ESSENTIALS OF DIAGNOSIS
Extremely painful inflammatory swelling based on a hair follicle that forms an abscess.
Coagulase-positive S aureus is the causative organism.
Predisposing condition (diabetes mellitus, HIV disease, injection drug use) sometimes present.
A furuncle (boil) is a deep-seated infection (abscess) caused by S aureus and involving the entire hair follicle and adjacent subcutaneous tissue. The most common sites of occurrence are the hairy parts exposed to irritation and friction, pressure, or moisture. Because the lesions are autoinoculable, they are often multiple. Diabetes mellitus (especially if using insulin injections), injection drug use, allergy injections, and HIV disease all increase the risk of staphylococcal infections by increasing the rate of carriage. Certain other exposures including hospitalization, athletic teams, prisons, military service, and homelessness may also increase the risk of infection.
A carbuncle consists of several furuncles developing in adjoining hair follicles and coalescing to form a conglomerate, deeply situated mass with multiple drainage points.
Recurrent furunculosis (three or more episodes in 12 months) tends to occur in those with direct contact with other infected individuals, especially family members.
Pain and tenderness may be prominent. The abscess is either rounded or conical. It gradually enlarges, becomes fluctuant, and then softens and opens spontaneously after a few days to 1–2 weeks to discharge a core of necrotic tissue and pus. The inflammation occasionally subsides before necrosis occurs.
There may be slight leukocytosis, but a white blood cell count is rarely required. Pus can be cultured to rule out MRSA or other bacteria. Culture of the anterior nares and anogenital area (including the rectum to test for gastrointestinal carriage) may identify chronic staphylococcal carriage in cases of recurrent cutaneous infection.
The most common entity in the differential is an inflamed epidermal inclusion cyst that suddenly becomes red, tender, and expands greatly in size over one to a few days (eFigure 6–89). The history of a prior cyst in the same location, the presence of a clearly visible cyst orifice, and the extrusion of malodorous cheesy material (rather than purulent material) helps in the diagnosis. Tinea profunda (deep dermatophyte infection of the hair follicle) may simulate recurrent furunculosis. Furuncle is also to be distinguished from deep mycotic infections, such as sporotrichosis; from other bacterial infections, such as anthrax and tularemia (rare); from atypical mycobacterial infections; and from acne cysts (eFigure 6–54). Hidradenitis suppurativa (acne inversa) presents with recurrent tender, sterile abscesses in the axillae and groin, on the buttocks, or below the breasts (eFigure 6–90). The presence of old scars or sinus tracts plus negative cultures suggests this diagnosis.
Inflamed epidermal inclusion cyst. (Used, with permission, from K Zipperstein, ...