Background
Scabies is a common infestation in the pediatric population caused by the arthropod Sarcoptes scabiei. The mite is transmitted via prolonged direct human contact and rarely by fomites. The adult female mite is 0.3-0.5 mm long and has 4 pairs of legs. The term scabies is believed to be derived from the Latin term scabere, which means to scratch, or possibly from the term scabs, which are secondary to bacterial infection.
The photographs below depict various presentations of scabies.
In patients with scabies, erythematous vesicles and papules are present on torso extremities, some with adjacent linear excoriations. Courtesy of Audra Malerba, DO.
Scabies on hand. Courtesy of William D. James, MD.
Norwegian scabies. Courtesy of William D. James, MD.
Scabies on leg. Courtesy of William D. James, MD.
Scabies on buttocks. Courtesy of William D. James, MD.
Scabies on penis. Courtesy of William D. James, MD.
Scabies on penis. Courtesy of Hon Pak, MD. Pathophysiology
The skin is the main organ involved in scabies. 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 as many as 90 eggs in her 30-day lifespan. The larvae hatch in 3-4 days; 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 hypersensitivity to the debris, eggs, and feces, rather than by the direct effects of the mite. The primary lesions appear 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 because the immune system requires time to mount a hypersensitive response. Nocturnal pruritus is characteristic of scabies infestation. An immunologic study analyzing the cellular infiltrate types and patterns in lesions of scabies concluded that T4 cell dominance is the cause of persistent itching and T8 increase leads to improvement in the pruritus.[1]
Epidemiology
Frequency
United States
Scabies may be observed in people of all ages and is not always a disease of overcrowding. Norwegian (crusted) scabies is observed in patients who are immunocompromised, human immunodeficiency virus (HIV) positive, or institutionalized.
International
A survey of children in a welfare home in Pulau Pinang, Malaysia found that the infestation rate for scabies was highest among children aged 10-12 years.[2] Scabies was more commonly evident in boys (50%) than girls (16%). The overall prevalence rate for scabies was 31%.
Of 200 dermatology outpatients in Sirte, Libya with scabies, the following distribution was found:[3]
- Females - 59%
- Children - 37.5%
- Military personnel - 18%
Mortality/Morbidity
Intense pruritus is the major morbidity associated with scabies. This often leads to excoriation and secondary bacterial infection.
Skin disease is among the most frequent causes of morbidity in impoverished sections of Ethiopia; in one study, scabies was the leading cause of hospital admission.[4]
Race
No racial predisposition to acquiring scabies has been noted.
Age
Scabies can infect people of all age groups from infancy to adulthood.
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