Infections of the Skin, Muscles, and Soft Tissues¶
Chapter 134 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Skin and soft tissue infections (SSTIs) are common globally, with rising incidence due to MRSA, natural disasters, and trauma.
- Erysipelas and cellulitis are distinct infections: erysipelas is a superficial streptococcal infection with well-defined margins, while cellulitis is a diffuse bacterial infection.
- Necrotizing fasciitis is a life-threatening infection requiring immediate surgical debridement and broad-spectrum antibiotics.
- Viral infections like herpes simplex, varicella, and molluscum contagiosum present with vesicles, bullae, or crusted lesions.
- Treatment varies by infection type, with MRSA requiring vancomycin or linezolid, and necrotizing fasciitis needing clindamycin + cephalosporin.
1. DEFINITION & OVERVIEW¶
Skin and soft tissue infections (SSTIs) encompass a wide range of bacterial, viral, fungal, and parasitic infections affecting the epidermis, dermis, subcutaneous tissues, muscles, and fascia. The epidermis acts as a mechanical barrier, but disruptions (e.g., abrasions, burns) allow pathogens to invade deeper structures. SSTIs range from mild (e.g., impetigo) to severe (e.g., necrotizing fasciitis).
Table 134-1 Skin and Soft Tissue Infections¶
| LESION, CLINICAL SYNDROME | INFECTIOUS AGENT(S) | SEE ALSO CHAP(S) | |
|---|---|---|---|
| Vesicles | Smallpox | S4 | |
| Vesicles | Chickenpox | 198 | |
| Vesicles | Shingles (herpes zoster) | 198 | |
| Vesicles | Cold sores, herpetic whitlow, herpes gladiatorum | Herpes simplex virus | 197 |
| Vesicles | Hand-foot-and-mouth disease | Coxsackievirus A16 | 210 |
| Vesicles | Orf | Parapoxvirus | 201 |
| Vesicles | Molluscum contagiosum | Molluscum contagiosum poxvirus | 201 |
| Vesicles | Rickettsialpox | Rickettsia akari | 192 |
| LESION, CLINICAL SYNDROME | INFECTIOUS AGENT(S) | SEE ALSO CHAP(S) | |
|---|---|---|---|
| Vesicles | Blistering distal dactylitis | Staphylococcus aureus or Streptococcus pyogenes | 152, 153 |
| Bullae | Staphylococcal scalded-skin syndrome | S. aureus | 152 |
| Bullae | Necrotizing fasciitis | S. pyogenes, Clostridium spp., mixed aerobes and anaerobes | 153, 159, 182 |
| Bullae | Gas gangrene | Clostridium spp. | 159 |
| Bullae | Halophilic Vibrio | Vibrio vulnificus | 173 |
| Crusted lesions | Bullous impetigo/ecthyma | S. aureus | 152 |
| Crusted lesions | Impetigo contagiosa | S. pyogenes | 153 |
| Crusted lesions | Ringworm | Superficial dermatophyte fungi | 225 |
| Crusted lesions | Sporotrichosis | Sporothrix schenckii | 225 |
| Crusted lesions | Histoplasmosis | Histoplasma capsulatum | 218 |
| Crusted lesions | Coccidioidomycosis | Coccidioides immitis | 219 |
| Crusted lesions | Blastomycosis | Blastomyces dermatitidis | 220 |
| Crusted lesions | Cutaneous leishmaniasis | Leishmania spp. | 233 |
| Crusted lesions | Cutaneous tuberculosis | Mycobacterium tuberculosis | 183 |
| Crusted lesions | Nocardiosis | Nocardia asteroides | 179 |
| Papular and nodular lesions | Fish-tank or swimming-pool granuloma | Mycobacterium marinum | 185 |
| Papular and nodular lesions | Creeping eruption (cutaneous larva migrans) | Ancylostoma braziliense | 238 |
| Papular and nodular lesions | Dracunculiasis | Dracunculus medinensis | 240 |
| Papular and nodular lesions | Cercarial dermatitis | Schistosoma mansoni | 241 |
| Papular and nodular lesions | Verruca vulgaris | Human papillomaviruses 1, 2, 4 | 203 |
| Papular and nodular lesions | Condylomata acuminata (anogenital warts) | Human papillomaviruses 6, 11, 16, 18 | 203 |
| Papular and nodular lesions | Onchocerciasis nodule | Onchocerca volvulus | 240 |
| Papular and nodular lesions | Cutaneous myiasis | Dermatobia hominis | 472 |
| Papular and nodular lesions | Verruca peruana | Bartonella bacilliformis | 177 |
| Papular and nodular lesions | Cat-scratch disease | Bartonella henselae | 177 |
| Papular and nodular lesions | Lepromatous leprosy | Mycobacterium leprae | 184 |
| LESION, CLINICAL SYNDROME | INFECTIOUS AGENT(S) | SEE ALSO CHAP(S) | |
|---|---|---|---|
