Arthropod infestations of humans are most commonly caused by head or body lice (pediculosis), pubic lice (pthiriasis), fly larvae (myiasis), or mites. Although many mite species may feed on human tissue, sarcoptid mites (scabies) are the most common mites living on human hosts. All of these arthropods can cause irritation and inflammation of the skin, but fly larvae may penetrate more deeply into the body. Diagnosis of each of these parasitic problems is dependent on accurate identification of the infesting arthropod. Lice and scabies mites are readily transmitted between close contacts, whereas myiasis is not a contagious condition.

PEDICULOSIS CAPITIS

The most common form of louse infestation in North America is caused by the head louse, Pediculus humanus (designated Pediculus humanus capitis in the past to differentiate it from the body louse, formerly designated Pediculus humanus humanus, which has now been found to be genetically identical to the head louse). The presence of louse eggs (nits) cemented to the hairs of the scalp, or of lice themselves, is diagnostic. Many other bite lesions are caused by arthropods, such as spiders, rodent or bird mites, bed bugs, or biting flies, such as mosquitoes, but none of these are found infesting the affected person. The stage of the louse most commonly seen is the nit. Each nit is oval, opaque, and white (about 0.8 × 0.3 mm) and attached individually to a single hair by the female louse (Figure 24.1).