Cellulitis

Cellulitis is an infection that occurs in the subcutaneous tissues. It can be caused by multiple bacteria, but this page will focus on cellulitis caused by Streptococcus pyogenes, which is also called group A Streptococcus (group A strep).

Etiology

S. pyogenes are one of the most common causative pathogens for cellulitis.

S. pyogenes are gram-positive cocci that grow in chains (see Figure 1). They exhibit β-hemolysis (complete hemolysis) when grown on blood agar plates. They belong to group A in the Lancefield classification system for β-hemolytic Streptococcus, and thus are also called group A streptococci.

Clinical features

Cellulitis affects structures that are deeper than areas affected by impetigo or erysipelas.¹ As a result, the affected skin usually has a pinkish hue with a less defined border, compared to erysipelas that presents with well-demarcated borders and a bright red color.¹

Local signs of inflammation (warmth, erythema, and pain) are present in most cellulitis cases.² Systematic symptoms, such as fever, chills, and malaise, may be present, and can be accompanied by lymphangitis and, less frequently, bacteremia.¹ An elevated white blood cell count may also be present.

Transmission

Direct person-to-person transmission of group A strep can occur through contact with skin lesions or exposure to respiratory droplets.³ People with active infection are more likely to transmit group A strep compared to asymptomatic carriers. Local dermatophyte infection (e.g., athlete’s foot) may serve as portal of entry for group A strep.¹

Risk factors

Disruption of the cutaneous barrier, such as presence of ulcers, wounds, or fungal skin infections (e.g., athlete’s foot), is a risk factor for developing cellulitis.^1,4,5 Previous history of cellulitis; venous insufficiency, presence of chronic edema, or impaired lymphatic drainage of the limbs; obesity; and injection drug use have also been identified as risk factors for cellulitis.^1,4,6

Diagnosis and testing

Diagnosis of cellulitis is usually made clinically.

For cellulitis, the Infectious Diseases Society of America (IDSA) does not recommend routine collection of cultures, including blood, cutaneous aspirates, biopsies, or swabs.⁷ However, blood culture and microbiologic examination and culture of cutaneous aspirates, biopsies, and swabs may help when atypical pathogens are suspected. These procedures are recommended by IDSA in those with immunocompromised status, immersion injuries, or animal bites.⁷ Waiting for culture results should never delay the initiation of treatment; however, when available, culture results can be used to tailor antibiotic therapy.

Treatment

For typical cases of non-purulent cellulitis, IDSA recommends treatment with an antibiotic that is active against streptococci.⁷ Due to the difficulty of determining the causative pathogen for most cellulitis cases, clinicians may select antibiotics that cover both Staphylococcus aureus and group A strep.

Group A strep remains susceptible to beta-lactam antibiotics. Mild cellulitis can be treated with oral antibiotics, including penicillin, cephalosporins (e.g., cephalexin), dicloxacillin, or clindamycin. If signs of systemic infection are present, then intravenous antibiotics can be considered, such as penicillin, ceftriaxone, cefazolin, or clindamycin.⁷

The recommended duration of antibiotic treatment for most cellulitis cases is 5 days.⁷ Cases in which there has not been improvement during this time period may require longer durations of treatment.⁷

In addition, elevation of the affected area and treating predisposing factors (e.g., edema, underlying skin disorders) is recommended to reduce the risk of recurrent infection.⁷

Prognosis and complications

Occasionally, cellulitis can result in bacteremia and rarely in deep tissue infections, such as septic thrombophlebitis, suppurative arthritis, osteomyelitis, and infective endocarditis. Patients with impaired lymphatic drainage of the limbs or those who have undergone saphenous vein removal for coronary artery bypass grafting are at increased risk of recurrent infection.¹

Prevention

The spread of all types of group A strep infection can be reduced by good hand hygiene, especially after coughing and sneezing, and respiratory etiquette (e.g., covering your cough or sneeze). Early identification and management of superficial skin lesions is also key to cellulitis prevention. Patients with recurrent lower-extremity cellulitis should be inspected for tinea pedis and should be treated if present. Traumatic or bite wounds should be cleaned and managed appropriately (e.g., antibiotic prophylaxis, surgical debridement if indicated) to prevent secondary infections.⁷

Epidemiology

Among the estimated 23,650 cases (7.26/100,000 population) of invasive group A strep in the United States that occurred in 2017, nearly 45% of patients had cellulitis according to Active Bacterial Core surveillance (ABCs) data. However, only a small proportion of cellulitis results in invasive disease. CDC does not track incidence of non-invasive group A strep infections. Therefore, the incidence of overall cellulitis due to group A strep is likely to be much larger.