Mastitis and Breast Abscess

• Lactational mastitis typically arises from milk stasis with nipple trauma providing a portal of entry. The usual organism is Staphylococcus aureus. It presents with a unilateral, wedge-shaped, tender, erythematous, warm area with fever and malaise.
• Continued emptying of the breast is part of the treatment, not a risk to the infant. Stopping worsens stasis and promotes abscess formation.
• Breast abscess is suggested by a fluctuant mass or failure to improve on appropriate antibiotics, is confirmed by ultrasound, and is treated with drainage — needle aspiration for many, incision and drainage for larger or loculated collections.
• Non-lactational and subareolar abscesses are associated with smoking and often involve anaerobes, and tend to recur.
• Red flag: erythema and peau d'orange without fever, or an inflammatory picture not responding to antibiotics, requires evaluation for inflammatory breast carcinoma with skin punch biopsy.

Infectious causes by setting

• Staphylococcus aureus: the dominant pathogen in lactational mastitis and abscess, entering through cracked or fissured nipples. Community-acquired MRSA is now a common abscess isolate in the US and should be assumed when an abscess forms or when empiric beta-lactam therapy fails.
• Streptococci (group A and B) and coagulase-negative staphylococci: less common; streptococcal disease tends to produce diffuse rather than wedge-shaped cellulitis.
• Mixed anaerobes with skin flora (Bacteroides, Peptostreptococcus, Prevotella): characteristic of non-lactational subareolar/periductal infection, hence the foul-smelling drainage and recurrence.
• ***Corynebacterium* species** (notably C. kroppenstedtii): linked to granulomatous lobular mastitis, which mimics infection and malignancy.

Modifiable risk factors (what the stem plants)

• Ineffective or infrequent milk removal: poor latch, nipple shields, missed feeds, abrupt weaning, tight bras or restrictive clothing, and oversupply — all produce milk stasis, the initiating event.
• Nipple trauma/fissures: the portal of entry; often from a shallow latch or improper pump flange fit.
• Cigarette smoking: the single strongest association with non-lactational subareolar abscess and periductal mastitis, because toxins damage the lactiferous duct epithelium.
• Nipple piercing: introduces skin flora directly into subareolar ducts.
• Maternal fatigue, stress, and poor nutrition: reduce host defence and feeding frequency.

Non-modifiable / host factors

• First few weeks postpartum, especially weeks 2–3, when latch is still being established — the classic timing.
• Prior mastitis or prior abscess: strongly predicts recurrence.
• Diabetes, obesity, and immunosuppression: impair neutrophil function and favour abscess.
• Nipple inversion and ductal ectasia: predispose to subareolar stasis and Zuska disease.

• Milk stasis is the initiating insult: incomplete or infrequent drainage raises intraductal pressure. Stagnant milk is a protein- and lactose-rich culture medium, and the pressure itself forces milk constituents across the ductal epithelium into surrounding stroma.
• Chemical (inflammatory) mastitis precedes infection: extravasated milk proteins act as foreign antigens, recruiting neutrophils and cytokines. This explains why early mastitis can present with fever and a tender wedge of breast without a true pathogen, and why some cases resolve with drainage and anti-inflammatories alone.
• Bacterial seeding: a nipple fissure or a damaged duct lining lets skin colonisers — chiefly S. aureus — ascend the lactiferous duct into the stagnant milk, where they replicate rapidly.
• Lobar anatomy explains the shape: infection spreads along a single duct system and its lobule, producing the classic unilateral, wedge-shaped erythema pointing toward the nipple rather than diffuse involvement.
• Systemic signs come from cytokines, not bacteraemia: absorbed IL-1, IL-6 and TNF-α produce high fever, rigors, myalgias and a flu-like prodrome out of proportion to the local findings.
• Abscess formation: S. aureus coagulase and alpha-toxin cause fibrin deposition and tissue necrosis, walling off a purulent cavity. Antibiotics penetrate this avascular collection poorly, which is why failure to improve after 48–72 hours of appropriate therapy implies pus that needs drainage.
• Subareolar (non-lactational) disease has a different chain: smoking-related toxins induce squamous metaplasia of the lactiferous ducts; keratin plugs obstruct the duct, it dilates, ruptures, and anaerobic-rich flora seed the periareolar tissue. Repeated rupture creates a chronic mammary duct fistula at the areolar margin — Zuska disease.
• Inflammatory breast carcinoma mimics all of this by a wholly different route: tumour emboli obstruct dermal lymphatics, causing erythema, oedema and peau d'orange without pus, without fever, and without response to antibiotics.

