What are the causes of breast abscesses?

Causes of Breast Abscesses

Breast abscesses are most commonly caused by Staphylococcus aureus, with methicillin-resistant S. aureus (MRSA) becoming increasingly prevalent, particularly in lactational abscesses. 1

Classification of Breast Abscesses

Breast abscesses can be classified into two main types:

1. Lactational (Puerperal) Abscesses

   • Occur during breastfeeding
   • More likely to be caused by S. aureus (statistically significant association) 2
   • Higher rates of MRSA compared to non-lactational abscesses 1
   • Usually develop within the first 100 days after delivery (68.8% within first 60 days) 3

2. Non-Lactational (Non-Puerperal) Abscesses

   • Associated with periductal mastitis 4
   • More diverse microbiology
   • May have no growth or other organisms in culture 1

Microbiology of Breast Abscesses

Common Causative Organisms

1. Staphylococcus aureus

   • Most common pathogen (32-51.3% of cases) 2, 5
   • MRSA rates have increased significantly:
     • 8.6% of S. aureus isolates in older studies 2
     • Up to 58% of S. aureus isolates in more recent studies 5
     • 50.8% of S. aureus isolates in a 2018 study 1

2. Other Common Organisms

   • Mixed anaerobes (13.7%) 2
   • Anaerobic cocci (6.3%) 2
   • Coagulase-negative Staphylococcus (16%) 5
   • Diphtheroids (16%) 5
   • Pseudomonas aeruginosa (8%) 5
   • Proteus mirabilis (5%) 5
   • Klebsiella pneumoniae 1
   • Bacteroides species 1
   • Streptococcus species 1
   • Mycobacterium tuberculosis (rare) 1
   • Propionibacterium acnes 5
   • Peptostreptococcus anaerobius 5

Polymicrobial Nature

• Approximately 39% of breast abscesses are polymicrobial 5
• Average of 1.4 isolates per specimen 5
• Some abscesses (approximately 17-39%) may yield no growth on culture 2, 5

Risk Factors and Pathogenesis

1. For Lactational Abscesses

   • Milk stasis
   • Cracked nipples allowing bacterial entry
   • Mastitis that progresses to abscess formation
   • Typically occurs within first 3 months postpartum 3

2. For Non-Lactational Abscesses

   • Periductal mastitis
   • Obstruction of mammary ducts
   • Smoking (associated with periductal mastitis)
   • Nipple inversion or retraction

Clinical Implications

1. Antibiotic Selection

   • Traditional first-line antibiotics (amoxicillin-clavulanate) may no longer be appropriate due to increasing MRSA rates 1
   • Empiric coverage should consider local MRSA prevalence
   • Recommended empiric antibiotics:
     • Clindamycin and ciprofloxacin as first choice 1
     • All MRSA isolates in some studies were sensitive to clindamycin, trimethoprim-sulfamethoxazole, and linezolid 5
     • Only 29% of MRSA isolates were sensitive to levofloxacin in one study 5

2. Treatment Approach

   • Incision and drainage remains the cornerstone of treatment for breast abscesses 6, 7
   • Ultrasound-guided needle aspiration is an effective alternative, even for abscesses >5cm 3
   • Antibiotic therapy should be targeted based on culture results

Preventive Measures

For lactational abscesses, prevention includes:

• Proper breastfeeding techniques to avoid milk stasis
• Early treatment of mastitis
• Prompt attention to cracked or damaged nipples
• Regular breast emptying during breastfeeding

For non-lactational abscesses, management of underlying conditions like periductal mastitis is important.

Key Points for Clinicians

• The microbiology of breast abscesses has evolved, with increasing prevalence of MRSA
• Empiric antibiotic choices should reflect local resistance patterns
• Obtain cultures before starting antibiotics when possible
• Consider MRSA coverage, especially for lactational abscesses
• Drainage (either by incision or aspiration) remains essential for definitive treatment