Basics
Scabies is a skin infestation caused by the mite Sarcoptes scabiei var. hominis. It is usually spread by skin-to-skin contact, most frequently among family members and by sexual contact in young adults. Occasionally, epidemics occur in nursing homes and similar extended-care institutions, where scabies is spread by person-to-person contact and possibly by mite-infested clothing and bed linen.
The diagnosis of scabies should be considered when an individual complains of intractable, persistent pruritus, especially when other family members, consorts, or fellow inhabitants of an institution such as a nursing home or school have similar symptoms.
Although scabies is found more commonly in poor, crowded living conditions, it occurs worldwide and is not limited to the impoverished or those who practice poor personal hygiene. African-American and Afro-Caribbean individuals less frequently acquire scabies; the reason is unknown.
Pathogenesis
A fertilized female mite (Fig. 29.13) excavates a burrow in the stratum corneum, lays her eggs, and deposits fecal pellets (scybala) behind her as she advances.
The eggs, scybala (Fig. 29.14), and other secretions act as irritants or allergens, which may account for the itching and the subsequent delayed type IV hypersensitivity reaction that occurs approximately 30 days after infestation.
Clinical Manifestations
Because the incubation period from initial infestation to the onset of pruritus is approximately 1 month, it is not uncommon for contacts to be asymptomatic, especially if they have been recently infested.
Itching, especially at night (nocturnal pruritus), has traditionally been considered a symptom that is characteristic of scabies; however, nocturnal pruritus commonly occurs in many other skin conditions because people are less distracted by their active daytime routines at night.
Description of Lesions
The initial lesions of scabies include tiny pinpoint vesicles and erythematous papules, some of which evolve into burrows, the classic telltale lesions of scabies.
The burrow is a linear or S-shaped excavation that is pinkish white and slightly scaly and ends in the pinpoint vesicle or papule. This is where the mites may be found. Burrows are easiest to find on the hands, particularly in the finger webs (Fig. 29.15) and flexor wrists (Fig. 29.16) in adults and on the palms and soles in infants.
Sometimes burrows can be highlighted by applying black ink with a felt-tipped pen to the suspected areas.
Distribution of Lesions
Lesions are most often located on the interdigital finger webs, sides of the hands and feet, flexor wrists, umbilicus, waistband area, axillae, ankles, buttocks, groin, and penis (Figs. 29.17 and 29.18).
Children and adults rarely have lesions above the neck; this is an important diagnostic sign.
Infants tend to have more widespread involvement, including the face and scalp and especially the palms and soles.
Norwegian, or crusted, scabies occurs in people with varying degrees of immune deficiency such as that seen in Down syndrome (Fig. 29.19), leukemia, certain nutritional disorders, and acquired immunodeficiency syndrome (HIV/AIDS) (see also Chapter 33: Cutaneous Manifestations of HIV Infection and Figs. 33.13 and 33.14).
The hands and feet may be scaly and crusted with a thick keratotic material that can also be seen under the nails.
There may be wart-like vegetations on the skin; these are hosts to thousands of mites and their eggs.
Initially, itching is rather mild and focal, but when lesions begin spreading rapidly, usually after 4 to 6 weeks, it can sometimes become intolerable.
A generalized distribution of lesions is probably the result of a hypersensitivity reaction. In this case, a more pleomorphic array of lesions, such as “juicy” papules and nodules, may be seen.
Hemorrhagic crusts and ulcerations may replace the primary lesions.
In men, itchy papules and nodules, particularly on the penis and scrotum, are virtually pathognomonic for scabies.
Differential Diagnosis Diagnosis
A conclusive diagnosis is made by finding scabies mites, eggs, or feces.
A drop of mineral oil is applied to the most likely lesion (usually a vesicle on the finger web or wrist is chosen). The site is then scraped with a surgical blade (Fig. 29.20), the scrapings are placed on a slide, and a cover slip is then applied.
Adults, who are more efficient scratchers than children, tend to remove the definitive evidence of scabies (i.e., mite) with their fingernails. Because mites are few and are particularly difficult to find in adults, the time and effort spent searching for the mite may be better used by taking a thorough history and counseling the patient and his or her contacts. Thus, if scabies is strongly suspected on clinical grounds, scabicidal treatment should be initiated.
Treatment is directed at killing the mites with a scabicide as well as providing rapid symptomatic relief using appropriate oral antihistamines and topical corticosteroids, if necessary. Permethrin (Elimite and Acticin)
Ivermectin
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Management
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