Infectious skin diseases

Bacterial Infections - Bugs on the Surface

• Impetigo

  • S. aureus (most common) or S. pyogenes.
  • Superficial pustules → rupture, forming a characteristic “honey-colored crust”.
  • Highly contagious, common in children.
  • Bullous type caused by exfoliative toxin A (S. aureus).

• Erysipelas

  • Superficial cellulitis involving upper dermis and lymphatics; primarily S. pyogenes.
  • Presents as a well-demarcated, raised, erythematous plaque with a "peau d'orange" texture.
  • Commonly affects the face and lower extremities.

• Erythrasma

  • Corynebacterium minutissimum.
  • Infects intertriginous zones (e.g., groin, axillae).

  ⭐ On Wood's lamp examination, the affected skin shows a characteristic coral-red fluorescence due to coproporphyrin III production.

Fungal Infections - Fungi Among Us

• Superficial Mycoses
  • Tinea Versicolor: Malassezia spp. Hypopigmented macules on the trunk; lipid-dependent yeast.
  • Dermatophytes (Tinea): Infections of keratinized tissues (skin, hair, nails). e.g., Tinea corporis (ringworm). 📌 Mnemonic: Try My Epidermis (Trichophyton, Microsporum, Epidermophyton).
• Subcutaneous Mycoses
  • Sporotrichosis: Sporothrix schenckii. Follows lymphatic drainage after trauma ("rose gardener's disease").
• Opportunistic Mycoses
  • Candida albicans: Satellite pustules in moist, intertriginous areas.
• Diagnostics
  • KOH Prep: Standard initial test to visualize hyphae/yeast.
  • Wood's Lamp: Microsporum fluoresces blue-green; Malassezia shows yellow-gold fluorescence.

⭐ On KOH microscopy, Malassezia furfur classically displays a “spaghetti and meatballs” pattern (hyphae and yeast).

Viral Infections - Viruses Gone Viral

• Herpesviridae (HSV, VZV):

  • Painful vesicles on an erythematous base.
  • Microscopy: Multinucleated giant cells with nuclear molding and intranuclear inclusions (Cowdry A bodies).
  • Tzanck smear is a rapid diagnostic test.

• Human Papillomavirus (HPV):

  • Verrucae (warts) characterized by epidermal hyperplasia.
  • Hallmark: Koilocytes (keratinocytes with perinuclear halos and wrinkled nuclei).

• Poxviridae:

  • Molluscum Contagiosum: Flesh-colored, umbilicated papules.
  • Histology: Intracytoplasmic eosinophilic inclusions (Henderson-Patterson bodies).

⭐ In adults, widespread molluscum contagiosum can be a sign of cellular immunodeficiency (e.g., HIV).

Parasitic Infestations - Unwanted Tiny Tenants

• Scabies (Sarcoptes scabiei)

  • Intensely pruritic burrows, especially in finger webs, wrists, axillae; worse at night.
  • Histo: Mites, eggs, or fecal pellets (scybala) in stratum corneum with superficial perivascular infiltrate containing eosinophils.
  • 

• Pediculosis (Lice)

  • Pediculus humanus (head/body), Pthirus pubis (pubic).
  • Nits (eggs) attached to hair shafts; pruritus from saliva.

• Cutaneous Larva Migrans

  • Caused by dog/cat hookworm larvae (Ancylostoma spp.).
  • Serpiginous, erythematous, pruritic tracks ("creeping eruption").

⭐ Crusted (Norwegian) Scabies: Hyperkeratotic plaques teeming with thousands of mites, seen in immunocompromised or neurologically impaired patients. Extremely contagious.

• Impetigo, caused by S. aureus or Strep. pyogenes, presents with classic "honey-crusted" lesions.
• Verrucae (warts) from HPV are characterized by epidermal hyperplasia and koilocytosis.
• HSV/VZV infections show multinucleated giant cells and intranuclear inclusions on Tzanck smear.
• Molluscum contagiosum (poxvirus) features pathognomonic intracytoplasmic eosinophilic inclusion bodies (molluscum bodies).
• Scabies involves mites burrowing in the stratum corneum, causing intense pruritus, worse at night.
• Fungal infections like tinea are diagnosed using KOH prep or visualized with PAS stain.