What Is Causing This Newborn’s Rash?

Angelica Wong DesPain, MD

A 2-week-old, full-term newborn boy presented with a pustular/vesicular type rash in his diaper area. His mother said he had developed a pustule in his diaper area on day 4 of life, the day after his circumcision. A total of 8 lesions of various sizes had developed with pus or blood in them during the 1½ weeks prior to having sought medical treatment. His mother had been using petroleum jelly at home after the lesions had spontaneously drained, and she believed the lesions were tender to palpation. The patient did not have any fevers in the first 2 weeks of life and had been taking formula normally.  

The boy had been delivered full-term via normal spontaneous vaginal delivery. The infant’s mother denied any history of sexually transmitted infections and reported negative infectious serology results prior to delivery. The physical examination revealed an alert, active newborn with normal vital signs. He had multiple bullae and erosions in various stages of healing on the scrotum, inner thigh, and mons (Figure). No other lesions were observed on his body. 

What is the cause of these lesions on this newborn?

A. Bullous impetigo
B. Herpes simplex virus lesions
C. Staphylococcal scalded skin syndrome
D. Pemphigus foliaceus

(Answer and discussion on next page)

Answer: Bullous impetigo 

The patient was seen by a dermatologist and received a diagnosis of bullous impetigo secondary to circumcision. Bacterial cultures taken from an intact pustule grew methicillin-resistant Staphylococcus aureus (MRSA) susceptible to clindamycin and trimethoprim-sulfamethoxazole (TMP-SMX). The lesions resolved with topical mupirocin applied 3 times daily for 1 week to the affected areas.

Bullous impetigo is characterized by fragile, fluid-filled vesicles and flaccid blisters caused by pathogenic strains of S aureus, which produces exfoliative toxins that target desmoglein-1 at the junction of the epidermis and dermis.1,2 Because the basal layer of the epidermis is still intact, bullous impetigo demonstrates a positive Nikolsky sign. In bullous impetigo, exfoliative toxins are restricted to the area of infection, and bacteria can be cultured from the blister contents. Whereas bullous impetigo is at the mild end of a spectrum of blistering skin diseases caused by a staphylococcal exfoliative toxin, the generalized form of bullous impetigo is staphylococcal scalded skin syndrome. This syndrome is caused by the hematogenous spread of exfoliative toxins from a source leading to widespread epidermal damage.1

The differential diagnosis for bullous impetigo is broad and includes insect bites, acute contact dermatitis, herpes simplex virus (HSV) 1 and 2, burns, human papillomavirus, pemphigus foliaceus or pemphigus vulgaris, and Stevens-Johnson syndrome.

HSV is a consideration in the neonatal period even in the case of maternal denial of prior infection. Most cases of vertical HSV transmission are from primary infections and can occur when maternal symptoms and lesions are absent.3 

Pemphigus foliaceus also presents with lesions in varying stages of healing, especially in areas of friction. It also demonstrates a positive Nikolsky sign, because the autoantibodies in this disease attack desmoglein-1 as well. However, pemphigus foliaceus usually affects adults, has an average age of onset between 40 and 60 years, and rarely affects neonates.4 This is because neonates express desmoglein-3 in their superficial dermis in addition to desmoglein-1, which compensates for the destruction of desmoglein-1.5

Treatment for bullous impetigo is a course of antibiotics for 5 to 7 days. For mild cases of impetigo, antibiotic creams and ointments such as mupirocin or retapamulin applied twice a day for 5 days are effective. More serious cases of bullous impetigo require oral antibiotics such as cephalexin or dicloxacillin for 7 days. However, in the case of suspected or confirmed MRSA, use of clindamycin, doxycycline, or TMP-SMX is recommended.6 

Angelica Wong DesPain, MD, is a resident at Children’s National Medical Center in Washington, DC.

Kirk Barber, MD, FRCPC—Series Editor, is a consultant dermatologist at Alberta Children’s Hospital and clinical associate professor of medicine and community health sciences at the University of Calgary in Alberta, Canada.


1. Johnston GA. Treatment of bullous impetigo and staphylococcal scalded syndrome in infants. Expert Rev Anti Infect Ther. 2004;2(3):439-446.

2. Yasushi H, Schechter NM, Lin C, et al. Molecular mechanisms of blister formation in bullous impetigo and staphylococcal scalded skin syndrome. J Clin Invest. 2002;110(1):53-60.

3. Brown ZA, Benedetti J, Ashley R, et al. Neonatal herpes simplex virus infection in relation to asymptomatic maternal infection at the time of labor. N Engl J Med. 1991;324(18):1247-1252.

4. James KA, Culton DA, Diaz LA. Diagnosis and clinical features of pemphigus foliaceus. Dermatol Clin. 2011;29(3):405-412.

5. Wu H, Wang ZH, Yan A, et al. Protection of neonates against pemphigus foliaceus by desmoglein 3. N Engl J Med. 2000;343(1):31-35.

6. Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clin Infect Dis. 2014;59(2):147-159.