Skin and Soft-Tissue Infections

The skin is an effective barrier to infection. The risk of skin and soft-tissue infection (SSTI) increases when the integrity of the skin is breached by trauma or underlying skin disease such as eczema. SSTIs can range from mild cellulitis that can be treated in the outpatient setting with oral antibiotics to life-threatening necrotizing soft-tissue infections. Management depends on the infection’s severity, presence or absence of purulence, and patient comorbidities.

Clinical Presentation

SSTIs are characterized by warmth, erythema, tenderness, and induration. Most patients do not have systemic signs of infection but may experience fever and malaise. SSTIs can range from cellulitis (a pyogenic infection of the skin, generally limited to the epidermis, dermis, and superficial subcutaneous tissues) to abscess (a focal purulent infection contained within the deep subcutaneous tissues). Differentiation of abscesses from cellulitis is generally based on physical examination and imaging. However, many cases of cellulitis may have an associated abscess. Lymphadenitis can progress to cellulitis. SSTIs can also lead to necrotizing fasciitis, which is characterized by severe pain that is not consistent with clinical signs. Necrotizing fasciitis can rapidly progress and even become fatal if early surgical debridement is not performed.

(Source: Cellulitis. N Engl J Med 2004.)

(Source: Cellulitis. N Engl J Med 2004.)

Microbiology

• Most SSTIs are caused by either Staphylococcus aureus (methicillin-susceptible and methicillin-resistant [MSSA and MRSA]) or Streptococcus pyogenes.

• Gram-negative pathogens complicating polymicrobial infections can also play an important role when SSTIs are located in the buttock or axillary regions or associated with bite wounds or a history of water exposure.

• Anaerobic pathogens should be considered in patients with bite wounds and history of soil exposure.

• Candida albicans is a common fungal cause of cellulitis in the groin of children who wear diapers.

• Pasteurella multocida should be considered when evaluating infection resulting from animal bite wounds, and Eikenella corrodens is a common pathogen complicating bites from another human.

• Immunocompromised patients are at risk of cellulitis secondary to other fungi and invasive molds (including Aspergillus fumigatus, mucormycetes, and other opportunistic fungi).

Epidemiology

Some data suggest that the incidence of SSTIs in children has increased during the past 2 decades due to the emergence of community-acquired MRSA and the organism’s predilection to cause skin abscesses. The estimated incidence of SSTIs in children (age <17 years) is 43 per 1000 person-years (based on International Classification of Diseases, 9th edition [ICD-9] billing codes). Most culture-positive cases are caused by S. aureus.

Diagnosis

SSTI is a clinical diagnosis based on physical examination and history. In patients with cellulitis, abscess should be ruled out through physical examination or imaging. Differentiating cellulitis from deeper infections, including abscesses, is important for optimal management. Point-of-care ultrasound (POCUS) is increasingly used to help differentiate cellulitis from an underlying abscess. Ultrasound is 90%-97% sensitive and 67%-83% specific for the diagnosis of abscesses in the emergency department. On physical exam, abscesses may have fluctuance or a pustule visible at the skin that can occasionally drain spontaneously. An area of necrosis may be mistaken for a spider bite, but it is most commonly not due to a bite but is rather a hallmark of MRSA infection. Serious complications of cellulitis (e.g., necrotizing fasciitis) should be considered, particularly if the patient is ill appearing and the lesion is extremely painful.

Management

Treatment of SSTIs should be geared toward the most likely pathogens involved based on clinical examination, exposure history, and patient comorbidities. Attempts should be made to determine the likely pathogen whenever feasible. If purulent fluid is expressed, a culture should be obtained.

Empiric antibiotics: Patients with cellulitis present a treatment challenge when the infection does not develop purulent drainage, making it difficult to identify specific causative organisms. Antibiotic treatment of cellulitis should be guided by the local epidemiology and susceptibility of S. aureus isolates in the community. Bacteremia is rare in patients with SSTIs and blood cultures are not routinely recommended; however, blood cultures should be obtained in patients who are ill appearing, are immunocompromised, or have complicated SSTI.

Exposure history should be obtained in all patients with a history of cellulitis to determine if broader-spectrum antibiotic coverage is needed. For example, patients who develop cellulitis following a bite (dog, cat, or human) should be treated with amoxicillin-clavulanate to cover a likely polymicrobial infection, including P. multocida (dog and cat bites) and E. corrodens (human bites), in addition to S. aureus, S. pyogenes, and oral anaerobes.

Pediatric patients with localized impetigo, a skin infection characterized by sores typically around the mouth and nose with honey-colored crusts, can be treated with topical mupirocin alone. Most other patients with SSTI should be treated with oral antibiotics.

(Source: Recent Advances in Dermatology. N Engl J Med 1992.)

Antibiotic treatment of SSTI is directed at the most likely pathogens. In situations where an organism is not obtained, treatment of abscesses should typically include coverage for MRSA with either trimethoprim-sulfamethoxazole (TMP-SMX) or clindamycin (based on local susceptibility patterns), since MRSA accounts for a significant proportion of abscesses in the United States. Patients with SSTIs who are immunocompromised; ill appearing; or have signs of tachycardia, hemodynamic instability, or evidence of a large abscess (>5 cm), require intravenous antibiotics and hospitalization.

Surgical drainage: In patients with skin abscesses, the mainstay of treatment is incision and drainage (see video). Some experts suggest that incision and drainage alone can be sufficient and alleviate the need for antibiotics. However, treatment with antibiotics after drainage is required in pediatric patients who are younger than 1 year, have a complicated abscess or signs of systemic illness, or who are immunocompromised. Recent clinical trial data in patients older than 12 years with simple MRSA abscesses demonstrated improved short-term outcomes in those treated with clindamycin or TMP-SMX versus placebo, following incision and drainage.

In 2014, the Infectious Diseases Society of America published guidelines for the diagnosis and treatment of SSTIs and provided the following algorithm for management:

(Source: Practice Guidelines for the Diagnosis and Management of Skin and Soft Tissue Infections: 2014 Update by the Infectious Diseases Society of America. Clin Infect Dis 2014. Reproduced with permission of the Infectious Disease Society of America.)

Complications

When cellulitis is treated early, the disease is usually uncomplicated. However, untreated cellulitis may progress to bacteremia and infections at other sites. Poststreptococcal glomerulonephritis is a known complication of streptococcal impetigo. Although antibiotic treatment is indicated for impetigo, it may not prevent the development of kidney disease.