Cellulitis

Cellulitis is a bacterial infection primarily involving the inner layers of the skin, specifically the dermis and subcutaneous fat. This condition manifests as an area of redness that expands over a few days, often accompanied by swelling, heat, and pain.

The redness typically turns white under pressure, although this isn't always the case. Fever and fatigue are common systemic symptoms.

Signs and Symptoms

The hallmark signs of cellulitis are an area that is red, hot, and painful. It may also involve the lymphatic system, indicated by red streaking from the infection site. Fever and fatigue are additional systemic symptoms.

Complications

Complications can include abscess formation, fasciitis, and sepsis. In severe cases, the infection can extend to deeper tissues, resulting in necrotising fasciitis, a medical emergency.

Causes

Cellulitis is generally caused by bacteria entering through breaks in the skin. The most common pathogens are Group A Streptococcus and Staphylococcus aureus. Conditions that predispose individuals to cellulitis include insect bites, animal bites, recent surgery, and chronic skin conditions like eczema.

Risk Factors

Individuals with weakened immune systems, such as the elderly and diabetics, are particularly susceptible to cellulitis. Diabetics are prone to foot cellulitis due to poor circulation and diabetic neuropathy, which can lead to unnoticed wounds becoming infected. Other risk factors include chronic venous insufficiency, lymphedema, and living in densely populated environments like nursing homes or dormitories.

Diagnosis

Diagnosis is primarily clinical, based on history and physical examination. Typical signs include rapidly spreading redness, swelling, and heat. Blood cultures and skin aspirates are rarely helpful, except in severe cases requiring differential diagnosis to exclude conditions like deep vein thrombosis or necrotising fasciitis. Bedside ultrasound can help distinguish between cellulitis and abscess.

Differential Diagnosis

Conditions that may mimic cellulitis include deep vein thrombosis, stasis dermatitis, erysipelas, and Lyme disease. Misdiagnosis can lead to unnecessary hospitalizations and healthcare costs. Dermatological evaluation can reduce misdiagnosis rates.

Prevention

For individuals with recurrent cellulitis, prophylactic antibiotics may be recommended. Preventative antibiotics have been shown to reduce the incidence of cellulitis in lower limbs, although the effects diminish after stopping the medication.

Treatment

Treatment primarily involves antibiotics, selected based on the suspected pathogen. For nonpurulent cellulitis, penicillinase-resistant penicillins or first-generation cephalosporins are recommended. In cases where an abscess is present, surgical drainage is required. Pain relief and elevation of the affected area are also recommended. Steroids may accelerate recovery when used alongside antibiotics.

Antibiotics

Common antibiotics include cephalexin, amoxicillin, and cloxacillin. For individuals allergic to penicillin, erythromycin or clindamycin may be used. In regions with high methicillin-resistant Staphylococcus aureus (MRSA) prevalence, doxycycline or trimethoprim/sulfamethoxazole may be recommended.

Epidemiology

Cellulitis is a significant global health burden, with around 16,900 deaths worldwide in 2015. In the United States alone, cellulitis accounts for substantial healthcare costs, with more than 650,000 admissions per year. Most cases are nonculturable, but when identified, Streptococcus and Staphylococcus aureus are common pathogens.

Other Animals

Horses can also develop cellulitis, usually secondary to a wound or deep-tissue infection. Treatment includes wound care, NSAIDs, cold hosing, and mild exercise.

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Self-assessment MCQs (single best answer)

Which layers of the skin are primarily involved in cellulitis?

What are common systemic symptoms of cellulitis?

Which bacteria are most commonly responsible for cellulitis?

Which of the following is a risk factor for developing cellulitis?

Which diagnostic tool can help distinguish between cellulitis and abscess?

What is a common complication of cellulitis?

Which condition may mimic cellulitis and needs to be excluded during diagnosis?

What is a recommended treatment for nonpurulent cellulitis?

In areas with high MRSA prevalence, which antibiotics are recommended?

How can recurrent cellulitis be prevented?