Mastitis is inflammation of the breast tissue and most commonly affects individuals who are lactating (puerperal mastitis). Nonlactional mastitis, although rare, can also occur. Mastitis typically manifests as pain, swelling, and redness of the affected breast(s) with or without systemic signs of infection. Diagnosis is usually clinical; studies such as breast milk culture and imaging may be indicated to rule out complications (e.g., breast abscess) and differential diagnoses (e.g., inflammatory breast cancer). Puerperal mastitis may resolve with supportive therapy. Patients with severe or persistent symptoms should additionally receive empiric antibiotics.
Mastitis is defined as inflammation of the breast, with or without infection. 
- Puerperal mastitis: mastitis associated with lactation
- Mastitis not associated with lactation
- May affect subareolar ducts (periareolar or periductal mastitis) or peripheral parenchyma
- Idiopathic granulomatous mastitis (rare): recurrent or persistent mastitis, often associated with a palpable mass; most commonly affects parous women 
- Puerperal mastitis occurs in up to 10% of nursing mothers (particularly 2–3 weeks postpartum). 
- Nonpuerperal mastitis is rare (approx. 1–2% of symptomatic breast conditions). 
Epidemiological data refers to the US, unless otherwise specified.
Infectious mastitis 
- Staphylococcus aureus (most common infectious cause of puerperal mastitis) 
- Other pathogens (e.g., Streptococcus, Escherichia coli, Corynebacterium, mycobacteria)
- Idiopathic 
- Foreign body reaction (e.g., to piercings, implants) 
- Periductal mastitis is associated with cigarette smoking. 
- Peripheral nonpuerperal mastitis may be idiopathic or associated with trauma, diabetes mellitus, or immunosuppression. 
- Nipple fissures facilitate the entry of bacteria located in the nostril and throat of the infant or on the skin of the mother into the milk ducts during breastfeeding.
- Prolonged breast engorgement; (due to overproduction of milk ) or insufficient drainage of milk; (e.g., due to infrequent feeding, quick weaning, illness in either the baby or mother) result in milk stasis, which creates favorable conditions for bacterial growth within the lactiferous ducts.
- Typically localized, tender, firm, swollen, erythematous breast (generally unilateral)
- Systemic symptoms (malaise, fever, and chills)
- Pain during breastfeeding 
- Reduced milk secretion
- Reactive axillary lymphadenopathy (less common)
Inflammatory breast cancer may manifest with features similar to mastitis and should be evaluated for in patients with inadequate response to empiric treatment of mastitis. 
- Diagnosis is usually clinical.
- Diagnostic studies are indicated to evaluate for complications or alternative diagnoses in patients with atypical presentation or poor response to initial empiric antibiotic therapy.
Breast milk culture 
- Inadequate response to initial empiric antibiotic therapy.
- Severe infection
- Immunosuppressed infant or infant in critical care receiving breast milk
- Recurrent mastitis
- Clean the nipple and areola with a topical antiseptic solution.
- Using sterile gloves to express breast milk, collect a 5–10 mL sample of breast milk in a sterile container for culture.
Indications (not routinely required) 
- Poor response to empiric antibiotic therapy (e.g., within 48–72 hours)
- Exclusion of alternative diagnoses
- Evaluation for complications (e.g., abscess)
- Breast ultrasound: preferred for palpable breast lump, suspected breast abscess, or periductal mastitis 
- Mammography or digital breast tomosynthesis : preferred when inflammatory breast cancer is suspected 
Supportive findings (of mastitis) 
On breast ultrasound
- Inflamed breast parenchyma (e.g., edema, increased echogenicity)
- Inflammatory axillary lymphadenopathy
- Hypoechoic fluid collections suggestive of a breast abscess may be present.
- On mammography: ill-defined architectural distortion and localized skin thickening
Mammography is not contraindicated during lactation. Nursing or expressing breast milk before imaging improves imaging sensitivity. 
- Core needle biopsy: may be preferable for patients with imaging features suspicious for malignancy 
- Punch biopsy: may be preferable for patients with features concerning for inflammatory breast cancer 
Puerperal mastitis 
- Initiate supportive therapy.
- Consider antibiotics if no improvement after 12–24 hours. 
- For inadequate response to initial treatment or recurrence of symptoms, consider:
- Breast milk cultures and/or imaging (see “Diagnostics”)
- Referral to breast surgery (e.g., for treatment of underlying breast abscess, which requires surgical drainage)
- Severe cases (e.g., sepsis): Admit to hospital and initiate IV antibiotics.
Supportive therapy 
- Rest, adequate hydration
- Warm and cold compresses
- Consider therapeutic ultrasound for symptomatic relief. 
- NSAIDs (e.g., ibuprofen ) 
- Acetaminophen 
- Breastfeeding; upon infant demand with alternate breasts 
Patients with mastitis should continue breastfeeding to reduce the risk of a breast abscess, but feeding should only be on-demand to avoid contributing to the inflammatory process. 
Empiric antibiotic therapy for breast infections
- Oral penicillinase-resistant penicillin (e.g., dicloxacillin or flucloxacillin ) 
- OR cephalexin ) 
Risk factors for MRSA infection:
- Clindamycin 
- Or trimethoprim-sulfamethoxazole (TMP-SMX) 
- IV vancomycin 
Avoid TMP-SMX in lactating mothers with newborns < 30 days old because of the risk of kernicterus. 
Nonpuerperal mastitis 
- All patients: Initiate empiric antibiotic therapy for mastitis. 
Periductal mastitis 
- Consult surgery for recurrence or development of complications (e.g., fistula).
- Encourage smoking cessation.
- Idiopathic granulomatous mastitis: specialist referral (e.g., breast surgery) to rule out breast cancer and determine management 
- Anticipatory lactational counseling 
- To prevent recurrence: Consider oral Lactobacillus probiotic.