Impetigo

Irina Margaritescu, MD, DipRCPath

Bruce R. Smoller, MD

Key Facts

Terminology

Definition
- Acute, contagious superficial pyogenic infection of skin caused by staphylococci, streptococci, or both

Clinical Issues

Epidemiology
- Peak incidence is during summer and fall
- Face and extremities are usually involved
Nonbullous impetigo
- Thin-walled vesicles on erythematous base that rupture rapidly, forming honey-colored crusts
Bullous impetigo
- Flaccid blisters and tender shallow erosions
Typically resolves with topical and oral antibiotics

Microscopic Pathology

Nonbullous impetigo
- Subcorneal pustule with a few acantholytic cells
- Gram-positive cocci sometimes found in pustule or scaly crust
- Moderate superficial perivascular and interstitial mixed-cell infiltrate
Bullous impetigo
- Subcorneal blister with a few neutrophils and some acantholytic cells
- Rare or absent cocci
- Sparse superficial perivascular inflammatory infiltrate

Ancillary Tests

A case of impetigo in a 7-year-old girl demonstrates an erythematous patch with peripheral collarette covered by a hemorrhagic crust.

H&E section shows a subcorneal blister with neutrophils

and sparse inflammatory cell infiltrate in the dermis

in a case of bullous impetigo.

TERMINOLOGY

Synonyms

ETIOLOGY/PATHOGENESIS

Infectious Agents

Staphylococcus aureus (S. aureus)
- Gram-positive, nonmotile, non-spore-forming, catalase-positive cocci
- Produce extracellular exfoliative exotoxins (exfoliatin A and B)
Streptococcus pyogenes (S. pyogenes)
- a.k.a. group A β-hemolytic streptococci (GABHS)
- Gram-positive, nonmotile, chain-forming, non-spore-forming, oxidase- and catalase-negative cocci

Pathogenesis

Staphylococcal pyodermas
- Occur in individuals who are carriers of organism in axillary, inguinal, and perianal areas and in anterior nares
- Predisposing conditions include atopic dermatitis, diabetes mellitus, dialysis, intravenous drug use, and HIV infection
- Insect bites, dermatophytoses, herpetic infections, varicella, abrasions, lacerations, and thermal burns also contribute to pathogenesis
Group A streptococcal pyodermas
- Occur following colonization of skin from skin of another individual or from patient’s nasopharynx
Nonbullous impetigo
- Currently, S. aureus is prominent pathogen responsible for nonbullous impetigo
  - Accounts for 50-60% of cases
  - In past, S.aureus and S. pyogenes occurred with equal frequency
- 20-45% of cases are due to combination of S. aureus and S. pyogenes
- S. pyogenes is still most common cause in developing countries
Bullous impetigo
- Causative agent is gram-positive, coagulase-positive, group II S. aureus, most often phage type 71
- S. aureus exotoxins cause loss of cell adhesion in superficial dermis, producing blisters in granular cell layer of epidermis
- Desmoglein-1 is 1 of target proteins for exotoxin A

CLINICAL ISSUES

Epidemiology

Incidence
- Most common bacterial skin infection and 3rd most common skin disease among children
- Peak incidence occurs during summer and fall
Age
- Both bullous impetigo and nonbullous impetigo affect all ages but mostly affect children < 6 years old
- Bullous impetigo is more common than nonbullous impetigo in neonates and infants

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Jul 8, 2016 | Posted by drzezo in PATHOLOGY & LABORATORY MEDICINE | Comments Off

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