| Papular and nodular lesions | Secondary syphilis | Treponema pallidum | 187 |
| Papular and nodular lesions | Tertiary syphilis | T. pallidum | 187 |
| Ulcers with or without eschars | Anthrax | Bacillus anthracis | S4 |
| Ulcers with or without eschars | Ulceroglandular tularemia | Francisella tularensis | 175, S4 |
| Ulcers with or without eschars | Bubonic plague | Yersinia pestis | 176, S4 |
| Ulcers with or without eschars | Buruli ulcer | Mycobacterium ulcerans | 185 |
| Ulcers with or without eschars | Leprosy | M. leprae | 184 |
| Ulcers with or without eschars | Cutaneous tuberculosis | M. tuberculosis | 183 |
| Ulcers with or without eschars | Chancroid | Haemophilus ducreyi | 162 |
| Ulcers with or without eschars | Primary syphilis | T. pallidum | 187 |
| Ulcers with or without eschars | Erysipelas | S. pyogenes | 153 |
| Ulcers with or without eschars | Cellulitis | Staphylococcus spp., Streptococcus spp. | various other bacteria |
| Ulcers with or without eschars | Necrotizing fasciitis | S. pyogenes, Clostridium spp., mixed aerobes and anaerobes | 153, 159, 182 |
| Ulcers with or without eschars | Streptococcal gangrene | S. pyogenes | 153 |
| Ulcers with or without eschars | Fournier gangrene | Mixed aerobic and anaerobic bacteria | 182 |
| Ulcers with or without eschars | Staphylococcal necrotizing fasciitis | Methicillin-resistant S. aureus | 152 |
1.1 Anatomic Relationships¶
The dermal capillary plexus beneath the dermal papillae plays a key role in infection localization and immune response. Disruption of the stratum corneum allows bacterial translocation via lymphatics, leading to conditions like erysipelas. Postcapillary venules are sites of leukocyte sequestration and chemotaxis.
1.2 Classification¶
SSTIs are classified by lesion type (vesicles, bullae, crusted lesions) and depth (superficial vs. deep). Deep infections (e.g., necrotizing fasciitis) involve fascia and muscles, while superficial infections (e.g., cellulitis) affect the dermis and subcutaneous tissues.
2. EPIDEMIOLOGY¶
SSTIs are common globally, with rising incidence due to MRSA, natural disasters, and trauma. MRSA (USA300 clone) is a major cause of severe infections. Risk factors include immunocompromise, diabetes, and trauma. Necrotizing fasciitis and gas gangrene are often associated with penetrating injuries or underlying conditions like diabetes.
2.1 Demographics¶
Erysipelas and cellulitis are more common in elderly and immunocompromised patients. Necrotizing fasciitis is more severe in patients with comorbidities like diabetes or peripheral vascular disease.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
SSTIs are caused by a wide range of pathogens, including bacteria (Staphylococcus, Streptococcus, Clostridium), viruses (HSV, VZV), fungi (dermatophytes), and parasites (Leishmania, Schistosoma). Pathogenesis involves disruption of the epidermal barrier, bacterial translocation via lymphatics, and immune-mediated inflammation.
3.1 Bacterial Pathogenesis¶
Staphylococcus aureus and Streptococcus pyogenes are common pathogens. Clostridium spp. cause gas gangrene. MRSA produces PVL toxin, contributing to necrotizing fasciitis. Vibrio vulnificus is associated with seawater exposure.
4. CLINICAL FEATURES¶
Clinical features vary by infection type. Vesicles (e.g., varicella), bullae (e.g., staphylococcal scalded-skin syndrome), and crusted lesions (e.g., impetigo) are common. Necrotizing fasciitis presents with severe pain, erythema, and systemic toxicity. Gas gangrene is characterized by gas formation and rapid progression.
5. DIFFERENTIAL DIAGNOSIS¶
Differential diagnoses include erysipelas vs. cellulitis, necrotizing fasciitis vs. cellulitis, and viral vs. bacterial infections. Key features include lesion morphology, progression, and systemic symptoms.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis involves clinical evaluation, imaging (CT/MRI), and laboratory tests (Gram stain, culture). Punch biopsy and frozen section analysis help differentiate conditions like SSSS vs. TEN. PCR and serology may be used for viral infections.
