Papular urticaria

Definition

Papular urticaria is a hypersensitivity reaction to the bites of insects and arachnids, with chronic or recurrent itchy papules, mainly in children. It is also called persistent insect bite reaction; "insect bite-induced hypersensitivity" has been proposed as a more precise term.

Occurrence & epidemiology

Children are affected most often because they have not yet become desensitised to insect bites. In adults it occurs mainly in travellers to new environments. Flares cluster in the warmer months.

Aetiopathogenesis

An immunological reaction to bites; the initial bite is rarely noticed. Fleas and mites from cats and dogs are most often responsible; mosquitoes, bed bugs, bird mites, carpet beetles and caterpillars have also been reported. As desensitisation develops, the reaction settles over months to years.

Clinical features

• Clusters of itchy red papules (0.2–2 cm) without systemic symptoms, each with a central punctum; the papule may surround a wheal, and crops of vesicles can occur.
• Mainly on the legs and other uncovered areas (forearms, face), sometimes in small groups all over the body.
• New lesions appear every few days as older ones fade; a new bite can reactivate old lesions.
• The papules last days to weeks and can leave postinflammatory pigmentation or hypopigmented scars, particularly after deep scratching; scratching can lead to secondary bacterial infection (impetiginisation).

Histology

Mild dermal oedema, extravasated erythrocytes, interstitial eosinophils and exocytosis of lymphocytes; vasculitic features may be present.

Diagnosis

• Usually a clinical diagnosis; asking about pets, travel and season is helpful.
• A biopsy can support the diagnosis, as insect bites have a characteristic microscopic appearance.
• Affected children are often misdiagnosed and subjected to extensive, sometimes invasive investigations; clinical features (the SCRATCH principles) are intended to aid early diagnosis.

Differential diagnoses

• Urticaria
• Scabies
• Atopic dermatitis (atopic eczema)
• Cimicosis (bed bug bites)
• Pediculosis capitis

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References (selection)

1. DermNet: Papular urticaria
2. PubMed: Papular urticaria (PMID 11534921)
3. PubMed: Insect bite-induced hypersensitivity and the SCRATCH principles: a new approach to papular urticaria (PMID 16751615)

Cross-references