Skin abscesses are collections of pus within the dermis and deeper skin tissues. A furuncle (or "boil") is an infection of the hair follicle in which purulent material extends through the dermis into the subcutaneous tissue, where a small abscess forms. A carbuncle is a coalescence of several inflamed follicles into a single inflammatory mass with purulent drainage from multiple follicles .
Most abscesses are caused by infection. However, sterile abscesses can occur in the setting of injected irritants. Examples include injected drugs (particularly oil based ones) that may not be fully absorbed and so remain at the site of injection, causing local irritation. Sterile abscesses can turn into hard, solid lesions as they scar.
The epidemiology, clinical manifestations, microbiology and treatment of skin abscesses, furuncles, and carbuncles due to infection will be reviewed here. Issues related to dental, vulvovaginal, and perianal abscesses are discussed separately. (See "Complications, diagnosis, and treatment of odontogenic infections" and "Perianal abscess: Clinical manifestations, diagnosis, treatment" and "Disorders of Bartholin gland".)
Skin abscesses, furuncles, and carbuncles can develop in healthy individuals with no predisposing conditions other than skin or nasal carriage of Staphylococcus aureus; spontaneous infection due to community-acquired methicillin-resistant S. aureus (CA-MRSA) may occur with greater frequency than abscesses due to other pathogens . Rectal colonization of S. aureus, including CA-MRSA of the USA300 clone, was strongly associated with skin abscess formation in one pediatric investigation . Individuals in close contact with others who have active infection with skin abscesses, furuncles, and carbuncles are at increased risk [4,5]. Individuals exposed to whirlpool footbaths at nail salons are at risk for mycobacterial furunculosis [6,7]. Additional risk factors include diabetes mellitus and immunologic abnormalities [6,8]. (See "Approach to the adult with recurrent infections" and "Approach to the child with recurrent infections" and "Epidemiology of methicillin-resistant Staphylococcus aureus infection in adults" and "Epidemiology and clinical spectrum of methicillin-resistant Staphylococcus aureus infections in children", section on 'Epidemiology and risk factors'.)
Any process leading to a breach in the skin barrier can also predispose to the development of a skin abscesses, furuncle, or carbuncle. Examples include primary dermatologic conditions as well as trauma related to abrasions, shaving, and insect bites. In addition, intravenous drug users are at particular risk due to needle injection, including either intravenous injection or subcutaneous and intramuscular injection ("skin popping") [9-11].