Impetigo

Definition: Impetigo is a common infection of the superficial layers of the epidermis that is very contagious and most often caused by gram-positive bacteria. This condition is characterized by pustules and honey-coloured crusted erosions. It can be classified into non-bullous and bullous impetigo. 

Etiology: Impetigo is caused by Staphylococcus aureus and less commonly, Streptococcus pyogenes. Nonbullous impetigo can be caused by either bacteria or a conjoint infection. Disruption in skin integrity allows for the invasion and colonization of bacteria on the surface. Bullous impetigo is caused by Staphylococcus aureus, which produces exfoliative toxins that target intracellular adhesion molecules of the epidermal granular layer. This results in the dissociation of epidermal cells, leading to blister formation.

Epidemiology: Impetigo accounts for approximately 10% of skin ailments in the pediatric population, most prevalent in children aged 2-5. The male-to-female ratio of incidence is approximately 1:1. Men are more commonly affected. Bullous impetigo is more common in children younger than two. Non-bullous impetigo caused by S. aureus accounts for approximately 80% of cases. Group A beta-hemolytic Strep (GABHS) accounts for 10% of cases. Methicillin-resistant S aureus (MRSA) has become more prevalent in hospitalized patients. 

Signs: Non-bullous impetigo can occur on the face, extremities or other body parts. It begins with a single erythematous macule that evolves into a pustule. This may be erythematous on lighter skin tones, and violaceous or brown on deeper skin tones. When this pustule or vesicle ruptures, it releases serous contents which dry to leave a typical honey-coloured crust.

Bullous impetigo is typically found on the face, trunk, extremities, buttocks and perianal regions. Autoinoculation may cause distal spread of lesions. Bullous impetigo appears as superficial small or large, thin-roofed bullae which spontaneously rupture to leave collarette.

Ecthyma is a deep ulcerated infection that can develop as a complication of bullous impetigo.

Symptoms: Individuals with non-bullous impetigo may experience some itching and regional lymphadenopathy. Individuals with bullous impetigo may experience systemic symptoms of fever, malaise and lymphadenopathy.

Differentials: scabies, atopic dermatitis, contact dermatitis, candidiasis, herpes simplex

Diagnosis: diagnosis of impetigo begins with a thorough history and physical examination. Bacterial cultures may be in the confirmation of diagnosis if MRSA is suspected or if there is an impetigo outbreak. Skin biopsy may be used in refractory cases. Human immunodeficiency virus testing should be considered when a previously healthy adult develops bullous impetigo.

Treatment: Specific measures in treating impetigo include topical or oral antibiotics, proper hand hygiene and avoiding close contact with confirmed cases.

References:

1.     Nardi NM, Schaefer TJ. Impetigo. In: StatPearls. Treasure Island (FL): StatPearls Publishing; July 31, 2023.

2.     Quirke K. Impetigo | DermNet NZ. dermnetnz.org. Published March 2022. https://dermnetnz.org/topics/impetigo