Infectious Disease and Microbiology – Superficial skin and soft-tissue infections

Superficial skin and soft-tissue infections (SSTIs) involve structures ranging from the epidermis and hair follicles to the dermis and subcutaneous tissues. Common manifestations include impetigo, folliculitis, furunculosis, simple abscesses, erysipelas, and cellulitis.

SSTIs are generally classified as uncomplicated or complicated. Uncomplicated infections are superficial and usually respond to a single course of antimicrobial therapy or simple drainage. Complicated infections extend into deeper tissues, require surgical intervention, involve infected ulcers or wounds, or occur in patients whose underlying illnesses make treatment more difficult.

Epidemiology

Impetigo occurs at an estimated rate of approximately 10–20 cases per 1,000 person-years. It may appear in outbreaks and is especially common among children, older adults, and populations living in crowded or poorly sanitized environments.

Erysipelas occurs less frequently and is particularly common among older adults. Most cases involve the lower extremities, and women are affected somewhat more frequently.

Cellulitis is relatively common, with many cases occurring in people in their sixth decade of life. The lower limbs are the most frequently involved sites.

Risk factors

Impaired immunity increases susceptibility to essentially all forms of SSTI.

Impetigo is associated with poor sanitation, crowding, and warm tropical climates, whereas folliculitis and furunculosis are strongly associated with Staphylococcus aureus carriage and poor hygiene.

Important risk factors for erysipelas and cellulitis include breaks in the skin barrier, lymphedema, chronic edema, venous insufficiency, obesity, and previous episodes of cellulitis.

Contact sports, crowded living conditions, and inadequate hygiene may increase colonization and transmission of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA).

Prevention

Preventive measures include improved hygiene and sanitation, prompt treatment of underlying disorders such as venous stasis and obesity, and careful management of chronic skin conditions.

Proper foot care is particularly important in people with diabetes, tinea pedis, lymphedema, or chronic venous insufficiency.

Healthcare personnel should follow standard infection-control practices to reduce transmission of MRSA.

Pathophysiology

Most SSTIs begin when bacteria enter through a disruption in the skin, such as an abrasion, insect bite, ulcer, traumatic wound, or underlying dermatologic disease.

The infection may remain localized, as occurs in folliculitis, furunculosis, and simple abscesses, or it may spread through surrounding tissues, as in erysipelas and cellulitis.

In erysipelas, infection may spread rapidly through the superficial lymphatic channels.

Etiology

Staphylococcus aureus is the most common overall pathogen, followed by group A Streptococcus (GAS).

Impetigo is commonly caused by S. aureus, GAS, or both.

Folliculitis and furunculosis are usually caused by S. aureus.

Simple abscesses are also commonly associated with S. aureus, although polymicrobial infection may occur.

Erysipelas and nonpurulent cellulitis are most often caused by β-hemolytic streptococci, particularly GAS.

Certain exposures suggest alternative organisms. Pasteurella multocida may follow animal bites, Aeromonas hydrophila may follow freshwater injuries, and Vibrio species may occur after exposure to seawater.

Impetigo

Impetigo commonly occurs on the face and extremities, particularly in warm and humid environments.

Nonbullous impetigo begins as thin-walled vesicles or pustules on an erythematous base, which subsequently rupture and crust.

Bullous impetigo produces superficial, flaccid, pruritic bullae and is typically caused by toxin-producing strains of S. aureus.

Folliculitis

Folliculitis is a superficial infection involving individual hair follicles.

It typically presents as small erythematous papules, vesicles, or pustules centered on hair follicles.

Furunculosis and carbuncles

A furuncle, or boil, represents a deeper infection of a hair follicle and appears as a firm, painful inflammatory nodule.

When several adjacent furuncles merge, they may form a carbuncle, which is a larger inflammatory and purulent mass.

Carbuncles commonly occur on the posterior neck, back, and thighs.

Erysipelas

Erysipelas typically produces a raised, bright-red, painful, indurated plaque with sharply demarcated advancing borders.

The affected skin may have a peau d’orange appearance because of superficial edema.

The lower extremities are most commonly involved, although facial erysipelas can affect the bridge of the nose and cheeks.

Systemic manifestations such as fever, chills, and malaise are common.

Recurrent erysipelas is particularly associated with chronic venous or lymphatic obstruction and may repeatedly affect the same limb.

Cellulitis

Cellulitis is characterized by erythema, warmth, swelling, and tenderness involving the deeper dermis and subcutaneous tissues.

Unlike erysipelas, the borders of cellulitis are usually poorly defined and not raised.

More severe disease may be accompanied by fever, chills, malaise, and systemic toxicity.

Diagnosis

Diagnosis of most superficial SSTIs is primarily clinical.

Laboratory studies such as a complete blood count, metabolic panel, and C-reactive protein may be appropriate when invasive infection or systemic illness is suspected.

Blood cultures and needle-aspiration cultures are not routinely required but may be considered in patients with diabetes, malignancy, neutropenia, immunodeficiency, animal bites, immersion injuries, or unusual clinical circumstances.

When an abscess is drained, the obtained purulent material can be sent for culture and susceptibility testing, particularly in severe or recurrent infections.

Imaging

Imaging is usually unnecessary in uncomplicated superficial infection.

Ultrasonography can help distinguish cellulitis from an underlying abscess and can guide aspiration or drainage.

