Scabies is an ectoparasitic skin infestation with the mite Sarcoptes scabiei var. humanus. First described in 1687 with the organism responsible being identified in the 18th century, this intensely pruritic disease was named from the Latin word for “scratch,” scabere. Scabies affect people of all races and age groups and the geographic distribution is worldwide. It is spread by skin-to-skin, via both sexual and nonsexual contact, and in some cases as fomites.

The adult mite is 1/3-mm long and has a life cycle of 30 days. An infestation occurs when a fertilized female mite burrows into the stratum corneum of the host’s skin. As it burrows, it lays eggs and expels fecal pellets behind it. A hypersensitivity reaction to the scybala may be responsible for skin irritation and itching because IgE titers are elevated and eosinophilia develop after initial infestation.

Patients with scabies infestation manifest intractable pruritus, stereotypically with nocturnal exacerbations. Lesions may be eczematous with impetiginization and are often excoriated. Those infected may be asymptomatic for one month following initial infestation by at least one scabies mite. Distribution of skin lesions includes digit web spaces (Fig. 8-1); sides of the fingers; buttocks; peri-umbilical areas; and flexor aspects of the wrists, elbows, and axillary folds. Other areas that may be affected include the penis (Fig. 8-2) and scrotum in men and areolae in women. If noticeable, visualization of a burrow (Fig. 8-3), which can be seen as a short, S-shaped, dark line on the surface of the skin, is pathognomonic for scabies. Family members of patients may also develop similar symptoms and signs. Manifestations of scabies infections are outlined in Table 8-1.

Medication is needed to both kill the mites and destroy the eggs. Scabicides must be applied thoroughly to areas behind ears and from the neck down to the soles of the feet. Regions to emphasize coverage are between the fingers and toes, the umbilicus, groin, between buttocks, and under fingernails and toenails. Topical creams ought to be washed off after the recommended time period to avoid toxicity. The morning after topical treatment, all linens, towels, and clothing in the vicinity of the affected individual should be machine-washed and dried using the hot cycles. This ensures that no mites are present to reinfest the host after treatment. Members of the affected household should also be treated with scabicides. Itching and eczema may persist for several months after treatment, even if all mites are successfully removed.

In cases of crusted scabies and scabies in conjunction with human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS), sequential use of two or more compounds may sometimes be necessary. The entire area of skin should be treated and patients should be isolated until treatment has been completed. The surrounding environment and personal items should be assiduously cleaned.

Complications are most often seen in patients with atopic dermatitis. In these individuals, there is an increase in eczematous lesions during the infestation that may persist even after treatment. Excoriations from these lesions can lead to secondary pyoderma and colonization by nephritogenic strains of streptococci, which are most prevalent in tropical regions. The resulting complication is glomerulonephritis. In nonatopics, postscabetic itch can last several months.

Pediculosis refers to the infestation of certain body areas by head, body, or crab lice. Lice are obligate human ectoparasites that cannot live without their hosts for more than 10 days. The three types of lice, Pediculosis humanis capitis, Pediculosis humanis corporis, and Pthirus pubis, affect different parts of the body, but have similar life cycles and features. Generally, they are 1 to 4 mm long, flat, wingless insects with terminal claws at the end of their three pairs of legs. The legs tend to be clustered toward the anterior aspect of the body, immediately behind the head. The lice live on the hair shafts and crawl down to the
skin to feed on human blood about five times a day. They pierce the skin with their claws, inject anticoagulants and other irritants into the site, and suck blood. Lice are active and travel quickly, resulting in high transmissibility of pediculosis. Most often noticeable in patients with pediculosis is the presence of nits, or eggs, laid by the female lice. Six eggs are laid by the female louse each day for about one month. The nits themselves are less than 1 mm long and are firmly attached to the base of the hair shafts or to fibers on clothing in the case of the body louse. Their proximity to the skin provides nits with the warmth needed to sustain continued growth. Nits are hard to remove, even with a fine-toothed comb.

The bite of the louse itself is painless, but hypersensitivity to the saliva and feces of the mite may lead to inflammation and pruritus. Immediate urticarial lesions may appear in some individuals, but most develop small macules or papules hours to days later. Intense scratching of these areas can result in trauma to the skin, excoriations, and secondary bacterial infections. Most often seen are the results of pruritus. A comparison of the different types of pediculosis is presented in Table 8-2.

Saliva and fecal material can induce a hypersensitivity reaction and inflammation. The resulting pruritus leads to scratching and the development of excoriations and secondary impetiginization of the affected area(s).

There are over 12 million cases per year of head lice in the United States alone. The area of infestation is limited to the human head and is commonly seen on the back of the head and neck and behind the ears. It occurs primarily in children, especially in girls. African Americans have very low incidence of head lice. Head lice are spread by close physical contact and sharing of head gear, combs, brushes, and pillows. Usually, an infestation on an individual will have less than 20 adult lice.

The structure of body lice looks similar to head lice, but body lice tend to be larger. It is an infestation associated with poverty and poor living condition. Therefore, it is most often seen in the poor, homeless, recent refugees, and soldiers in wartime conditions. The main mode of transmission is via contaminated bedding or clothing because body lice do not lay eggs on humans, but rather in clothes that the person wears.

The crab louse (Phthirus pubis) is physically distinctive from head and body lice. Its body is more rounded, resembling the shape of a crab, and has progressively larger claws from its front pairs of legs toward the rear pairs. This formation allows the louse to grip sparser, coarser hairs of the pubic area, eyebrows, eyelashes, thighs, and perianal regions. The incidence of crab lice is slightly higher in men, potentially due to larger amounts of coarse body hair. It is considered a sexually transmitted disease and is associated with other sexually transmitted infections. There are no racial differences in incidence of infestation, although pediculosis pubis often recurs in men who have sex with men (MSM).

There are signs of local resistance to topical medications for treatment of pediculosis. In resistant cases, systemic therapy with ivermectin (adult dosage [for >5 years; safety in <5 not established]: 150 to 200 mcg/kg/d PO as single dose; 12 mg PO repeated in 7 to 10 days) is indicated.