See full diagnosis

Look For:

Abscesses	Follicular pustules	Furuncles
Bullous impetigo	Non-bullous impetigo	Cellulitis

Best Tests:
Culture and sensitivity where pus or serous fluid is evident

Therapy:
Incise and drain any purulent lesions.
   See below for alternative antibiotics.

(The following information has been excerpted from VisualDx V.5.08 to be released in November 2007)

Diagnosis Synopsis
Skin and soft tissue infections (SSTIs) including cellulitis, abscesses, impetigo, folliculitis, and furunculosis can all be caused by infection with Staphylococcus aureus, including methicillin-resistant Staphylococcus aureus (MRSA). Recent studies report the increasing incidence of MRSA within the community (community-associated MRSA, or CA-MRSA) as compared to 10 years ago when MRSA was primarily limited to hospital-acquired infections. In most parts of the United States, over 50% of community-associated S. aureus are MRSA. Hence, S. aureus infections and MRSA are becoming synonymous.

MRSA was first identified in 1961, shortly after the introduction of methicillin. Since that time, MRSA has widely been recognized as an important agent of disease in hospitals, nursing homes, and other long-term care facilities. Beginning in the 1990s, however, outbreaks of MRSA were recognized among healthy children, athletes, prisoners, military recruits, and intravenous drug users in the community at large. Most of those with MRSA infections lacked "traditional" MRSA risk factors, such as exposure to the health care system.

The most common manifestation of CA-MRSA infections are skin and subcutaneous infections. Many infections are attributed by patients as spider bites, although an exposure to a spider is typically not recalled by patients. MRSA skin infections occur in patients of all ages and among people with no "traditional" risk factors, or even recently described risk factors. Clearly, any person, regardless of background or exposures, can acquire a CA-MRSA infection.

Virtually all CA-MRSA isolates retain susceptibility to vancomycin, linezolid, daptomycin, and trimethoprim–sulfamethoxazole (TMP-SMX). Most isolates are susceptible to clindamycin and tetracyclines, although susceptibility can vary by geography. While CA-MRSA has a propensity for causing skin and soft tissue infections, it can also cause more severe and invasive disease syndromes including necrotizing pneumonia, empyema, necrotizing fasciitis, pyomyositis, septic thrombophlebitis with pulmonary embolization, osteomyelitis, and severe sepsis syndrome, among other manifestations. "Spider bite"-like skin infections and the more severe syndromes may be due in large part to a specific exotoxin present in many strains of CA-MRSA, Panton-Valentine leukocidin (PVL). PVL, a virulence factor that is lethal to neutrophils, has also been implicated in cases of invasive CA-MRSA infections such as necrotizing pneumonia.

Look For

• Abscesses
• Follicular pustules
• Furuncles (boils)
• Bullous impetigo – pus or fluid-filled bullae and vesicles
• Non-bullous impetigo – crusted plaques and erosions
• Cellulitis – circumscribed erythema and/or edema, tissue warmth, and tenderness

(See individual synopses of abscess, folliculitis, impetigo, and cellulitis within the VisualDx system.)

Diagnostic Pearls
In most geographic areas, assume MRSA when there are severe furuncles or abscesses. Consider MRSA in almost any skin or soft tissue infection. Many patients with Staphylococcal furunculosis believe they were bitten by a spider despite not having seen the spider or felt the bite.

Differential Diagnosis & Pitfalls

• Streptococcal ecthyma
• Pseudomonas folliculitis
• Herpes simplex infections
• Ruptured epidermoid cysts
• Foreign body reactions
• Hidradenitis suppurativa
• Necrotizing fasciitis
• Pyomyositis
• Tinea infections including deep infections (Majocchi's granuloma)
• Rickettsialpox (endemic areas)
• Arthropod bites

Best Tests
Culture and sensitivity where pus or serous fluid is evident.

Management Pearls
Incision and drainage is the cornerstone of therapy of purulent MRSA infections. Recent studies suggest that in selected hosts (eg, those with smaller infections, no signs of systemic toxicity, no underlying immunosuppression, and no infection in a critical body part such as the hand, genitalia, or face) systemic antibiotics may not be required for purulent MRSA SSTIs if complete surgical drainage can be performed. These investigations have important therapeutic implications for otherwise healthy patients with SSTIs amenable to surgical intervention.

If a ß-hemolytic streptococcal infection is suspected, therapy with a tetracycline and probably TMP-SMX should be augmented to include a penicillinase-resistant ß-lactam antibiotic.

