eMedicine Specialties > Infectious Diseases > Skin and Soft-Tissue Infections
Cellulitis
Updated: Sep 23, 2009
Introduction
Background
Cellulitis refers to an inflammatory process caused by bacterial infection of the dermis and underlying subcutaneous tissues of the skin. A local and systemic host immune response leads to signs of inflammation in the affected tissue.
Pathophysiology
The human integument provides a very effective barrier against environmental pathogens. Squamous epithelial cells, with their tight intercellular bonds, are the first line of defense against the outside environment. When this barrier is breached owing to trauma or underlying dermatitis, bacteria are able to penetrate to the deeper dermis, where infection occurs. Bacteria commonly found on the skin are most often the cause of cellulitis, although bacteria from the environment may also cause disease.
Frequency
United States
Recent evidence suggests that the incidence of cellulitis is increasing in the United States. Outpatient visits for cellulitis increased from 32 to 48 per 1000 population from 1997 to 2005.1
International
The frequency of cellulitis is unknown.
Mortality/Morbidity
The vast majority of cellulitis and soft-tissue infections can be treated on an outpatient basis with oral antibiotics and do not result in lasting sequelae. However, just as the incidence of cellulitis is increasing, so is the severity. Although the exact reason for this is unknown, certain host and pathogen factors play a role in increasing the risk of severe infection.
Perhaps the most important contribution to the increasing severity of cellulitis is the emergence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), specifically the USA 300 clone, as a leading pathogen in cellulitis and soft-tissue infections associated with purulence.2 Infections caused by CA-MRSA tend to be more severe and are resistant to many of the antibiotics commonly used to treat cellulitis.
This is a case of cellulitis without associated purulence. Note the presence of lymphedema, a risk factor for cellulitis (photo courtesy of Amy Williams).
Patient with cellulitis of the left ankle. Note the area of drainage. This cellulitis was caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). (Photo courtesy of Texas Dept. of Public Health)
Abscess and associated cellulitis caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). (Photo courtesy of Texas Dept. of Public Health)
Hand cellulitis caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). (Photo courtesy of Texas Dept. of Public Health)
Certain strains of bacteria, most notably group A beta-hemolytic Streptococcus (GABHS) and S aureus, produce toxins. These toxins may mediate a more severe systemic infection, leading to septic shock and death.3,4
Certain host factors predispose to severe infection. Individuals with comorbid conditions such as diabetes mellitus, immunodeficiency, cancer, venous stasis, chronic liver disease, peripheral arterial disease, and chronic kidney disease appear to be at a higher risk for both recurrent and more severe infection owing to an altered host immune response. Concurrent intravenous or subcutaneous “skin popping” drug use also predisposes to cellulitis. Infections in this setting are most often polymicrobial, but CA-MRSA is also a common pathogen.
Race
Cellulitis has no predilection for any race or ethnic group.
Sex
Cellulitis has no predilection for either sex.
Age
In general, no specific age group is at a higher risk for cellulitis. However, certain age groups are at a higher risk in some unique scenarios.
- Historically, buccal cellulitis caused by Haemophilus influenzae type B was more common in children younger than age 3 years. Vaccination against this organism may have decreased the incidence of this infection, but recent data suggest that this remains something to be considered, even in vaccinated cohorts.5
- Facial cellulitis is more common in adults older than 50 years.
- Perianal cellulitis, usually with GABHS, occurs in children younger than 3 years.
- Group B Streptococcus cellulitis occurs in infants younger than 6 months. Underlying osteomyelitis or septic arthritis must be excluded in these infants.
- Elderly patients with cellulitis are predisposed to thrombophlebitis.
Clinical
History
A comprehensive history is vital to diagnosing cellulitis. The patient may or may not relate an episode of trauma that has preceded their symptoms. Cellulitis develops several days after the inciting trauma; this intervening period often means that the patient does not recollect any trauma, which is often minor, at the time of presentation. Rapid progression is concerning sign of a more severe infection, such as necrotizing fasciitis, and should be managed promptly.
- If the patient recalls an episode of trauma, the clinician should ask about circumstances surrounding the incident that may elicit clues to a particular etiology. Identifying the specific inciting cause helps the clinician to identify the most likely pathogen, to choose appropriate antibiotic therapy, and to offer appropriate immunization, if indicated.
- The presence of foreign bodies, including indwelling intravenous catheters, external orthopedic pins, and other surgical devices, predisposes to infection in the local area.
