10-07-2025
A 10-Year-Old Presents With ‘Itchy Bumps' on His Extremities
A 10-year-old boy presented to the dermatology clinic for evaluation of 'itchy bumps' on the arms and legs over several months. The child is medically well with a history of mild asthma. The exam shows erythematous papules and vesicles on the upper and lower extremities, along with scattered crusted lesions and hypopigmented ring lesions (Figures 1 and 2).
Figure 1.
Figure 2.
The correct diagnosis is bug bite hypersensitivity.
Discussion
The clinical evaluation showed several scattered erythematous papules that appeared in a linear morphology, along with scattered vesicles. The vesicles had clear fluid, without purulence (Figure 2). These clinical findings suggest a clinical diagnosis of hypersensitivity to bug bites, also known as papular urticaria. The pattern on the arms is consistent with the 'breakfast, lunch, and dinner' sign of bug bite reactions (Figure 1).
Both flea bites and mosquito bites can elicit a skin reaction that typically consists of edematous papules with associated pruritus. There are two phases to the reaction: immediate and delayed. The immediate reaction results in an edematous, erythematous papule within 20-30 minutes of the mosquito bite. The delayed reaction results in pruritic papules for up to 1-3 days. These itchy papules can cause sequela such as possible scarring, hyperpigmentation, and infection from secondary excoriations. Typically, they do not cause systemic reactions, but this can occur in rare occurrences. Hypersensitivity reactions may result in vesicles that can range from 1-2 mm up to several centimeters in size. Other arthropods, such as bedbugs, may cause similar lesions.
It is thought that the mechanism of action is an immunologic response elicited from an anticoagulant in mosquito saliva. The exposure activates mast cells via immunoglobulin (Ig)E antibodies; however, some findings also demonstrate recruitment of non-IgE mediated immune responses. In a small subset of individuals, an amplified immune response can occur, causing a large local reaction, defined as an erythematous wheal larger than 5 mm. These individuals are thought to have mosquito allergy. One study found that children with atopy were more likely to have amplified reactions to mosquito bites: 35% of those with mosquito allergy had atopy compared with only 12% of control subjects ( P <.001). Additionally, 32% of children with intense bite reactions were found to have other atopic diseases such as asthma, allergic rhinitis, or eczema. Skeeter syndrome is a large, local inflammatory response to mosquito bites that is usually accompanied by fever and lymphadenopathy
Additionally, studies have shown that there is evidence of varying human susceptibility to mosquito bites and associated itch. Monozygotic and dizygotic twin studies have demonstrated that there is a stronger genetic association to mosquito bite susceptibility in identical twins. This suggests that the relationship between genetics, skin microbiome, and body odor can alter mosquito attractivity to individuals.
An important consideration when approaching management of hypersensitivity to bug bites is avoidance. Mosquitoes are found near areas with standing water because of their life cycle. Therefore, avoiding these areas and increasing physical barriers with protective clothing and mosquito nets is useful in decreasing mosquito bites. Insect repellants, including DEET, can be highly effective but in rare cases can cause adverse reactions.
The mainstay treatment for mosquito bites is second-generation antihistamines and topical steroids. It may also be reasonable to treat with topical mupirocin if signs of impetigo are present. Management of fleas often includes treating pets and applying household treatments, which can include vacuuming and washing of bedding and carpets, and pest control services.
The differential diagnoses for hypersensitivity to bug bites can share similarities in presentation, but ultimately there are key differences in the morphology and clinical course.
Allergic contact dermatitis is a type of skin inflammation mediated by an allergic response after exposure to a substance. It typically presents as an eczematous dermatitis with vesicles, oozing, crusting, and sometimes bullae formation in areas of the skin with direct exposure to the allergen. Given this presentation, the location and morphology are often signs of this reaction, such as a rash on the wrist after wearing nickel-containing items or linear streaks from brushing against poison ivy, poison oak, or poison sumac.
Cellulitis is caused by a bacterial infection of the dermis and subcutaneous tissue introduced via a break in the superficial skin. It presents as an area of well-defined erythema associated with pain, swelling, and warmth.
Folliculitis is caused by inflammation that affects hair follicles. Because this process targets the hair follicle unit, folliculitis typically presents as small, itchy, erythematous papules in areas with significant hair growth that can progress into erythematous pustules.
Varicella is characterized by an acute fever and widespread pruritic erythematous papules that progress into vesicles on an erythematous base ('dewdrop on a rose petal') before finally crusting.