Therapy
Incise and Drain any Purulent Lesions!
Standard cephalosporins and penicillins are of no benefit in treating MRSA. In a recent study, CA-MRSA demonstrated a high degree of susceptibility to trimethoprim-sulfamethoxazole (100%), clindamycin (95%), and tetracycline (92%). Inducible resistance to clindamycin should be excluded by performing a D-zone disk-diffusion test on any isolates. While most isolates are rifampin susceptible, there are no data to suggest adjunctive rifampin therapy provides additional benefit for treatment of skin infection. Rifampin monotherapy should be avoided because of the high likelihood of developing resistance on therapy.

Possible antibiotic regimens for MRSA include:

• Clindamycin 300–450 mg p.o. 3 times daily (adults), or 30 mg/kg/d in 3–4 divided doses (children)
• Trimethoprim-sulfamethoxazole 1–2 double strength tabs p.o. twice daily (adults), or trimethoprim, 8–12 mg/kg/day, and sulfamethoxazole 40–60 mg/kg/day in 2 divided doses (children)
• Doxycycline or minocycline 100 mg p.o. 2 times daily (adults), or 2–4 mg/kg/day in 2 divided doses (children, use 4 mg/kg/day for minocycline); note tetracyclines are contraindicated in children less than 9 years of age.
• Clindamycin 600 mg IV every 8 hours or 300–450

Critically ill patients with MRSA or suspected MRSA should receive vancomycin, linezolid, or daptomycin:

• Vancomycin 30 mg/kg/day IV divided twice daily (adults), or 40 mg/kg/day, in 3–4 divided doses (children)
• Linezolid 600 mg IV or p.o. every 12 hours (adults), or 30 mg/kg/day in 2–3 divided doses (children)
• Daptomycin 4 mg/kg IV every 24 hours (adults); dosing unknown in children

Choices of Parenteral Therapy
In systemically ill patients or those with multiple comorbidities, hospitalization and parenteral antibiotic administration is warranted.

Although at the present time many still consider vancomycin the drug of choice for invasive MRSA infections, newer anti-Gram positive agents linezolid and daptomycin are as effective for many conditions. Some experts consider these newer agents to be superior to vancomycin, as there is evidence that vancomycin susceptibility among MRSA has decreased over time. Linezolid appears to have improved outcomes for treatment in subgroups of patients with MRSA complicated skin infections and pneumonia, although overall it has not been demonstrated to be superior to vancomycin for either of these conditions when looking at treatment of all etiologies. Daptomycin can also be used as an alternative to vancomycin in patients with severe MRSA infection. Daptomycin is approved for treatment of skin infections and MRSA bacteremia. However, daptomycin is ineffective in cases of pneumonia due to MRSA because it binds to surfactant and is inactivated in the lung.

Because some patients suffer from recurrent skin infections, many patients and providers are interested in prevention of recurrent infections. However, there are almost no data on indications for recurrent therapy or optimal regimens. For those with 2 or more infection episodes, some experts recommend a topical nasal antibiotic with a body wash for 7–14 days. Some also recommend adding systemic antibiotics to the above regimen. Systemic antibiotics, especially rifampin, may improve the rate of decolonization, although the additional benefit of this aggressive approach, if any, is unclear. Rifampin should be used with caution in patients on medical therapy because of its high rate of drug interactions; the risk/benefit of its use should be carefully considered. Resistance to intranasal antibiotics (especially mupirocin), while relatively uncommon, have been well described and may be increasing.

Nasal antibiotics (applied intranasally twice daily):

• Bacitracin, alone or in combination with polymyxin and neomycin twice daily
• Mupirocin (Bactroban Nasal)
• Retapamulin

Body decolonization:

• Chlorhexidine 4% solution applied to the body and scalp daily. Do not use soap, creams, lotions, or other skin products immediately after use. Do not use in the eyes, nose, ears, or mouth.
• Dilute bleach baths (one cup of bleach per bathtub); do not use in the eyes, nose, ears, or mouth.

Systemic antibiotics:

• See "possible antibiotic regimens for MRSA," above, plus:
• Rifampin 20 mg/kg/day in 1 dose or 2 divided doses; maximum dose, 600 mg/day (adults and children)

Medical Disclaimer:
The information contained in this web page is intended to be an adjunct to traditional medical information sources. It is not intended to be a substitute for professional medical judgment.