Skin infections

Definitions:

• Cellulitis involves deeper dermis and subcutaneous tissue. Can get petechiae, ecchymosis, bullae. Indistinct borders. Beta-haemolytic strep and stap aureus - see below for a table of other organisms based on inoculating mechanism.
• Erysipelas involves upper dermis and epidermis - clear demarcation on skin, often raised edges, more rapid onset, brighter red appearance. Involvement of the ear is a distinguishing feature of erysipelas - no deeper dermal tissue. Mostly caused by beta-haemolytic strep.
• Skin abscess is a collection of pus within dermis or subcutaneous tissue
• Furuncle (boil) is a deep infection of a hair follicle
• Carbuncle is a collection of multiple coalesced furuncles. Commonly seen on back, axila, buttocks

Differential diagnosis

• Necrotizing fasciitis - rapidly-progressive erythema or pain out of proportion to exam findings
• Toxic shock syndrome
• Clostridial myonecrosis

Complications

• Bacteraemia
• Endocarditis
• Metastatic infection
• Sepsis
• Toxic shock syndrome
• Septic arthritis or OM

Principles of management:

• Antibiotics
  • Treat for cellulitis whenever you are uncertain whether it is cellulitis or erysipelas
  • Indications for MRSA coverage
    • Systemic toxicity
    • Cellulitis with purulent wound drainage
    • Known MRSA colonisation or infection
    • Injection drug use
    • High-risk neutropaenia
  • Regimens:
    • No MRSA factors: cefazolin/cefalexin
    • MRSA factors: Bactrim OR Augmentin DF + doxycycline 100mg BD. Generally avoid clindamycin due to C diff risk and MRSA resistance.
    • MRSA and needs IV: vancomycin
    • Erysipelas can generally be managed as an outpatient: Augmentin DF, cefalexin, or as above if risk of MRSA
• Symptomatic improvement usually within 24-48 hours, but skin manifestations can take longer. Look for reductions in pain, fever, brightness of erythema, and WCC. Skin can begin to weep, blister, or crack as cellulitis evolves - not generally a marker of worsening infection
• Consider decolonisation for MRSA carriers
• Surgery
  • Excision of boil/carbuncle and primary closure - be careful not to spill pus into the wound - incise an ellipse to the extent of the skin changes, then tunnel the subcutaneous resection slightly outwards to ensure you don't go into the cavity

Bacteriology

Exposure	Pathogen
Most common pathogens (regardless of exposure)	Group A Streptococcus (ie, Streptococcus pyogenes) Non-group A, beta-hemolytic streptococci (groups B, C, G, and F) Staphylococcus aureus (S. aureus)
Cirrhosis	Gram-negative bacilli: Klebsiella spp Escherichia coli Vibrio vulnificus and Vibrio parahaemolyticus* Aeromonas spp*
Splenic or humoral immune dysfunction	Encapsulated bacteria: Streptococcus pneumoniae Haemophilus influenzae
Neutropenia	Pseudomonas aeruginosa and other Gram-negative bacilli Clostridium spp Invasive fungal infections
Fresh water (lakes, rivers)	Aeromonas hydrophila Plesiomonas shigelloides Edwardsiella tarda Pseudomonas aeruginosa Shewanella spp
Salt water	Vibrio vulnificus and Vibrio parahaemolyticus Erysipelothrix rhusiopathiae
Shellfish ingestion, especially oysters	Vibrio vulnificus and Vibrio parahaemolyticus
Animal bite	See separate topic 'bites'
Human bite	See separate topic 'bites'
Traumatic wound contaminated by soil	Clostridium spp Pseudomonas aeruginosa and other Gram-negative bacilli Fungi (eg, mucormycosis)
Nail puncture through sneakers	Pseudomonas aeruginosa
Recent travel	Depends on the location of travel