By Gary M. White, MD
The term papular urticaria refers to the urticarial bumps that occur with insect bites. Blisters are common as well (see below). Humans are often bitten by various insects, e.g., mosquitos, spiders, etc., which induces an immune response at the bite site. See also bed bugs, flea bites, bird mites and gallery.
The most common response is an intensely urticarial papule or nodule. Lesions may have a scab or disruption in the center from the bite. Targetoid or bullous lesions may be seen. The lesion usually occurs in an exposed area. Linear lesions may occur in groups of three (or four) and have been called "breakfast, lunch, and dinner (and a snack)." Bullous lesions are usually tense, filled with clear fluid, and with an uninflamed base. Asymmetry is almost always part of the pattern, helping with the diagnosis.
If lesions continue to develop, then ongoing exposure in the house must be considered. Has anyone in the family stayed in a hotel recently or has anyone slept over recently (bed bugs)? Any nests in the eves (bird mites)? Are there any animals in the house?
Sometimes the patient is adamant that s/he is not being bitten. In that case, the clinician must always consider other possibilities. These include bacterial folliculitis (often pustules are present and the lesions are more painful than itchy), lymphomatoid papulosis and "Insect Bite-like reaction associated with hematologic neoplasms" [Arch Dermatol 1999;135;1503].
An exaggerated response may occur to an arthropod bite [JAAD 1993;29;269]. Clinically, one sees an urticarial plaque or ring centered about a bite, 5-20 cm in size. It may be bullous and typically lasts 1-2 weeks. Occasionally, it is reported in association with chronic Epstein-Barr virus infection. See hypersensitivity to mosquito bites.
It is always helpful to try to identify the source of the insect and avoid future exposure. The patient may need to enlist an exterminator. The entry below of a study from Italy shows to collection indoor dust can be helpful. Sometimes the clinician is sure it is bites, but the patients is skeptical. A biopsy to confirm can be helpful here. A potent topical steroid (e.g., clobetasol) BID for 3-5 days can rapidly reduce the swelling and itching.
Some insect bite reactions can last months. Biopsy of these lesions may indeed show Well's syndrome. Topical, intralesional and even systemic steroids may be indicated [PD 2017;33;677].
A study of 105 subjects with dermatitis induced by arthropods in a domestic environment has been reported [JEADV 107;31;1526]. The authors used the term strophulus to describe this condition (from the Greack "turn of bowels"). Major criteria for inclusion in their study were a multiple, scattered, red bumps; itch; and remission with patient removal from the home. Their method for identifying the responsible organism involved indoor dust collection and subsequent examination. The following is from their methods section: "Indoor dust was manually collected by the patients themselves, after shaking the sheets, pillows, couch and chair upholstery, with an electrostatic cloth from the floor of each room of their house. The collected dust was placed into hermetic plastic jars." Mites were found in 56% and insects 44%. Woman were more frequently affected (73%) than men. Summer infestation was much more common than winter.
The classic pattern: grouped inflammatory papules. The second photo shows bruising from scratching.
An infant with multiple bites that caused an allergic, urticarial reaction.
Blisters are very common.
Sometimes, the blisters are blood-tinged.
Juicy papules and plaques may result from a bite.
Exaggerated bite reactions. Occasionally, larger inflammatory, red, or urticarial rings may form.
For more photos, see insect bite gallery.
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