Skin Disorders Guide

Diagnostic Hallmarks

1. Distribution - pattern depends on the insect
2. Sudden onset
3. Spontaneous resolution in hours to days

Clinical Presentation

Most insect bites result in the development of nonscaling, dome-shaped, red papules 4 to 8 mm in size. A central punctum, if present, is diagnostic. Itching is usually, but not invariably, present. The papules often appear within minutes, but some reactions are delayed and are not apparent until 6 to 12 hours later. In the case of these delayed reactions, patients are often totally unaware that they have been bitten. In such instances the number and distribution of the papules may offer considerable diagnostic help. For instance, bedbug bites are characterized by the clustering of several bites on the trunk, flea bites are most often found oil the lower legs, and chigger bites are clustered where clothing binds. Ant bites and bee stings usually do not cause much diagnostic difficulty, since the lesions are clinically apparent immediately after the insult. Black widow spiders have almost no mark at the bite site, whereas local reactions to brown recluse spiders may be massive.

Most bites and stings resolve spontaneously over several days, but in unusually reactive patients, an urticarial response may be perpetuated for weeks by repeated scratching. Such lesions, known as papular urticaria, are particularly common in children. Anaphylaxis is an uncommon but potentially fatal complication of some insect bites and stings.

Pathogenesis

Bites fall into three categories from a standpoint of pathogenesis: (1) toxic reactions that cause pain (bee stings), (2) toxic reactions that cause itching as a result of pharmacologic release of histamine (mosquito bites), and (3) allergic reactions that cause itching as a result of antigen deposition and immunologic release of inflammatory mediators (flea bites). These allergic reactions can be particularly troublesome from a diagnostic standpoint, since the reaction does not occur in everyone who is exposed. Thus most patients will be surprised and reluctant to consider insect bites when only one member of an exposed family develops lesions.

Therapy

Most bites require no specific therapy. Soaks and topically applied steroids may offer symptomatic improvement for those with unusually severe reactions. For the most part, identification of the cause allows the patient to take appropriate steps (such as personal application of repellents and, in some cases, treatment of animal hosts) to avoid future problems. Those insect repellents with high concentrations of diethyltoluamide (DEET) such as Cutter’s and Off seem to work best. A variety of folk remedies to repel insects are touted in the lay literature, but proof of efficacy is lacking.