Arthropods are common in the environment. Although many can bite or sting humans, only a few infest humans. Arachnids (mites) are the most common, parasitizing humans and animals by burrowing into the skin and depositing eggs within the skin.
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Scabies is caused by the mite Sarcoptes scabiei. The female mite burrows into the epidermis and deposits her eggs, which mature in 10 to 14 days. The disease is highly contagious because infested humans do not manifest the typical signs or symptoms for 3 to 4 weeks, facilitating transmission. An immunocompetent person with scabies typically harbors only 10 to 20 mites.
The clinical presentation varies depending on the age of the patient, duration of infestation, and immune status of the patient. Severe and paroxysmal itching is the hallmark, with complaints of itching that is frequently worse than the eruption would suggest. Most children exhibit an eczematous eruption composed of red, excoriated papules and nodules. The classic linear papule or burrow is often difficult to find. Distribution is the most diagnostic finding; the papules are found in the axillae, umbilicus, groin, penis, instep of the foot, and web spaces of the fingers and toes (Fig. 196-1). Infants infested with scabies have diffuse erythema, scaling, and pinpoint papules. Pustules, vesicles, and nodules are much more common in infants and may be more diffusely distributed. The face and scalp usually are spared in adults and older children, but these areas are usually involved in infants. Nodular lesions may represent active infection or prolonged hypersensitivity lesions following resolution of infestation. Immunocompromised or neurologically impaired persons may develop a severe form of the disease known as Norwegian or crusted scabies, with infestation of 2 million live mites at one time.
Figure 196-1 Scabies (hand).
The diagnosis of scabies can be confirmed by microscopic visualization of the mite, eggs, larvae, or feces in scrapings of papules or burrows examined under oil immersion. Skin biopsy is rarely necessary but may be useful if lesions have become nodular.
Curative treatment is achieved by a 12-hour (overnight) application of permethrin 5% cream applied to the entire body. Because permethrin is not effective against the eggs, the treatment should be repeated 1 week later to kill any subsequently hatched larvae. All household members and close contacts should be treated simultaneously, even if asymptomatic. Bed linens, towels, and clothes worn for the previous 2 days before treatment should be machine-washed in hot water and machine-dried using high heat; heat is the most effective scabicide. Items that are not washable may be dry-cleaned or placed in a sealed plastic bag for 7 days.
Secondary bacterial infection may occur but is uncommon. In contrast to the pediculoses, scabies is not a vector for infections. Pruritus may persist for 7 to 14 days after successful therapy because of a prolonged hypersensitivity reaction, which does not indicate treatment failure. Inadequate treatment or reinfestation should be suspected if new lesions develop after treatment.
Three species of lice infest humans: Pediculus humanus capitis, the head louse; Pthirus pubis, the pubic louse or crab louse; and Pediculus humanus humanus (also known as Pediculus humanus corporis), the body louse. Lice are wingless insects 2 to 4 mm in length that cannot fly or jump. Transmission usually occurs by direct contact with another infested individual. Indirect spread through contact with fomites or personal belongings, such as hairbrushes, combs, or caps, is much less frequent.