Purulent Skin and Soft Tissue Infections: Furuncles, Carbuncles, and Cutaneous Abscesses

Definitions Top

Furuncles, carbuncles, and abscesses are the more common types of purulent skin and soft tissue infections (SSTIs).

Furuncles (or “boils”) are infections of the hair follicle that extend into the dermis and subcutaneous tissue. These usually extend from an area of folliculitis.

Carbuncles form when multiple furuncles coalesce through the subcutaneous tissue with purulent drainage from follicles. They are often divided by a septum.

Cutaneous abscesses refer to the collection of purulent material that can develop in the dermis, subcutaneous tissue, or both. They can also develop as a complication of furuncles and carbuncles.

Etiology and PathogenesisTop

Furuncles and carbuncles are typically caused by Staphylococcus aureus (both methicillin-susceptible Staphylococcus aureus [MSSA] and methicillin-resistant Staphylococcus aureus [MRSA]). Infections secondary to a penetrating trauma or involving the perioral or perineal area increase the likelihood of a polymicrobial infection, including S aureus, Streptococcus pyogenes, gram-negative bacilli, and anaerobes. Depending on host factors and exposures, other causes can include blastomycosis, cryptococcosis, nontuberculous mycobacterial infection, and nocardiosis.

Purulent SSTIs often affect healthy, immunocompetent adults. Risk factors are similar to those for nonpurulent SSTIs (see Table. Risk factors for nonpurulent skin and soft tissue infections). Additional risk factors include immunocompromised state (particularly neutrophil defects), suboptimal hygiene (eg, sharing contaminated hygiene items like shaving razors), hot and humid climates, MRSA carriage, and additional MRSA risk factors (see Nonpurulent Skin and Soft Tissue Infections: Erysipelas and Cellulitis).

Clinical FeaturesTop

SSTIs should be categorized as mild, moderate, or severe (see Table. Severity of skin and soft tissue infections).

A furuncle is a well-demarcated, painful nodule involving a hair follicle that frequently occurs in areas of friction (neck, axillae, thighs, buttocks). Its extension into dermal and subcutaneous tissue can result in suppurative abscesses and cellulitis. Carbuncles can occur anywhere but often develop at the back of the neck. If a carbuncle forms, patients can have associated systemic symptoms (eg, fevers, chills, rigors). Both furuncles and carbuncles (especially the latter) can be further complicated by bacteremia, toxic shock syndrome, necrotizing fasciitis, and sites of metastatic infection (eg, osteomyelitis, endocarditis).

Other considerations include the location of the abscess, whether it is amenable to drainage, and if there are any predisposing conditions. For example, the presence of perianal abscesses in a patient with chronic diarrhea should prompt consideration of Crohn disease, whereas recurrent abscesses since young childhood should prompt consideration of underlying immunodeficiency, such as chronic granulomatous disease.

DiagnosisTop

The diagnosis is generally clinical. Laboratory investigations are not required in a mild, uncomplicated infection in an immunocompetent host. If there is evidence of purulent drainage, wound samples can be obtained and sent for Gram staining and culture. However, this does not preclude the initiation of treatment, if indicated.

Cultures of blood, wounds, and drainage fluid at the time of incision and drainage (I&D) are indicated prior to initiation of antimicrobial therapy in the following circumstances:

1) The patient being systemically unwell.

2) Failure of prior antibiotic therapy.

3) History of recurrent or multiple abscesses.

4) Immersion injury or animal bite.

5) Immunocompromise and other comorbidities (eg, malignancy, neutropenia, splenectomy, immunodeficiency).

Ultrasonography should be performed if there is clinical suspicion of an abscess. Radiography can be considered to assess for subcutaneous gas but it will not exclude a deeper SSTI.

Differential Diagnosis

1. Infectious:

1) Nodular lymphangitis (eg, sporotrichosis, nocardiosis, Mycobacterium marinum infection, cutaneous leishmaniasis, dimorphic fungal infections).

2) Botryomycosis (a chronic granulomatous infection secondary to S aureus infection, often seen in immunocompromised hosts).

3) Kerion (a form of tinea capitis presenting as a painful, exudative mass on the scalp).

2. Noninfectious:

1) Erythema nodosum and pyoderma gangrenosum.

2) Epidermoid cyst.

3) Hidradenitis suppurativa.

Many of these noninfectious causes can become secondarily infected.

TreatmentTop

All purulent SSTIs should have definitive source control. Furuncles will often rupture and spontaneously begin draining with application of warm compresses. Larger furuncles and carbuncles often require I&D, and >80% of furuncles resolve with I&D. Antibiotics should be used if there is evidence of cellulitis beyond the furuncle itself (Table 1) or systemic signs of infection. Ultrasound-guided aspiration alone is insufficient and often results in treatment failure.

Depending on the location of the abscess, the risk of a polymicrobial infection may be increased (eg, perirectal abscesses) and this will alter antimicrobial therapy choices.

Recurrent skin abscesses at the same site should prompt evaluation of other local causes, such as hidradenitis suppurativa, foreign body/material, or pilonidal cysts. Recurrent abscesses should be drained and samples should be sent for culture. Underlying immunodeficiencies, such as chronic granulomatous disease, should be investigated in adults with recurrent abscesses that began in early childhood.

TablesTop