Cellulitis is an inflammatory bacterial infection of the dermis and subcutaneous tissues often caused by Group A streptococci (Streptococcus pyogenes) or Staphylococcus aureus. Haemophilus influenzae was formerly a common cause of head and neck cellulitis (including periorbital and orbital cellulitis) in children until routine H influenzae type B (HiB) vaccination was established in developed countries. Other bacterial agents may be considered based on immunization status and age.

In cellulitis, bacteria invade through breaks in the skin, including insect bites, puncture wounds, lesions from dermatitis, varicella, and burns. The clinical manifestations of cellulitis include rapidly progressive areas of skin edema, redness, warmth, and pain with or without associated lymphangitis or lymphadenitis. In children, cellulitis often occurs on the lower extremities and buttocks. Systemic symptoms of fever, malaise, and chills are common. In immunosuppressed individuals, the infection can spread to cause large abscesses, necrosis, and dissemination into blood. Predisposing factors include conditions that compromise the barrier function of the skin (such as atopic dermatitis) or weakened host defenses (such as malnutrition, obesity, trauma, chronic edema, cancer, and HIV infection).

Children with facial cellulitis are more often admitted to the hospital and may require multidisciplinary care, particularly if there is periorbital or orbital cellulitis present.

Recurrent cellulitis occurs less frequently in children as compared to adults; risk factors include lymphedema and rhinosinusitis (for periorbital cellulitis).

See the Infant / Neonate summary for information on cellulitis-adenitis syndrome.

Although many cases of cellulitis are attributable to Streptococcus spp, S aureus is another cause of cellulitis, and it is important to be cognizant of the rising prevalence of methicillin-resistant S aureus (MRSA) in communities. S aureus should be considered for purulent infections. In the outpatient setting, MRSA coverage should be added for patients whose cellulitis has not responded to antistaphylococcal beta-lactam therapy.