Why is eczema herpeticum unexpectedly rare?

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease of humans, affecting approximately 17% of children. AD patients are especially susceptible to cutaneous bacterial and viral infections, and may develop severe or fatal herpes simplex virus (HSV) infection (eczema herpeticum, EH), requiring intensive antiviral therapy. However, even though a majority of adults show serologic evidence of previous HSV exposure, EH occurs in less than 3% of AD patients. The unexpected rarity of AD patients with EH (ADEH+) suggests that multiple host factors play a role in the clinical expression of this complex phenotype. Recent studies comparing ADEH+ versus ADEH- patients reveal that patients prone to ADEH+ have more severe AD skin disease, biomarkers associated with Th2 helper cell responses (reduced interferon levels, circulating eosinophil counts, increased serum IgE and allergen sensitization) and decreased epidermal expression of filaggrin and antimicrobial peptides. ADEH+ subjects are also more likely to have a history of food allergy or asthma, early onset of AD and a history of other cutaneous infections with Staphylococcus aureus or molluscum contagiosum.