or around or after puberty, with 82% of cases in girls.

Etiology and Pathophysiology

  • Microabscesses along milk ducts and surrounding tissues
  • Inflammatory cell infiltration of breast parenchyma and surrounding tissues
  • Nonpuerperal (infectious) S. aureus (including methicillin-resistant S. aureus [MRSA]), Bacteroides spp., Peptostreptococcus, Staphylococcus (coagulase negative), Enterococcus faecalis, Histoplasma capsulatum, Salmonella enterica, rare case of Actinomyces europaeus
  • Puerperal (infectious) S. aureus (including MRSA), Streptococcus pyogenes (group A or B), Enterobacteriaceae, Corynebacterium spp., Bacteroides spp., Staphylococcus (coagulase negative), Escherichia coli, Salmonella spp. (1)
  • Rare secondary site for tuberculosis in endemic areas (1% of mastitis cases in these areas): single breast nodule with mastalgia
  • Tuberculosis mastitis in nonendemic areas has also been reported in patients with exposure to TNF-α inhibitors and other immunomodulating compounds (2)[C].
  • Corynebacterium spp. associated with greater risk for development of chronic cystic mastitis
  • Granulomatous mastitis
    • Idiopathic
      • Predilection for Asian and Hispanic women
      • Association with α1-antitrypsin deficiency, hyperprolactinemia with galactorrhea, oral contraceptive use, Corynebacterium spp. infection, and breast trauma
      • Most women have a history of lactation in previous 5 years.
      • New cases have been reported in male-to-female transgender patients in setting of exogenous progesterone and estrogen treatment (3)[B].
    • Lupus; autoimmune
  • Puerperal
    • Retrograde migration of surface bacteria up milk ducts
    • Bacterial trapping behind plugged milk in the ductal outflow tracts
    • Bacterial migration from nipple fissures to breast lymphatics
    • Secondary monilial infection in the face of recurrent mastitis or diabetes
    • Seeding from mother to neonate in cyclical fashion
  • Nonpuerperal—a variety of causes including: ductal ectasia, breast carcinoma, inflammatory cysts, chronic recurring SC or subareolar infections, parasitic infections (Echinococcus, filariasis, guinea worm in endemic areas), herpes simplex, cat-scratch disease, and, in older patients, smoking. Lupus is a rare cause.

Risk Factors

  • Breastfeeding
  • Milk stasis: inadequate emptying of breast (scarring due to previous breast surgery [breast reduction, biopsy, or partial mastectomy], scarring of breast due to prior mastitis), breast engorgement: interruption of breastfeeding, milk oversupply, plugged ducts
  • Nipple trauma increases risk of transmission of bacteria to deeper breast structures: S. aureus predominant organism.
  • Neonatal colonization with epidemic Staphylococcus
  • Neonatal—occurs more commonly in bottle-fed babies; may be related to manual expression of “witch’s milk” and can lead to lethal necrotizing fasciitis
  • Maternal diabetes
  • Maternal HIV
  • Smoking

General Prevention

Regular emptying of both breasts and nipple care to prevent fissures when breastfeeding; also good hygiene including hand washing and washing breast pumps after each use

Commonly Associated Conditions

Breast abscess

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