Local findings (from lobar inflammation)

• Unilateral, wedge-shaped erythema with warmth, firm induration and exquisite tenderness — the wedge follows one duct-lobule unit and points toward the nipple.
• Nipple fissure or cracking on the affected side: the portal of entry.
• Breast engorgement proximal to the involved segment, reflecting the underlying stasis.
• Fluctuant, well-demarcated mass with overlying thinning skin: indicates abscess rather than simple mastitis. Purulent nipple discharge may be expressible.

Systemic findings (cytokine-mediated)

• Fever ≥38.5°C, rigors, myalgias, malaise — a flu-like illness in a postpartum woman is mastitis until proven otherwise.
• Tachycardia and, less commonly, ipsilateral axillary lymphadenopathy from regional drainage.

Non-lactational/subareolar pattern

• A periareolar tender mass in a smoker, often a woman in her 30s–40s who is not breastfeeding, with foul-smelling discharge (anaerobes), nipple retraction from ductal fibrosis, and a history of prior drainage. A draining sinus at the areolar edge is the mammary duct fistula of Zuska disease.

Red-flag presentation — inflammatory breast carcinoma

• Diffuse erythema covering a large portion of the breast, **skin oedema with *peau d'orange*, nipple retraction, a firm non-fluctuant breast, little or no fever, and no improvement after a full antibiotic course**. Typically an older, non-lactating woman.

Step 1 — Mastitis is a clinical diagnosis

• Focal unilateral erythema, warmth and tenderness plus systemic symptoms in a lactating woman is sufficient to begin treatment. No imaging or culture is required for uncomplicated first-episode lactational mastitis.

Step 2 — When to culture

• Expressed milk culture and sensitivities are recommended for severe or hospital-acquired infection, recurrent mastitis, failure of first-line antibiotics, or a penicillin-allergic patient in whom the choice depends on susceptibility. Purulent aspirate from an abscess should always be sent for Gram stain and culture, because it drives MRSA versus MSSA therapy.

Step 3 — Imaging when abscess is suspected

• Ultrasound is the initial and confirmatory test (consistent with ACR Appropriateness Criteria for a palpable breast abnormality in a young or lactating woman): it is radiation-free and shows a hypoechoic collection with posterior acoustic enhancement, internal debris and swirling movement on compression, often with a hyperaemic rim on Doppler. Cellulitis alone shows only skin thickening and increased echogenicity of fat without a drainable cavity.
• Ultrasound also guides aspiration and documents cavity size, which determines aspiration versus incision and drainage.

Step 4 — When infection does not fit

• Any inflammatory breast picture that fails to resolve after an appropriate antibiotic course (roughly 48–72 hours for improvement, days for resolution) requires evaluation for malignancy.
• Full-thickness skin punch biopsy of the involved skin is the confirmatory test for inflammatory breast carcinoma, demonstrating tumour emboli in dermal lymphatics; a core biopsy of any underlying mass establishes histology and receptor/HER2 status. Diagnostic mammography with ultrasound should accompany this in a non-lactating woman or any woman over 30.
• Persistent granulomatous inflammation on biopsy with negative routine cultures suggests idiopathic granulomatous mastitis or Corynebacterium infection.

Foundation of therapy (Academy of Breastfeeding Medicine Protocol #36)

• Continue breastfeeding or effective milk removal from the affected breast — this relieves the stasis that drives the disease and is safe for the infant, including with MRSA. Do not advise weaning; abrupt cessation worsens stasis and promotes abscess.
• Supportive measures: cold compresses, adequate hydration and rest. ABM's 2022 protocol cautions against aggressive deep breast massage and over-pumping, which increase tissue oedema and oversupply.
• NSAIDs: ibuprofen for pain and inflammation; acetaminophen as adjunct. Both are compatible with lactation.

First-line antibiotics (anti-staphylococcal, per IDSA skin and soft tissue infection principles)

• Anti-staphylococcal penicillin — dicloxacillin, or a first-generation cephalosporin — cephalexin, for 10–14 days when symptoms persist beyond 24 hours of improved milk removal or the patient is systemically ill.
• Penicillin allergy: cephalexin is acceptable for non-anaphylactic reactions (true cross-reactivity is roughly 1–3%, driven by shared R1 side chains); clindamycin if severe allergy.
• Suspected MRSA (abscess, prior MRSA, no improvement): trimethoprim-sulfamethoxazole or clindamycin. Avoid TMP-SMX in the mother of an infant under about one month, a jaundiced or preterm infant, or G6PD deficiency.
• Severe/inpatient disease: IV vancomycin, dosed to a 24-hour AUC targeting AUC/MIC 400–600 per the 2020 IDSA/ASHP consensus — not a 15–20 mcg/mL trough.

