Powell Perng, BSE, BA, MS4

	The mother of a 12-year-old girl brings her daughter to the office for evaluation of recurrent, itchy red bumps and blisters on her legs. The mother gets the same rash recurrently as well. 

	Papular urticaria is a common but underrecognized cutaneous hypersensitivity reaction arising from insect bites in children and less commonly in adults. The condition is characterized by chronic linear eruptions of itchy, red, edematous papules, typically found on the extensor surfaces of arms and legs. Unlike classic urticaria, which resolve within hours, papular urticaria persist for days to weeks. It is most often seen in the spring and summer, with children aged between 2 and 7 years comprising the majority of cases.

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	Papular urticaria is less common in infants

 who have not yet been sufficiently sensitized to insect antigens, and beyond adolescence, at which point hypersensitivity to insect antigens declines because of years of repeated exposures.

 Arthropods, including spiders, lice, ticks, mites, fleas, and mosquitos, are common culprits.

 Risk factors include warmer seasons of the year (spring and summer), being of lower socioeconomic status, and exposure to household pets.

Lesions begin as 3 mm to 10 mm pruritic, erythematous wheals that progress over hours to brownish red or violaceous papules. In highly sensitized individuals, papules can progress to tense vesicles and subepidermal bullae within 1 to 3 days of onset.

 Lesions are often symmetric in distribution

 and arranged in irregular clusters or quasilinear patterns (eg, the famous breakfast, lunch, and dinner arrangement). Face, neck, and extensor surfaces of the extremities are most commonly involved, whereas acral, genital, perianal, and axillary involvement is less common and should prompt alternative diagnoses.

 Lesions heal over 4 to 6 weeks, leaving residual hyperpigmented macules, often with a hypopigmented center, particularly in dark-pigmented individuals. Because sensitivity to insect antigens varies among individuals, it is often the case that close contacts of a patient with papular urticaria are asymptomatic.

Papular urticaria can be difficult to diagnose and are easily mistaken for other dermatoses with similar presentations. Scabies is frequently invoked but is associated with J-shaped superficial burrows, and lesions localize to interdigital and flexural spaces. Contact dermatitis can also present with pruritic papules and vesicles but is often asymmetric in distribution with underlying patches of erythema.

	Papular urticaria can be mistaken for early-stage varicella but importantly will lack the constitutional symptoms of chickenpox and is often chronic and recurrent. Papular urticaria with vesicles and/or bullae may be confused with bullous impetigo, which is identified by golden-brown crusting, or immune-mediated bullous dermatoses, which tend to be truncal in distribution and feature superficial, asymptomatic, 2-mm to 3-mm pustules that quickly rupture, leaving expanding crusts.

 Other considerations include papular drug eruption, pityriasis lichenoides, and urticaria pigmentosa.

	Identifying and eliminating the inciting cause is preventive but often difficult to achieve. Fumigating the home, laundering bedding in hot water, and treating household pets for parasites may lead to dramatic improvements.

 However, the source of biting insects may be outside a patient’s home.

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	Short-term topical steroids and systemic antihistamines such as cetirizine and Benadryl can be used for symptomatic management.

 Patients are advised to wear protective clothing and use insect repellent with 30% DEET while outdoors to prevent recurrent eruptions. 

	The patient and her mother were counseled about prophylaxis with protective clothing and insect repellent and watching for secondary infection. For symptomatic relief, they agreed to try cool compresses and to apply a topical steroid. However, they were warned that the blisters could continue to itch for several weeks and new lesions would do the same.

	1. Steen CJ, Carbonaro PA, Schwartz RA. Arthropods in dermatology. 

	2. Stibich AS, Schwartz RA. Papular urticaria. 

	3. Shaffer B, Jacobson C, Pori PP. Papular urticaria; its relationship to insect allergy. 

	4. Demain JG. Papular urticaria and things that bite in the night. 

	5. Hernandez RG, Cohen BA. Insect bite-induced hypersensitivity and the SCRATCH principles: a new approach to papular urticaria. 

	Mr Perng is a fourth-year medical student at Johns Hopkins University School of Medicine, Baltimore, Maryland. Dr Cohen, section editor for Dermcase, is professor of Pediatrics and Dermatology, Johns Hopkins University School of Medicine, Baltimore. The author and section editor have nothing to disclose in regard to affiliations with or financial interests in any organizations that may have an interest in any part of this article. Vignettes are based on real cases that have been modified to allow the author and editor to focus on key teaching points. Images also may be edited or substituted for teaching purposes.

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The mother of a 12-year-old girl brings her daughter to the office for evaluation of recurrent, itchy red bumps and blisters on her legs. The mother gets the same rash recurrently as well.

Author | Powell Perng, BSE, BA, MS4

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