Papular Urticaria


Key Points

  • Papular urticaria refers to a clinical presentation of hypersensitivity reactions to insect bites that results in a distinct cutaneous morphology and natural history.
  • The classic morphology is a symmetric urticarial to papular or papulovesicular eruption, each lesion usually 0.3-1 cm in diameter, which is intensely pruritic, occurs in crops, and lasts a few days to a week.
  • Fleas, bedbugs, mosquitos, and mites are the most common causes of papular urticaria.
  • There is increased seasonal incidence of papular urticaria during the spring and summer months.
  • It is important to note that the bites of many insects can trigger hypersensitivity reactions only in certain individuals. Children are most commonly affected.
  • With repeated exposure, the hypersensitivity reaction wanes and the skin eruption spontaneously resolves or ceases to flare. This may take weeks, months, or even years.
  • The therapeutic strategy is to identify and eliminate the arthropod source and to suppress the resultant inflammatory reaction.

Introduction

Papular urticaria refers to a clinical presentation of hypersensitivity reactions to insect bites that results in a distinct cutaneous morphology and natural history. The classic morphology is a symmetric urticarial to papular or papulovesicular eruption, each lesion usually 0.3-1 cm in diameter, which is intensely pruritic, occurs in crops, and lasts a few days to a week. They may have a central punctum and typically occur on exposed areas of skin (arms, legs). Though they can be caused by a broad spectrum of insect causes, these lesions are termed papular urticaria, independent of the inciting organism. They are commonly caused by arthropod bites. Other common causes include bites by mosquitos, followed by bites by fleas, chiggers, bedbugs, or blackflies (“no-see-ems”). When caused by arthropods, pets or other animals are often the primary host (e.g., Cheyletiella, animal scabies, mite dermatitis, fleas). Exposure to penetrating body parts (setae) of certain moths and caterpillars may cause similar reactions. This chapter discusses those hypersensitivity reactions to arthropod bites and exposures from species that do not infest (e.g., live on) humans; scabies and lice are discussed elsewhere.

There is increased seasonal incidence of papular urticaria during the spring and summer months. It is important to note that the bites of many insects can trigger hypersensitivity reactions only in certain individuals. This contributes to a key clue to the natural history of this process—only certain individuals within a household will be affected by papular urticaria, though all household individuals are exposed to the inciting bites. Thus often only a single individual within a household is affected. Children are most commonly affected. They typically present without systemic symptoms and are well-appearing. Over time, with repeated exposure, the hypersensitivity reaction wanes and the skin eruption spontaneously resolves or ceases to flare; this may take weeks, months, or even years. In rare cases, resensitization—marked by increased numbers of lesions and symptoms such as pruritus—may occur.

The therapeutic strategy is to identify and eliminate the arthropod source and to suppress the resultant inflammatory reaction. The most difficult step in therapy is identifying the offending arthropod; three factors are needed in this identification:

  • Know the offending arthropods in your geographic region.
  • Know the distribution pattern of bites (i.e., lower leg: fleas, chiggers; grass-exposed areas of arms and face: biting flies, mosquitos).
  • Take a careful history from the patient or family members: Are there pets in the home? Have arthropods or potential arthropod vectors been seen or collected? Is there occupational, travel, or recreational exposure? In difficult cases, this information is critically important and aids in identifying unusual causes of papular urticaria (e.g., rodent mite dermatitis).
  • Exclude human scabies and lice infestation by careful examination of the patient’s body and clothing.

Because papular urticaria can clinically mimic other inflammatory conditions, and may have a widespread eruption, many patients will undergo extensive diagnostic evaluation. It is therefore critical for clinicians to have a high level of suspicion for this entity and recognize the typical morphology and natural history. In classic cases, the diagnosis is clinical and additional diagnostic evaluation is not needed.

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