Abscess — drainage is definitive

• Ultrasound-guided needle aspiration (repeat as needed) for most collections; incision and drainage for large, loculated, or skin-compromised abscesses. Antibiotics alone will not cure a walled-off collection.

Of the disease

• Breast abscess: the most common complication, from coagulase-mediated walling-off of necrotic tissue. Signalled by a fluctuant mass or failure to defervesce after 48–72 hours of appropriate antibiotics. Requires drainage, not more antibiotics.
• Sepsis and bacteraemia — an emergency: fever with hypotension, tachycardia and altered mentation. S. aureus bacteraemia mandates IV therapy and a search for endocarditis or metastatic seeding.
• Toxic shock syndrome — an emergency: toxin-mediated diffuse macular erythroderma, hypotension and multiorgan involvement with later desquamation.
• Necrotising soft tissue infection — an emergency: pain out of proportion, crepitus, bullae, dusky skin; needs immediate surgical debridement, not aspiration.
• Premature weaning and low milk supply: pain and misinformation lead to cessation; the resulting stasis paradoxically raises recurrence risk.
• **Recurrent subareolar abscess and mammary duct fistula (Zuska disease)**: from persistent squamous metaplasia in a smoker; a chronically draining periareolar sinus signals it and requires duct excision plus smoking cessation.
• Missed inflammatory breast carcinoma — the most consequential error: dermal lymphatic tumour emboli produce erythema and peau d'orange that are treated as infection for weeks while the disease progresses. Any non-resolving inflammatory breast warrants skin punch biopsy.

Of treatment

• Milk fistula: incision or biopsy through a lactating duct creates a persistent milk-draining tract; favours ultrasound-guided aspiration over open incision when possible.
• Scarring, cosmetic deformity and nipple distortion after incision and drainage; the periareolar approach mitigates this.
• Antibiotic-associated candidiasis of the nipple/areola: burning shooting pain with shiny or flaking nipple skin after a course of therapy, with oral thrush in the infant.
• ***Clostridioides difficile* colitis**, particularly with clindamycin.
• Infant effects of maternal drugs: TMP-SMX risks hyperbilirubinaemia/kernicterus in the neonate under about one month and haemolysis with G6PD deficiency.
• Vancomycin nephrotoxicity when AUC-guided dosing is exceeded.

• The classic vignette: a primiparous woman 2–6 weeks postpartum with a cracked nipple, fever, rigors, and a unilateral wedge-shaped area of erythema and tenderness. Organism: Staphylococcus aureus.
• Best next step is almost always "continue breastfeeding and start dicloxacillin or cephalexin." Stopping breastfeeding is the most frequently chosen wrong answer — it worsens stasis and causes abscess. Milk from the infected breast is safe for the infant.
• Fluctuant mass or no improvement after 48–72 hours of antibiotics → get a breast ultrasound, then needle aspiration (or incision and drainage for large/loculated collections). Antibiotics cannot sterilise a walled-off cavity.
• **Erythema plus peau d'orange without fever, or an "infection" that will not resolve → skin punch biopsy looking for tumour emboli in dermal lymphatics** (inflammatory breast carcinoma). This is the single association examiners test most.
• **Smoker + recurrent periareolar abscess + draining sinus at the areolar edge = *Zuska disease* (subareolar/periductal mastitis) from squamous metaplasia of lactiferous ducts**, anaerobe-rich, treated with amoxicillin-clavulanate, smoking cessation, and duct excision for recurrence.
• Distinguish from engorgement: engorgement is bilateral, diffuse, in the first days postpartum, without focal erythema or high fever.
• Antibiotic pitfalls: cover MRSA with TMP-SMX or clindamycin if abscess or treatment failure — but avoid TMP-SMX with a neonate under about one month, jaundice, or G6PD deficiency. Cephalexin remains reasonable in non-anaphylactic penicillin allergy (cross-reactivity ~1–3%, side-chain driven).
• Post-treatment burning nipple pain with infant thrush = candidal infection after antibiotics, not recurrent bacterial mastitis.