Skin and Soft Tissue Infection (STI)

Cellulitis

DDx: erysipelas, pyomyositis, necrotizing fasciitis, osteomyelitis, venous stasis, shingles, gout
Pathogens: Streptococcus species: Group A (most common), B, C, G, Staphylococcus aureus (including MSSA and MRSA)
- Non-purulent, lymphangitis, or erysipelas? Think Streptococcus
- Purulence (abscess or boil)? Think Staphylococcus
- Strep anginosus is a strep species that can cause pus formation
Unique clinical scenarios and associated organisms/organisms to consider:
- Dog/cat bite: Pasteurella multicoda, Capnocytophaga canimorsus
- Human bite: Eikenella corrodens, oral anaerobes, S. aureus
- Fresh water exposure: Aeromonas hydropholia, Plesiomonas shigelloides
- Salt water exposure: Vibrio vulnificus
- Neutropenia, presence of ecthyma: Gram negatives (Pseudomonas aeruginosa)
- Immunocompromised: Fungal (Candida spp, Cryptococcus), Nocardia, nontubercular mycobacteria)
- Burn pts: Pseudomonas, Acinetobacter, Fusarium

Outline border of erythema and obtain urgent surgery consultation if rapid spread of infection, crepitus, air in tissues, or pain out of proportion to exam
Blood cultures (BCx): ONLY needed if systemic signs of infection or immunocompromised (most pts will not need BCx or imaging)
US for underlying abscess
CT/MRI w/contrast: if necrotizing fasciitis, pyomyositis or osteomyelitis suspected
Bilateral lower extremity cellulitis is RARE and warrants consideration of noninfectious etiologies
Elevation test: if erythema improves after elevating leg above the level of the heart for 1- 2 minutes, less likely to be infectious cellulitis

Abx for 5 days for uncomplicated; can extend to 10-14 days if little to no improvement, more extensive/serious infection, or if immunosuppressed
Typically improvement is not seen until >48 hours of antibiotics, usually longer
Provide anti-Staphylococcal antibiotics for purulent cellulitis in addition to I&D, if abscess present
Clinical appearance may often appear to worsen initially despite adequate therapy
Always elevate the extremity for more rapid clinical improvement!

	No Staph suspected	MSSA	MRSA
Mild/Moderate (Outpt)	Cephalexin 500 QID Amoxicillin 500 TID Cefadroxil 1g BID	Cephalexin 500 QID Cefadroxil 1g BID Dicloxacillin 500 QID *Clindamycin 300-450 mg q6h	TMP/SMX 1-2 DS tabs BID Doxycycline 100 mg BID
Severe (Inpt)	Cefazolin 2g q8h CTX 2g q24h	Cefazolin 2g q8h PO step down: cephalexin 500 mg q6h or 1000mg q8h	Vancomycin (dose per PK) PO step down: TMP/SMX 1-2 DS tabs BID Doxycycline 100 mg BID Linezolid 600 mg BID
Severe (Inpt)	Cefazolin 2g q8h CTX 2g q24h	Stepdown to above PO options once: Staph aureus bacteremia is ruled out Clinical stability obtained > 24 hours Patient is tolerating oral therapy
*Consider for PCN allergy; check antibiogram (VUMC vs VA) for Staph sensitivities; clindamycin should NOT be used for strep coverage

Infection of the deeper soft tissues that causes necrosis along the muscle fascia and overlying subcutaneous fat that is rapidly progressive and lethal if not addressed
Clinical cues include rapid spread, pain out of proportion to exam, crepitus and hemorrhagic bullae
LRINEC score used to screen for necrotizing soft tissue infection

SURGICAL EMERGENCY!
- STAT consult to surgical service for emergent debridement (generally EGS vs ortho)
- Imaging does NOT rule out necrotizing fasciitis and should not delay these consultations
- CT is the best imaging modality
ID consult
Empiric antibiotics
Preferred: linezolid 600 mg IV BID + piperacillin-tazobactam 3.375g IV q8h extended infusion
For severe penicillin allergy: linezolid 600 mg IV BID + cefepime 2g IV q8h + metronidazole 500 mg IV BID
For severe penicillin and cephalosporin allergy: linezolid 600 mg IV BID + levofloxacin 750 mg IV q24h + metronidazole 500 mg IV BID
If patient cannot receive linezolid due to allergies, or multiple serotonergic drug interactions: vancomycin + piperacillin-tazobactam 3.375g IV q8h extended infusion + clindamycin 900mg IV q8h