Pathophysiology: The skin is the main organ involved. The lesion is caused by the gravid female mite burrowing beneath the stratum corneum. She leaves behind a trail of debris, eggs and feces (scybala), which induces an immunologic response. The female can lay up to 90 eggs in her 30-day lifespan. It takes 3-4 days for the larvae to hatch; they mature to adult forms over the next 2 weeks and continue the cycle. The average patient is infected with 10-15 live adult female mites at any given time.

Pruritus, the main clinical manifestation, is caused by the phenomenon of hypersensitivity to the debris, eggs and feces, rather than by direct effects of the mite. The primary lesions appear between 3-10 days after exposure to the mite. These lesions include burrows, papules, vesicles and pustules. The rash usually becomes intensely pruritic several days later as the immune system requires time to mount a hypersensitive response. Nocturnal pruritus is characteristic of scabies infestation.

Frequency:

• In the US: Scabies may be seen in all ages. It is not always a disease of overcrowding. Norwegian (crusted) scabies is seen in immunocompromised, HIV positive and institutionalized patients.

Mortality/Morbidity: Intense pruritus is the major morbidity associated with scabies. This often leads to excoriation and secondary bacterial infection.

Race: There appears to be no racial predisposition to acquiring scabies.

History:

• Main presenting features include rash and intense itching.

• In young infants, pruritus may be difficult to detect. Irritability, especially during sleep, may be the only symptom.

• History of involvement of other family members and contacts is often present and helps in establishing the diagnosis.

Physical:

• Primary and secondary lesions

• The classic rash of scabies includes primary and secondary lesions.

• The primary lesions include burrows, papules, vesicles and pustules.

• The secondary lesions occur from scratching and include excoriated papules and crusted areas.

• Rash distribution

• In infants, the most commonly affected areas are the palms, soles, axilla and scalp.

• Involvement of the face is uncommon after 5 years of age.

• In older children and adults, lesions are usually confined below the neck and involve the web spaces between the fingers, flexor surfaces of the arms, wrists, axillae and the waistline. The umbilicus, nipples, penis and scrotum may also be affected.

• Norwegian (crusted) scabies

• Norwegian scabies is characterized by crusted lesions and scaly plaques located mainly on the hands, feet, scalp and other pressure-bearing areas. These may sometimes generalize. Hyperkeratosis may occur in these lesions.

• Patients with Norwegian scabies can be infected with hundreds to millions of adult female mites. As a result, this type of scabies is highly contagious and may spread rapidly through patients in an institutionalized setting.

• Nodular scabies

• Orange-red nodules located in the axillae and groin define nodular scabies. These nodules are pathognomonic of scabies infection.

Lab Studies:

• The diagnosis can often be made on clinical grounds in patients with a pruritic rash and characteristic linear burrows. Definitive diagnosis of scabies is made by direct visualization of the mite, eggs or feces. Mineral oil should be placed on the end of a burrow, preferably where a black dot is seen. The area should then be scraped with a number 5 scalpel blade and the scrapings shed onto a slide. The mite can be visualized under low power microscopy. In infants and young children, 75% of mites can be found on the hands and feet making this the best site to examine for a burrow.

Histologic Findings: The female mite is 0.3-0.5 mm in length. She has a tortoise-shaped body with 4 pairs of very short legs, 2 pairs in front and 2 in back. The mite also has bristles on its dorsal surface.

Scabies is treated with topical antiparasitic medications. The treatment of choice is permethrin 5% lotion. Alternative drug therapy includes precipitated sulfur in 6% petrolatum, lindane, crotamiton and ivermectin. Topical antibiotics may be used to treat secondarily infected lesions. Compliance to the prescribed regimen is essential to prevent reinfestation or resistant scabies.

Drug Category: Scabicidal agents -- Mechanisms of action include depolarization and paralysis of the pests or nervous system stimulation.

Drug Category: Topical Antibiotics -- Used to treat lesions with secondary infection.

Drug Category: Topical corticosteroids -- May be applied to help control intense pruritus caused by scabies.

Further Inpatient Care:

• Patients may need to be reexamined at 2 weeks and again one month after treatment. Persistent pruritus after treatment does not necessarily reflect treatment failure. Hypersensitivity reaction may outlast the presence of live parasites. Antihistamines and topical corticosteroids may be used to control pruritus until symptoms resolve. If a patient has persistent lesions at the one month check-up, reinfection or persistent infection should be suspected. In this case, treatment should be reinitiated. The family or any close contacts should also be examined to check for a source of reinfection.

Further Outpatient Care:

• All family members and close contacts should be treated even if asymptomatic to prevent reinfection. All bed linen and clothing should be washed in order to remove eggs and mites. Fomites can exist for 2-3 days without a host. Articles that cannot be washed should be kept in sealable plastic bags for 3 days.

Complications:

• Secondary lesions may occur from scratching and include excoriated papules and crusted areas. These lesions may become secondarily infected. Topical antibiotics can be used to treat minor superinfection, and oral antibiotics may be required for more extensive infection. The most common infectious agents are skin flora including Staphylococcus spp. and Streptococcus spp.

Prognosis:

• Scabies has an excellent prognosis. If one medication fails, multiple treatments with scabicides or sequential use of several agents can be curative.

Patient Education:

• Education is extremely important. The mode of transmission is via human contact as well as fomites. All close contacts must be treated or reinfection will occur.