Pathogenesis and presentation of insects and mite bites

Insects and mites produce their effects on the skin by mechanical trauma and injection of irritant, toxic or allergenic substances. Insect and mite bites most often present as an itchy wheal that develops into a firm, itchy papule—this papule may persist for days to weeks. Occasionally, insect bite and mite reactions persist for months. The centre of the papule may show a punctum or vesicle. Papules are often grouped or in lines, and appear most often after known exposure or likely exposure (eg after camping, being outdoors).

The number and distribution of lesions depend on the type of insect or mite, and degree of exposure. Exposed skin is usually affected, but fleas and mites can cause rashes underneath clothes. New bites by the same species can often reactivate old lesions.

Examples of allergic reactions to insect and mite bites include papular urticaria (blistering reactions that are common on the lower legs, particularly in children) and cercarial dermatitis. In elderly people, insect bites on the lower legs may bleed or ulcerate. Occasionally, a severe reaction to a bite causes an area of induration and swelling that mimics cellulitis.

See also Bites, stings and rashes in the wilderness for more information (including information on stings).

Some patients present with a sensation of their skin crawling, which they attribute to insects; however, this may be due to other causes such as recreational drug use (eg methamphetamine [‘ice’]) and skin dysesthesia. Other causes can be psychiatric—consider psychiatric referral, if appropriate.