At the visit, the girl’s skin scrapings were analyzed under the microscope with potassium hydroxide (KOH) and no fungal elements were seen. A culture from one of the lesions was positive for methicillin-sensitive Staphylococcus aureus.
She was diagnosed with bullous impetigo (BI).
Impetigo is the most common superficial skin infection and can present as a nonbullous (most common) and bullous (least common) form.1 Nonbullous impetigo is usually caused the Staphylococcus aureus or Streptococcus pyogenes and tends to occur at sites of prior trauma like insect bites, scratches, atopic dermatitis, or varicella. On the other hand, bullous impetigo is caused by the local production of exfoliative toxins (ETA or ETB) by phage group II of Staphylococcus aureus. The exfoliative toxin binds to desmoglin-1, one of the desmosomal proteins of the skin, causing acantholysis at the level of the granular layer and blister formation. Different from nonbullous impetigo, bullous impetigo tends to occur in normal, undamaged skin. Lesions are more common in neonates and young infants but children also can be affected.
The characteristic lesions in bullous impetigo are small blisters that enlarge to 1-cm to 5-cm bullae that easily rupture, leaving an erythematous plaque with a collarette of scale or “double ring scale,” with minimal crust and mild erythema. They commonly occur on the face, trunk, buttocks, and intertriginous areas. The lesions heal within 4-6 weeks, leaving no scarring. Associated systemic symptoms are rare but some patients can present with weakness, fever, and diarrhea. The toxin can disseminate and cause staphylococcal scalded skin syndrome in neonates or older patients with renal failure or immunodeficiency.
The transmission of Staphylococcus aureus can occur from colonized or infected family members, children in contact sports, as well as contact with animals such as dogs, cattle, and poultry.2 Transmission from a pet rabbit also has been reported. In our patient, transmission from her pet hamster could have occurred as the areas on the body where there were lesions were areas where she was holding and cuddling her new pet.
The differential diagnosis of the type of lesions our patient presented with includes tinea corporis, and bullous tinea, which also can be transmitted by animals such as kittens. A KOH analysis ruled out this diagnosis. Tinea skin lesions tend to be more scaly than bullous impetigo lesions, which are more inflamed and crusted. Bullous arthropod reactions should be considered in the differential diagnosis as well. Bullous bite reaction lesions present with tense bullae, as they are subepidermal in nature and are pruritic. Subacute cutaneous lupus lesions present as annular scaly plaques with an erythematous border and central clearing usually in sun exposed areas similar to the distribution of our patient. Severe contact dermatitis reactions also can blister and form similar lesions as seen in our patient but with the difference that our patient didn’t complain of pruritus, which is a characteristic feature of allergic contact dermatitis. In neonates or young infants with bullous lesions other conditions such as herpes simplex infection, epidermolysis bullosa, bullous pemphigoid, linear IgA bullous dermatosis, bullous mastocytosis, and bullous erythema multiforme should be considered in the differential diagnosis.
First line treatment for impetigo consists of the use of topical application of mupirocin (Bactroban) 2% ointment, retapamulin (Altabax) 1% ointment, or fusidic acid 2% cream. A Cochrane review compared systemic versus topical treatment for impetigo concluding that topical treatment with either mupirocin or retapamulin is equally if not more effective than oral antibiotics.3 Ozenoxacin (Xepi), a new nonfluorinated topical quinolone has recently been Food and Drug Administration approved for the treatment of localized impetigo in patients 2 months of age and older.4 When there is treatment failure with topical antibiotics, widespread disease, or systemic symptoms, oral antimicrobials should be consider, such as beta-lactamase–resistant penicillin, first-generation cephalosporins, or clindamycin. The use of bleach baths and general hygiene measures for 4 months can reduce the risks of recurrence in 20% of the patients as noted by a study by Kaplan et al.5
Our patient was treated with oral cephalexin for 7 days as well as topical mupirocin with fast resolution of the lesions. Sadly, the parents gave her hamster pet away.
Dr. Matiz is a pediatric dermatologist at Southern California Permanente Medical Group, San Diego. Email her at firstname.lastname@example.org