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.

Initial Evaluation

Differential Diagnosis

Urticaria

Scabies

Pediculosis (Lice)

Tick Bite

Bullous Arthropod

Treatment

First-line therapy: The first-line therapy for papular urticaria is to identify and eliminate the arthropod source and to then suppress the resultant inflammatory reaction.

First-line management of the inflammatory reaction involves three basic steps:

  • Treat the patient’s hypersensitivity reaction: Prescribe oral antihistamines (e.g., hydroxyzine 25-50 mg before bed and 10-25 mg 3 times daily consistently for 1 week). The nightly dose is not given as needed, but rather is given every night, so the antihistamine is at therapeutic doses when a bite occurs in order to block the allergic cascade induced by the bite. A nonsedating antihistamine such as loratadine, fexofenadine, or cetirizine may be helpful and may be prescribed twice daily in addition to a sedating antihistamine at bedtime (e.g. strategy of antihistamine stacking).
  • Apply a high- to superpotency topical steroid cream (such as fluocinonide 0.05% or clobetasol 0.05%) 2 to 3 times daily to each new papule until it resolves.
  • Secondary infection is common; treat it with appropriate antibiotics (e.g., oral dicloxacillin or cephalexin 250 mg 4 times daily for 7 days).

First-line management to eliminate bites:  Make the patient less attractive to the biting arthropod or insect by instructing the patient to apply an insect repellant containing diethyltoluamide (DEET) daily. Permethrin 5% may also be used as an insect repellent. It repels many biting arthropods (but not fleas) for about 1 week. Instruct the patient when outdoors to cover as much exposed skin as possible.

First steps

Instruct the patient to eliminate infestations of pets and/or the environment: this may require consultation with a veterinarian or professional exterminator. The basic principles of flea control are discussed here, as this is one of the most common problems.

(i)  Treat all dogs and cats in the household. Cats especially may harbor fleas without visible signs. Many of the available oral and topical agents are highly effective.
(ii) Spray or wash the bedding on which the pets sleep or lie for significant periods.
(iii) If the pet frequently lies in one area outdoors, treat this area with an effective insecticide (e.g., malathion). It is probably not necessary to treat the entire yard, although this can be done and should be considered in situations of severe hypersensitivity.
(iv) Use flea bombs (foggers) in all the rooms of the house, especially in carpeted areas. Most effective are those containing an agent that kills adult fleas (e.g., pyrethrin) plus an insect growth regulator to stop development of immature forms (e.g., fenoxycarb ormethoprene). Additional spraying may be required under furniture and in crevices along the wallboards.
(v) Vacuum the entire house and all furniture to pick up eggs and adult fleas. Dispose of the vacuum bags immediately after vacuuming.
(vi) Re-treatment of the pets and environment may be required several weeks after the initial treatment to kill any fleas that may have hatched.

Subsequent steps

  • Most patients who fail to respond to the above measures are still being bitten. Additional effort to remove the cause of the papular urticaria should be undertaken. The patient should consult an exterminator and have pets examined by a veterinarian. In some patients, the papular urticaria will continue for weeks to months, and in rare cases years, after the inciting agent has been removed.
  • Rarely patients will require a short course of systemic steroids for severe reactions.
  •  More potent antihistamines (e.g., doxepin 25-50 mg at bedtime) may be helpful. They should be continued as suppressive therapy for as long as lesions persist. Doxepin is a tricyclic antidepressant, and if used in higher doses (typically above 75 mg daily in adults), cardiac arrhythmias may result.
  •  Phototherapy (broadband UVB, narrowband UVB, PUVA) 2 to 3 times weekly can be effective in patients in whom topical steroids and antihistamines are inadequate. Response is noted after about 10 treatments. Phototherapy can be continued as maintenance treatment.

Pitfalls

  • Misdiagnosis is the major pitfall. Patients with papular urticaria are frequently misdiagnosed as having human scabies. Frequent applications of scabicides may exacerbate the symptoms.
  • Cercarial dermatitis (swimmer’s itch), sea bather’s eruption/sea lice dermatitis, and moth and caterpillar dermatitis all have similar morphologies and symptomatology to papular urticaria. A careful history is required to identify these conditions.
  • Occupational exposure to fiberglass produces a dermatitis very similar to papular urticaria.
  • Patients with delusions of parasitosis present with no primary lesions, but a fixation regarding infestation.
  • Pesticides can be toxic. Repeated or incorrect use can produce complications. Follow directions carefully or consult a professional exterminator.
  • Papular urticaria should not be excluded as a diagnosis if only limited members of a family are affected. Because papular urticaria is a hypersensitivity reaction, only sensitized household members will have clinical lesions, even though all family members are bitten.

When to refer to a dermatologist

  • • When the diagnosis of papular urticaria is not clear.
  • • If a skin biopsy is needed to rule out an alternative diagnosis, or additional diagnostic evaluation is needed.
  • • For chronic cases without improvement over months to years.
  • • For consideration of systemic therapy or phototherapy to control symptoms of pruritus.

Clinical Case

Case 1

  • 12-year-old female
  • No significant past medical history and she takes no medications
  • Review of systems is notable only for pruritus and occasional lack of sleep in association with the rash
  • Presents for management of pruritic papules on the legs and arms that started in the springtime and recur in crops
  • No other household members are affected
  • There is a cat living with the family

 Initial evaluation

  • Tired but otherwise well-appearing female
  • Firm erythematous papules with central punctum scattered in a symmetric eruption on arms and legs, few scattered on trunk without mucosal lesions
  • No linear burrows or other stigmata of scabies
  • No evidence of lice on the body or on clothing
  • Diagnosis: favor papular urticaria
  • There is no evidence of bacterial superinfection of the skin. A regimen of nonsedating antihistamines during the day (loratadine 10 mg twice daily) and sedating antihistamine (hydroxyzine 10 mg at bedtime) and topical corticosteroid cream (fluocinonide cream 0.05%) are prescribed. The family is counseled, advised to seek consultation with a veterinarian for evaluation of their pet cat and also a professional exterminator to determine whether the home is infested by fleas. Strategies for avoiding bites are discussed
  • Follow-up in 4 weeks

 Follow-up evaluation

  • The rash is much improved, however the family reports that the lesions are less frequent in number and less intense in symptoms
  • The family reports that the veterinarian and exterminator identified a flea infestation involving the pet cat and decontamination of the pet and household environment was ongoing.
  • Follow-up in 8 weeks (Resolved—few lesions only. The daytime nonsedating antihistamines are continued until full resolution of the recurrent lesions.)

References

Hernandez RG, Cohen BA (2006) Insect bite-induced hypersensitivity and the SCRATCH principles: a new approach to papular urticaria, Pediatrics, 18(1):e189-96.

Shmidt E, Levitt J (2012) Dermatologic infestations, Int J Derm, 51(2):131-141.