Causes and Risk Factors of Carbuncles

A carbuncle is a red, painful cluster of furuncles (boils) that forms under the skin when hair follicles become infected with bacteria. The bacterium that most often causes carbuncles, and the boils from which they form, is Staphylococcus aureus (S aureus). However, there has been an increase in the incidence of carbuncles caused by MRSA, staph bacteria that are resistant to antibiotics.

Risk factors for carbuncles vary widely, from health factors such as being overweight or having diabetes to lifestyle practices such as using injectable drugs.

Common Causes

A carbuncle is a type of skin abscess. Carbuncles generally begin as furuncles—infections of single hair follicles and their surrounding tissues. As the infection spreads, it affects other follicles and extends into the deeper layers of the skin (i.e., subcutaneous fat). The infected mass develops several heads of pus. Several furuncles that are densely packed together and reach this stage are what make a carbuncle.

Carbuncles tend to occur in areas with thicker skin like the nape of the neck, the back, or the thighs.

Staphylococcus Aureus

While other bacteria are sometimes responsible, the vast majority of carbuncles are caused by a Staphylococcus aureus, or staph, infection.

Staph is a very common bacteria often found on the skin, inside of the nose, and in the genital area. In the majority of cases, it's harmless.

The problem develops when the bacterium enters the skin through a broken area. Here, the bacteria begin to multiply, infecting several follicles and coalescing into a mass of infected tissue and pus.

There doesn't have to be a large injury for the infection to take hold. In some cases, it could be a simple scratch, scrape, insect bite, or even a microtear.

Friction also plays a role in carbuncle development, as it can lead to irritation and inflammation of the hair follicle (folliculitis) that allows the infection to more easily take hold.

MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is becoming a fairly common cause of carbuncles, especially recurrent ones. These infections most commonly occur in the groin, buttocks, and upper-thigh region.

Since the 1980s, there has been a sharp increase in the incidence of infections caused by a special strain of Staphylococcus aureus that is resistant to the normal penicillin-based antibiotic treatment.

Until about the 1970s, MRSA was an uncommon bacterial strain that occurred in nursing homes and other long-term care facilities. But with the overuse of antibiotics for conditions that don't require them, MRSA infections are now, unfortunately, more common in the general population in certain regions of the United States.

Personal Health Risk Factors

Carbuncles can affect anyone at any age and are most common in men who are middle-aged or older.

You are more likely to experience carbuncles if any of the following apply:

• Obesity: Being overweight increases your susceptibility to skin infections. This may be because bacteria thrive in moist environments such as skin folds, or that being overweight makes it harder to fight infection. The risk is highest in those with a BMI over 30.
• Diabetes: Carbuncles are especially common in those with uncontrolled diabetes.
• Chronic skin conditions: Chronic skin conditions that leave the skin's barrier compromised make it easier for bacteria to enter and set up shop. This includes common conditions such as acne, folliculitis, eczema, and other types of dermatitis.
• Being immunocompromised: Those with chronic infections, HIV, cancer, and other conditions that weaken the immune system are at greater risk for developing carbuncles. This is because the body has a harder time fighting off infection, allowing bacteria to rapidly grow unchecked.
• Certain medications: Along the same lines, medications that weaken the immune system also prevent your body from fighting carbuncle-causing infections efficiently. Examples include long-term oral steroids and chemotherapy.
• Having multiple or recurrent boils: Certain people are more susceptible to being infected by staph. Those prone to recurrent cases are likely to develop more serious abscesses like carbuncles.
• Other health conditions: Carbuncles are also more common in those with health conditions like heart failure, liver disease, and kidney disease.

Lifestyle Risk Factors

Even if none of the health risk factors above apply to you, you may be at increased risk for carbuncles simply because of living circumstances and life choices such as:

• Close contact with a person who has a staph or MRSA infection: The bacteria that cause carbuncles are contagious. Living or caring for someone with a staph infection puts you at greater risk of developing one yourself. It's not unusual for several members of the same family to have furuncles or carbuncles at the same time.
• Living in crowded quarters: Staph can run rampant in communal living places such as dorms and military barracks, where infections can easily pass from person to person and via surfaces. Homeless shelters and prisons are also prime places for the bacteria that cause carbuncles to spread.
• Being an athlete: From sharing athletic equipment or personal effects in the locker room, to close-contact sports like wrestling, staph can also spread easily among athletes.
• Poor hygiene: The bacteria that cause carbuncles are common and can live almost anywhere. Good hygiene practices such as hand washing, showering, and laundering clothing can help prevent infection. On the flip side, poor hygiene practices put you more at risk.
• Injected drug use: According to a 2018 study, those who use injected drugs (be them prescription or non-prescription) are 16 times more likely to develop serious MRSA infections.

A Word From Verywell

Carbuncles occur most often in middle-aged and older men, especially those who have health conditions such as diabetes and heart disease or weakened immune systems. But remember that carbuncles are fairly common skin infections that can happen to anyone. If you think that you may have one, call your primary care provider or a dermatologist.