Insect Bite Reactions

Initially, an insect bite can cause a local area of erythema, edema, and pruritus at the site of the bite. The reaction is triggered by irritant substances in insect saliva and often occurs within minutes of the offending bite.
This initial localized inflammatory reaction is short-lived and may go unnoticed.
In sensitized individuals, a local reaction may be followed by a delayed skin reaction consisting of urticarial wheals, itching, swelling, redness, firm red papules or nodules that may last for hours to several days.
The area of redness and swelling can extend several centimeters from the bite.
An overlying central punctum may be present.
Occasionally, especially in young children, a vesicular, bullous, indurated, or hemorrhagic lesion can occur (Fig. 8.11).
Lesions are characteristically grouped and when in a linear distribution are referred to as the insect's “breakfast, lunch, and dinner” (Fig. 8.12 [see also Fig. 29.2]) on exposed areas of skin, typically the extremities.
It is often not possible to determine the exact type of offending insect from the skin reaction.

Papular urticaria

Papular urticaria occurs when there are recurrent and chronic itchy papules on exposed areas of skin that are triggered by a hypersensitivity reaction to a variety of insect bites. Papular urticaria typically occurs in young children in the late spring and summer.
Lesions present as grouped, 3 to 10 mm urticarial or firm pink papules some with central punctum or with an overlying excoriation or crusting.
Typically found on face, and exposed areas of the extremities (Fig. 8.13). Covered areas and body folds are typically spared.
Individual lesions usually heal over 1 to 2 weeks but scratching may lead to reactivation and persistent itching of older lesions, which leads to a chronic itch-scratch cycle that can last from months to years.

Diagnosis is based on history and clinical examination and ruling out other etiologies.

Differential Diagnosis

Infantile Acropustulosis (see Chapter 2: Neonatal and Infantile Eruptions)

Eosinophilic Pustular Folliculitis (see Chapter 2: Neonatal and Infantile Eruptions)

Cellulitis

Bacterial infection of the skin and subcutaneous tissues characterized by expanding erythema, warmth and tenderness.
Often accompanied by fever.
As opposed to bite reactions erythema expands and deepens in color if the cellulitis is untreated.

Management

Prevention

Management should primarily emphasize prevention with the use of protective clothing and insect repellents.
Scented hair products, perfumes, and colognes should be avoided as they may attract insects. Mosquitoes are attracted to bright clothing as well as human odors.
DEET (N,N-diethyl-3-methylbenzamide) is the most effective and most widely used insect repellent and is recommended by the American Academy of Pediatrics at concentrations <30%.
The duration of protection is directly related to DEET concentration. Concentrations between 10% and 30% will provide adequate protection in most circumstances.
Multiple DEET-containing insect repellants are available in varying concentrations and vehicles including: Off! Deep Woods® Insect Repellent (25% DEET), Off! FamilyCare Smooth (15% DEET), and 3M Ultrathon Insect Repellant (25% DEET).
Picaridin is a newer insect repellent that is odorless and has comparable efficacy to DEET. It is used in concentrations up to 20%. Examples include Natrapel 8-Hour (20% picaridin), Sawyer Premium Insect Repellent (20% picaridin), Repel Sportsmen Gear Smart Pump Spray (15% picaridin).
Extensive studies of DEET and picaradin found no significant toxicologic risks from the typical use of these repellents.

Treatment

For insect bite reactions, cooling (with an ice or cold pack) can reduce local edema.
Potent topical corticosteroids such as mometasone 0.1% ointment or fluocinoinde 0.05% gel can be helpful for decreasing inflammation and relieving pruritus.
Topical antipruritic creams, gels, and lotions, such as those containing calamine or pramoxine (Sarna sensitive lotion or Caladryl) may be beneficial.
Topical anesthetic and antihistamine preparations can induce allergic contact sensitivity and routine use should be avoided.
Nonsedating oral antihistamines such as cetirizine or loratadine may be helpful for itching but ideally should be taken prophylactically to help prevent pruritus and whealing.

Papular Urticaria

Management of papular urticaria is particularly challenging because parents need to be persuaded that the condition is the result of insect bites. Many times the bites occurred days to weeks before, and in addition constant scratching and picking at the lesions contributes to their persistence.
Treatment for papular urticaria includes education of parents and prevention of bites as well as mid to potent topical corticosteroids such as mometasone 0.1% ointment or triamcinolone 0.1% ointment applied to individual lesions and antihistamines.
Cordran tape provides the added benefit of occlusion and a physical impediment to scratching.
If itching and scratching is interfering with sleep, a sedating antihistamine such as hydroxyzine (0.5 mg/kg/dose every 6 hours and prior to sleep) may be helpful.
Papular urticaria can be frustrating for both affected families and physicians because of its significant associated itch and chronic nature; however parents should be reassured of its benign nature and eventual resolution.