- The patient should also be questioned about the presence of other skin disorders, including various types of dermatitis and especially preceding fungal infection, which serve as a portal of entry for bacterial pathogens.6
- The past medical history should focus on the presence of comorbid conditions that increase the risk for cellulitis, the most common of which include diabetes mellitus, HIV infection/AIDS, chronic kidney disease, and chronic liver disease.
Documented histoplasmosis cellulitis in a patient with AIDS.
- The surgical history may include a recent surgery that resulted in wound infection. Alternatively, a remote surgical history involving lymph node dissection or venotomy continues to predispose to cellulitis, even years after the surgery, owing to resultant edema.7
Physical
The physical examination should first focus on the area of concern. Most skin and soft-tissue infections have the 4 cardinal signs of infection: erythema, pain, swelling, and warmth. Several physical examination findings may help the clinician to identify the most likely pathogen and to assess the severity of the infection, facilitating appropriate treatment.
- Skin infection without underlying drainage, penetrating trauma, eschar, or abscess is most likely caused by streptococci. On the other hand, S aureus, often CA-MRSA, is the most likely pathogen when these factors are present.8
- Lymphangitic spread (red lines streaking away from the area of infection), crepitus, and hemodynamic instability are indications of severe infection, requiring more aggressive treatment.
- Circumferential cellulitis or pain that is disproportional to examination findings should prompt consideration of more severe soft-tissue infection.
Causes
The most common cause of cellulitis is infection with gram-positive cocci, specifically GABHS and S aureus, whether it be MRSA or methicillin-sensitive S aureus (MSSA). In certain circumstances, organisms such as anaerobes, gram-negative bacteria, and fungi may also cause cellulitis.
Some unique clinical situations to be aware of include the following:
- Individuals with diabetes are predisposed to skin infection. Infections in these patients are often polymicrobial. Common cellulitis pathogens, S aureus and streptococci, are still common in such patients.9
- Mammalian bite wounds represent a specific subset of cellulitis with unique pathogens. These infections are usually polymicrobial.10 Human, dog, cat, and wild-animal bites all predispose to cellulitis with unique pathogens, but dogs are the most commonly encountered bite wound in both the primary care and emergency setting.11 Several organisms are of particular interest in animal bites, including the following:10
- Capnocytophaga canimorsus (dog)
- Eikenella corrodens (human)
- Pasteurella multocida (dog or cat)
- Streptobacillus moniliformis (rat)
- Vibrio vulnificus may cause cellulitis following a traumatic injury in brackish or salt water. Individuals with chronic liver disease are particularly susceptible to V vulnificus infections.12
Cellulitis due to documented Vibrio vulnificus infection. Image courtesy of Kepler Davis.
Cellulitis caused by Vibrio vulnificus. Image courtesy of Kepler Davis.
- Cellulitis in infants is usually caused by hematogenous spread of group B Streptococcus. These patients often have additional sites of infection, such as the blood, joints, bone, kidneys, or central nervous system.
- Immunocompromised individuals are at an increased risk for gram-negative or fungal cellulitis.
- Cellulitis occurring around surgical wounds less than 24 hours postoperatively may result from GABHS or Clostridium perfringens infection. C perfringens produces gas, which may be appreciated on examination as crepitus.
- Puncture wounds, especially through the bottom of athletic shoes, may cause Pseudomonas osteomyelitis and/or cellulitis.
- Mycobacterium marinum is an atypical mycobacterium that is ubiquitous in aquatic environments and usually inoculates humans after fish-tank exposure.13
More on Cellulitis |
 Overview: Cellulitis |
| Differential Diagnoses & Workup: Cellulitis |
| Treatment & Medication: Cellulitis |
| Follow-up: Cellulitis |
| Multimedia: Cellulitis |
| References |
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References
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Further Reading
Keywords
cellulitis, gram-positive bacteria, group A beta-hemolytic Streptococcus, GABHS, Staphylococcus aureus, S aureus, MRSA, methicillin-resistant Staphylococcus aureus, CA-MRSA, community-acquired MRSA, Streptococcus pyogenes, S pyogenes, systemic toxins, bacteremia, sepsis, buccal cellulitis, Haemophilus influenzae type B, HIB, facial cellulitis, perianal cellulitis, group B Streptococcus cellulitis, Pseudomonas osteomyelitis, septic arthritis, thrombophlebitis, Pasteurella multocida, P multocida, Vibrio vulnificus, V vulnificus, Aeromonas species, Clostridium perfringens, C perfringens, crepitus, crepitation, Escherichia coli cellulitis, E coli, septic shock