7. MANAGEMENT & TREATMENT¶
Management includes surgical debridement, antibiotics, and supportive care. MRSA requires vancomycin or linezolid. Necrotizing fasciitis needs clindamycin + cephalosporin. Antivirals (e.g., acyclovir) are used for HSV infections. Antifungals are required for fungal infections.
Table 134-2 Treatment of Common Infections of the Skin¶
| DIAGNOSIS/CONDITION | PRIMARY TREATMENT | ALTERNATIVE TREATMENT | SEE ALSO CHAP(S) |
|---|---|---|---|
| Animal bite (prophylaxis or early infection)a | Amoxicillin–clavulanate (875/125 mg PO bid) | Doxycycline (100 mg PO bid) | 146 |
| DIAGNOSIS/CONDITION | PRIMARY TREATMENT | ALTERNATIVE TREATMENT | SEE ALSO CHAP(S) |
|---|---|---|---|
| Animal bitea (established infection) | Ampicillin–sulbactam (1.5–3 g IV q6h) | Clindamycin (600–900 mg IV q8h) | 146 |
| Animal bitea (established infection) | plus | Ciprofloxacin (400 mg IV q12h) or cefoxitin (2 g IV q6h) | |
| Bacillary angiomatosis | Erythromycin (500 mg PO qid) | Doxycycline (100 mg PO bid) | 177 |
| Herpes simplex (primary genital) | Acyclovir (400 mg PO tid for 10 days) | Famciclovir (250 mg PO tid for 5–10 days) or valacyclovir (1000 mg PO bid for 10 days) | 197 |
| Herpes zoster (immunocompetent host >50 years of age) | Acyclovir (800 mg PO 5 times daily for 7–10 days) | Famciclovir (500 mg PO tid for 7–10 days) or valacyclovir (1000 mg PO tid for 7 days) | 198 |
| Cellulitis (staphylococcal or streptococcalb,c) | Nafcillin or oxacillin (2 g IV q4–6h) | Erythromycin (0.5–1 g IV q6h) or clindamycin (600–900 mg IV q8h) | 152, 153 |
| MRSA skin infectiond | Vancomycin (1 g IV q12h) | Linezolid (600 mg IV q12h) | 152 |
| Necrotizing fasciitis (group A streptococcalb) | Clindamycin (600–900 mg IV q6–8h) plus penicillin G (4 million units IV q4h) | Clindamycin (600–900 mg IV q6–8h) plus a cephalosporin (first- or second-generation) | 153 |
| Necrotizing fasciitis (mixed aerobes and anaerobes) | Ampicillin (2 g IV q4h) plus clindamycin (600–900 mg IV q6–8h) plus ciprofloxacin (400 mg IV q6–8h) | Vancomycin (1 g IV q6h) plus metronidazole (500 mg IV q6h) plus ciprofloxacin (400 mg IV q6–8h) | 182 |
| Gas gangrene | Clindamycin (600–900 mg IV q6–8h) plus penicillin G (4 million units IV q4–6h) | Clindamycin (600–900 mg IV q6–8h) plus cefoxitin (2 g IV q6h) | 159 |
7.1 Antibiotic Therapy¶
Empirical treatment for cellulitis includes penicillin or cephalosporin. Necrotizing fasciitis requires clindamycin + cephalobacillin. MRSA infections need vancomycin or linezolid. Anaerobic coverage is essential for mixed infections.
8. PROGNOSIS & COMPLICATIONS¶
Prognosis varies widely. Necrotizing fasciitis and gas gangrene have high mortality (20–50%) without prompt treatment. Complications include sepsis, multiorgan failure, and amputation. Chronic conditions like leprosy or tuberculosis may require long-term management.
9. SPECIAL CONSIDERATIONS¶
Pregnancy, pediatrics, and elderly patients require special attention. HIV-infected individuals are at higher risk for recurrent infections. Immunocompromised patients may present with atypical features (e.g., disseminated fungal infections).
10. KEY POINTS & CLINICAL PEARLS¶
- Early surgical debridement is critical for necrotizing fasciitis.
- MRSA is increasingly common and requires vancomycin or linezolid.
- Viral infections (e.g., HSV, VZV) present with characteristic vesicles or bullae.
- Imaging (CT/MRI) is essential for assessing depth and extent of infection.
- IVIg may reduce mortality in severe group A streptococcal infections.