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Initial infection in carbuncle

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Initial Infection in Carbuncle

Causative Organism

The initial infection in a carbuncle is caused by Staphylococcus aureus - the same organism responsible for the preceding furuncle (boil). Community-acquired MRSA (CA-MRSA) strains are now the most common cause of skin and soft-tissue infections presenting to emergency departments and must be suspected in any skin abscess.
  • Medical Microbiology 9e, p. 216

How a Carbuncle Develops - The Progression

A carbuncle does not arise de novo. It follows a clear stepwise progression:
StageDescription
FolliculitisSuperficial infection of the hair follicle opening (earliest event)
Furuncle (boil)Deeper infection of the hair follicle, sebaceous gland, or sweat gland; blockage of the gland duct with inspissation of its contents predisposes to infection
CarbuncleMultiple, closely set furuncles coalesce and extend into deeper subcutaneous tissue, forming multiple communicating sinus tracts
The furuncle is the primary/initial infection unit - the carbuncle is its extension. The infected patient is often a nasal carrier of the offending S. aureus (anterior nares carriage).
  • Sherris & Ryan's Medical Microbiology, 8th Ed., p. 979

Pathogenesis of the Initial Infection

  1. S. aureus colonizes a hair follicle (or sebaceous/sweat gland)
  2. Blockage of the gland duct with inspissated secretions creates a nidus
  3. The bacteria multiply locally, producing leukocidin (Panton-Valentine Leukocidin, PVL) and other toxins that destroy neutrophils and tissue
  4. A localized abscess (furuncle) forms with a central necrotic plug
  5. When suppuration occurs in thick, inelastic skin and multiple furuncles coalesce, the carbuncle forms
  • Fitzpatrick's Dermatology, 8th Ed.

Clinical Features at Onset

  • Site: Nape of neck, back, thighs, buttocks - areas of thick inelastic skin
  • Pain: Extremely painful lesion
  • Systemic signs: Fever, malaise, and chills - these distinguish a carbuncle from a simple furuncle and indicate systemic spread via bacteremia
  • Appearance: Red, indurated area with multiple pustules draining around multiple hair follicles; central yellow-gray irregular crater develops
Carbuncle - multiple confluent furuncles draining from multiple openings at the nape of the neck
Carbuncle at the nape of neck showing multiple confluent furuncles draining pus (Fitzpatrick's Dermatology)

Predisposing Factors for Initial Infection

  • Diabetes mellitus (process is more extensive)
  • Obesity
  • Blood dyscrasias
  • Defects in neutrophil function
  • Immunosuppression (systemic glucocorticoids, chemotherapy, immunoglobulin deficiency)
  • Anterior nasal carriage of S. aureus
  • Chronic furunculosis (same strain repeatedly causing infection, as there is little acquired immunity)

Key Summary

The initial infection in a carbuncle starts in a hair follicle infected by Staphylococcus aureus. Blockage of the follicle/gland duct creates a predisposition to infection. The furuncle (boil) forms first; when multiple furuncles in thick inelastic skin coalesce and extend into subcutaneous tissue with multiple sinus tracts, the result is a carbuncle. The presence of systemic features (fever, chills, malaise) marks extension beyond the local site, reflecting bacteremia.
Sources: Sherris & Ryan's Medical Microbiology 8e | Fitzpatrick's Dermatology | Medical Microbiology 9e
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