Chapter 18 – Skin Infections Flashcards

Staph. aureus Infections

• Folliculitis & Abscess Formation:
  • Causative Agent: Staphylococcus aureus (Gram-positive).
  • Signs & Symptoms: Red, swollen bumps; pus; fever; cellulitis.
  • Pathogenesis: Attachment to hair follicle leads to infection spreading into sweat glands, causing inflammation. Virulence factors include coagulase (clots), exfoliative toxin, hyaluronidase (penetration), Protein A (blocks phagocytosis), and TSST (superantigen).
  • Epidemiology: Direct (skin-to-skin) and indirect (fomites) contact; high risk in athletes, wounds, or preexisting infections.
  • Prevention & Treatment: Drain lesion, culture wound, prescribe antibiotics like Vancomycin or Doxycycline if culture is positive. Prevention includes handwashing, careful handling of bandages, and not sharing towels/razors/needles.
  • Complications: Can lead to systemic infection → osteomyelitis.

Scalded Skin Syndrome

• Causative Agent: Staphylococcus aureus.
  • Signs & Symptoms: Reddened skin that peels in sheets.
  • Pathogenesis: Exfoliative toxin separates layers of epidermis.
  • Epidemiology: Newborns, elderly, immunocompromised individuals; often via umbilical cord infection.
  • Prevention & Treatment: Antibiotics. No vaccine available.
  • Complications: Secondary infections: Pseudomonas, Candida.

Flesh-Eating Streptococcal Infection (Necrotizing Fasciitis)

• Causative Agent: Streptococcus pyogenes (Group A, β-hemolytic).
  • Signs & Symptoms: Skin swelling, muscle pain, high fever, confusion, shock within 24 hrs.
  • Pathogenesis: Exotoxin A (superantigen → shock), M protein (blocks phagocytosis), proteases destroy tissue, hemolysins and streptolysins (lyse RBCs and neutrophils), hyaluronidase (tissue spread), streptokinase (dissolves clots).
  • Epidemiology: Sporadic; predisposing factors include diabetes, cancer, AIDS, surgery; transmission via minor skin trauma or needles.
  • Prevention & Treatment: Surgical debridement + high-dose penicillin. Prevention: Proper wound care.
  • Complications: High mortality rate (>25%). Can cause cellulitis, myositis, necrotizing fasciitis, and streptococcal TSS. 11,000 US cases/year.

Chickenpox

• Causative Agent: Varicella Zoster Virus (VZV).
  • Signs & Symptoms: Fever, rash (vesicles) on face/trunk; can become severe with pneumonia.
  • Pathogenesis: Virus enters respiratory tract → multiplies → viremia → skin/nerve cells; virus remains latent in nerves.
  • Epidemiology: Common; highly contagious; dangerous for pregnant women (vertical transmission).
  • Prevention & Treatment: Vaccine: Varivax (live attenuated); 2 doses (12-15 months, 4-6 years). Treatment: Antivirals like Acyclovir (early use).
  • Complications: Can reactivate later as shingles; risk of pneumonia, especially in adults.

Shingles (Herpes Zoster)

• Causative Agent: Reactivation of latent VZV.
  • Signs & Symptoms: Painful vesicles localized to a dermatome (belt-like distribution).
  • Pathogenesis: Latent VZV reactivates in sensory nerve ganglia.
  • Epidemiology: Older adults (>50), immunocompromised.
  • Prevention & Treatment: Vaccines: Zostavax (live attenuated booster), Shingrix (subunit vaccine—preferred for >50 years). Treatment: Antivirals like Acyclovir/Valtrex (within 24-48 hrs).
  • Complications: Can lead to post-herpetic neuralgia (chronic pain).

Pseudomonas aeruginosa infection

• Causative Agent: Pseudomonas aeruginosa (Gram-negative, biofilm-former).
  • Signs & Symptoms: Opportunistic: Burn wound infections, hot tub dermatitis, swimmer’s ear.
  • Pathogenesis: Toxins impair healing, Exotoxin A necrosis, pyocyanin (blue-green pigment, siderophore - iron collection).
  • Epidemiology: Found in water, soil, flower vases, disinfectants; burn patients, cystic fibrosis patients.
  • Prevention & Treatment: Treatment: Quinolones; very antibiotic-resistant. Prevention: Good wound care and hygiene.
  • Complications: Chronic infections, especially in immunocompromised individuals.