These terms refer to diffuse, spreading skin infections, excluding infections associated with underlying suppurative foci, such as cutaneous abscesses, necrotizing fasciitis, septic arthritis, and osteomyelitis.

Erysipelas is distinguished clinically from other forms of cutaneous infection by the lesions raised above the level of the surrounding skin, and there is a clear line of demarcation between involved and uninvolved tissue.

Cellulitis is an acute spreading infection of the skin, extending more deeply than erysipelas to involve the subcutaneous tissues as well.

Streptococcus species usually group A (S. Pyogenes, most cases).

Other:Streptococcus groups B, C, G (especially group G). Staphylococcus aureus -- MRSA now dominates both in and out of the hospital.

Dog/cat bite:Pasteurella multocida, Capnocytophaga canimorsus

Salt water exposure:Vibrio vulnificus

Fresh or brackish water exposure:Aeromonas hydrophila,Plesiomonas shigelloides

Neutropenia:P. Aeruginosa, other GNB.

Human bite:Eikenella corrodens, anaerobes, S. Aureus.

Occasional causes:other Vibrio spp. (saltwater exposure), other Aeromonas spp. (freshwater exposure), S. Pneumoniae, H. Influenzae,Legionella spp., Helicobacter cinaedi (immunocompromised), Erysipelothrix rhusiopathiae (meat/fish exposure), Staphylococcus epidermidis (immunocompromised patients), Group B streptococci (infants), fungal.

Predisposing conditions:Trauma, lymph stasis (prior radiation,mastectomy, saphenous vein harvest), injection drug use, ulcers, wounds,dermatophytic infections.

Exam: Red, hot, tender skin with edema - fever and adenopathy.

Differential diagnosis: Allergy, gout, zoster, erythroderma, insect bite,panniculitis, Lyme disease (erythema migrans), Sweet's syndrome, pyoderma, fixed drug reaction, thrombophlebitis, necrotizing fasciitis.

Lab: Blood cultures are < 5% positive; needle aspirates - usually negative; punch biopsy yield is 20-30%. Typically, culture test not performed with presumption that most cases are due to streptococci or staphylococci.

Blood cultures indicated but usually negative. Obtain especially if: significant systemic signs and symptoms present in immunocompromised patients, unusual pathogen suspected, no response to adequate antibiotic therapy.

Usually clinical diagnosis based on appearance and symptoms.Imaging helpful in some cases: ultrasound (to differentiate DVT), CT (if suspecting necrotizing fasciitis).

• Streptococci (only consider if erysipelas): Penicillin V 500mg PO four times a day x 10 days, amoxicillin 500mg PO three times a day x 10 days or penicillin G benzathine 1.2mil U IM x 1, cephalexin 500mg PO four times a day x 10 days.
• Penicillin allergy: Azithromycin 500mg PO x 1 day, then 250mg PO daily x 4 days, clarithromycin 250mg PO twice-daily x 7-10 days, clindamycin 300mg PO three times a day x 7-10 days.
• Streptococci and Staphylococcus aureus (most cases of cellulitis, must presume MRSA): Clindamycin 300mg PO three times a day x 7-10 days.

• Streptococci and Staph aureus (presume MRSA): Clindamycin (if D test neg)600mg IV q 8h, vancomycin 15mg/kg IV q 12h, linezolid 600mg IV q12h or daptomycin 4mg/kg IV q24h.
• Streptococci only (e.g., erysipelas): Penicillin G 2-4 mil U IV q 4-6h, cefazolin 0.5-1.5gm IV q 8h, cefotaxime 1-2gm IV q 8h, ceftriaxone 1-2gm IV q24h,clindamycin 600 mg q 8h IV or 300 mg PO four times a day, or penicillin +clindamycin.
• Penicillin allergy: Clindamycin or vancomycin (above doses).

• Erysipelas: consider prednisone 30mg with taper over 8 days.
• Elevation of affected site.
• Treat associated conditions: Tinea pedis, venous stasis, lymphedema, eczema,trauma sites.
• Dermatophytic infections: topical terbinafine or clotrimazole.

• Prevent edema (diuretics, limb elevation, compression stockings, decongestion therapy).
• Keep skin hydrated (emollients).
• Treat dermatophytic infections.
• Prevention of recurrent cellulitis especially with lymphedema: penicillin V 500mg PO twice-daily, amoxicillin 250-500mg PO twice-daily, clindamycin 150-300mg PO daily, erythromycin 250mg PO 1-2 x/d.

• Symptoms typically dissipate within first few days of antibiotic therapy or longer even though the symptoms may disappear earlier.
• Cellulitis may appear to worsen the first 24-48 hrs despite antibiotics. This may be due to toxins and/or bacterial lysis that drive inflammation even though antibiotics have achieved bacteriocidal effect.
• Severe cellulitis may predispose to repeat bouts; "cellulitis begets cellulitis."

Most common form of cellulitis - leg (tibial area) with breach in skin usually due to intertrigo.

Usual pathogen: Gr A Streptococci, especially if Legionella, perianal or buttock area;cellulitis complicating edema or lymphedema.

Treatment - always cover Streptococci which is always sensitive to penicillins.