CT or MRI may be required when there is concern for osteomyelitis, a deep abscess, or necrotizing soft-tissue infection.

Differential diagnosis

Infectious mimics include herpes simplex, herpes zoster, erysipeloid, ecthyma gangrenosum, and necrotizing soft-tissue infection.

Important noninfectious mimics include contact dermatitis, gout, insect bites or stings, drug reactions, superficial thrombophlebitis, deep venous thrombosis, eosinophilic cellulitis, lipodermatosclerosis, and lymphedema.

Treatment of impetigo

Mild localized impetigo can usually be treated with a topical antimicrobial such as mupirocin.

More extensive disease may require oral therapy with agents active against staphylococci and streptococci, such as an appropriate antistaphylococcal penicillin or first-generation cephalosporin.

Gentle cleansing with soap and water and removal of infected crusts can assist healing.

Treatment of furunculosis

Small furuncles may improve with warm moist compresses.

Large furuncles and carbuncles generally require incision and drainage.

Systemic antibiotics are usually reserved for patients with systemic illness, extensive surrounding cellulitis, multiple lesions, immunosuppression, or other high-risk features.

For recurrent S. aureus infections, decolonization measures such as intranasal mupirocin and antiseptic skin cleansing may be considered.

Treatment of abscesses

The main treatment of a simple abscess is incision and drainage.

Adjunctive antimicrobial therapy may be required when there are multiple abscesses, impaired host defenses, significant cellulitis, systemic symptoms, or increased risk of complications.

When MRSA coverage is required, commonly used outpatient agents may include doxycycline, trimethoprim-sulfamethoxazole, or clindamycin, depending on local susceptibility patterns and individual patient factors.

Treatment of erysipelas

Typical erysipelas is primarily a streptococcal infection, so penicillin-class therapy is generally appropriate.

If S. aureus is suspected, treatment should include adequate antistaphylococcal activity.

Severe infection may require intravenous antimicrobial therapy.

Treatment of cellulitis

Mild uncomplicated cellulitis is usually treated with an oral antimicrobial active against streptococci and methicillin-susceptible S. aureus.

Severe cellulitis may require intravenous therapy with agents such as cefazolin or an antistaphylococcal penicillin, while MRSA-active therapy is added when indicated.

Selection of antibiotics should take into account local resistance patterns, purulence, previous MRSA infection, exposure history, allergies, immune status, and severity of disease.

General measures

Elevation of an affected limb helps reduce edema and discomfort and may accelerate clinical improvement.

Underlying predisposing conditions such as tinea pedis, chronic edema, venous insufficiency, obesity, or lymphedema should be treated whenever possible.

Physiotherapy may be useful in selected patients to improve muscle function and venous return.

Recurrent infection

Patients with recurrent cellulitis or erysipelas should be evaluated for chronic edema, venous disease, lymphedema, skin breakdown, and fungal infection of the feet.

For patients with frequent recurrent episodes despite correction of risk factors, antibiotic prophylaxis may be considered.

Recurrent furunculosis may warrant attempts to eradicate S. aureus carriage, particularly when multiple household or community cases are occurring.

Surgery

Large furuncles, carbuncles, and abscesses generally require incision and drainage.

Immediate surgical consultation is required if the infection progresses rapidly, causes tissue necrosis, or raises concern for a necrotizing soft-tissue infection.

Features such as pain out of proportion to examination findings, rapidly spreading erythema, bullae, crepitus, skin necrosis, severe toxicity, or hemodynamic instability should prompt urgent assessment.

In-patient considerations

Most patients with uncomplicated erysipelas or cellulitis can be treated as outpatients.

Hospitalization should be considered for severe systemic illness, rapid progression, inability to tolerate oral medication, major immunosuppression, failure of outpatient therapy, or suspected deep or necrotizing infection.

Patients initially receiving intravenous therapy can generally be switched to oral treatment when there is clear clinical improvement and systemic manifestations have resolved.

Marking the outer border of erythema can help monitor progression or improvement.

Follow-up

Patients with cellulitis should generally be reassessed within 48–72 hours to confirm an appropriate clinical response.

Patients with impetigo should be reassessed if lesions fail to improve, become more extensive, or develop systemic manifestations.

Prognosis

Promptly treated uncomplicated SSTIs generally have an excellent prognosis.

Recurrence can occur, particularly when underlying conditions such as chronic edema or venous insufficiency persist.

Rarely, a superficial infection can progress into a life-threatening necrotizing soft-tissue infection.

Complications

Impetigo can rarely be followed by post-streptococcal glomerulonephritis.

Erysipelas and cellulitis may lead to abscess formation, lymphangitis, thrombophlebitis, recurrent lymphedema, bacteremia, endocarditis, or infection at distant sites.

Repeated episodes of cellulitis may further damage the lymphatic system and increase susceptibility to future episodes.

The most serious complication is extension into deeper tissue producing a necrotizing soft-tissue infection, which requires immediate surgical treatment.

High-Yield Pattern

Superficial crusted lesions → impetigo

Pustules centered on hair follicles → folliculitis

Painful infected follicular nodule → furuncle

Coalescing furuncles forming a large purulent lesion → carbuncle

Fluctuant collection of pus → abscess

Raised, sharply demarcated fiery-red plaque → erysipelas

Warm, tender, poorly demarcated erythema → cellulitis

Rapid progression + severe pain + systemic toxicity → suspect necrotizing soft-tissue infection