24 - Scabies, lice, and myiasis

Summary

Arthropod infestations of humans are most commonly caused by mites, head or body lice (pediculosis), pubic lice (pthiriasis), or fly larvae (myiasis). Although many mite species may feed on human tissue, 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.

Scabies

Scabies is a common parasitic infection caused by the mite Sarcoptes scabiei var. hominis, an arthropod of the order Acarina (Fig. 24.1). The worldwide prevalence has been estimated at about 100 million cases annually. In general, transmission occurs by direct skin-to-skin contact. In crusted scabies, transmission may also occur through infected clothing or bedding. Skin eruption with classical scabies is attributable to both infestation and hypersensitivity reaction to the mite. Moreover, because the eruption is usually itchy, prurigo and superinfection are common. The main symptom is pruritus that typically worsens at night, and it is often associated with itching experienced by other family members in the household or amongst people in close physical contact with an infested individual. The lesions are commonly located in the finger webs, on the flexor surfaces of the wrists, on the elbows, in the axillae, and on the buttocks and genitalia. The elementary lesions are papules, burrows, and nodules. In crusted scabies, clinical signs include hyperkeratotic plaques, papules, and nodules, particularly on the palms of the hands and the soles of the feet, although areas such as the axillae, buttocks, scalp, and genitalia in men, and breasts in women may also be affected.