What causes inflammatory mastitis?

Why Inflammatory Mastitis Happens

Inflammatory mastitis occurs when the mammary gland's immune system responds to triggers including bacterial invasion, tissue damage, or inflammatory signals—activating a cascade of inflammatory mediators that cause the characteristic pain, redness, warmth, and swelling. 1, 2

Primary Mechanisms of Inflammation

Pattern Recognition and Immune Activation

The inflammatory process begins when mammary epithelial cells and local immune cells detect threats through pattern recognition receptors, particularly toll-like receptors (TLRs). 3 These receptors respond to:

• Microbe-associated molecular patterns (MAMPs) from bacterial pathogens such as Staphylococcus aureus and Streptococcus species 4, 3
• Danger-associated molecular patterns (DAMPs) released from damaged mammary tissue, which can trigger inflammation even without significant bacterial infection 3

Inflammatory Cascade

Once activated, TLR4 signaling and downstream nuclear factor kappa B (NFkB) pathways drive both local mammary inflammation and systemic immune responses. 3 This cascade results in:

• Release of cytokines and inflammatory mediators that recruit leukocytes and plasma components from the bloodstream 2, 5
• Increased vascular permeability causing tissue edema, redness, and warmth 1
• Epithelial cell apoptosis and reduced milk protein synthesis, which can progress toward partial involution 3

Context-Specific Risk Factors

Lactational Mastitis

In breastfeeding women, mastitis predominantly results from: 1, 4

• Poor infant latch and improper lactation technique that causes tissue trauma and milk stasis 1
• Overstimulation of milk production from excessive pumping or aggressive breast massage, which increases tissue damage 4
• Perturbed milk flow creating an environment conducive to bacterial colonization 3

The condition typically occurs in the first 3 months postpartum, with approximately 10% incidence in the United States. 4

Non-Lactational Mastitis

In non-breastfeeding women, the etiology differs significantly: 6, 1

• Periductal mastitis and duct ectasia are the most common benign causes, particularly in women in their fourth decade of life 1
• Heavy smoking is linked to periductal inflammation and duct ectasia 7
• The condition is predominantly inflammatory rather than hormonal in nature, presenting as focal, unilateral pain often in the subareolar area or nipple region 6, 1

The Inflammation-Infection Balance

A critical concept is that most mastitis cases are driven primarily by inflammation rather than true bacterial infection. 4, 3 The severity of disease correlates more strongly with markers of inflammation than with bacterial load. 3 This explains why:

• Conservative anti-inflammatory measures (NSAIDs, ice) often resolve symptoms within 1-2 days without antibiotics 4
• Heightened host inflammatory signaling may act in concert with even commensal bacterial species to cause both inflammation and lactation insufficiency 3
• Genetic predisposition and maternal stress can amplify TLR4-mediated inflammatory responses, increasing susceptibility and severity 3

Pathophysiological Progression

The inflammatory process follows a predictable sequence: 1, 2

1. Focal breast tenderness typically precedes visible skin changes 1
2. Inflammation extends from breast parenchyma to potentially involve dermal lymphovascular structures 1
3. Clinical signs emerge: induration, erythema, warmth, and potentially fever 6, 4
4. Without adequate resolution, approximately 10% of cases progress to breast abscess formation 7

Critical Pitfall to Avoid

The inflammatory response must be tightly regulated—a precarious balance between pro-inflammatory and pro-resolving mechanisms is essential for optimal bacterial clearance without bystander damage to milk-synthesizing tissues. 2 Suboptimal or dysfunctional mammary defenses contribute to severe acute inflammation or chronic mastitis that adversely affects milk quantity and quality. 2 This is why aggressive interventions like excessive pumping, heat application, and vigorous breast massage are no longer recommended, as they worsen inflammation rather than resolve it. 4