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Therapeutics in Dermatology
A reference textbook in dermatology

Scabies

14 October 2019, by ALPALHAO M.

1 - ACKNOWLEDGEMENTS

Chapter written with the help of the EADV, the Fondation René Touraine and the Therapeutics in Dermatology

2 - OVERVIEW

Scabies is an infectious disease caused by a parasitic mite named Sarcoptes scabiei. This mite feeds on nutrients from human skin, where it reproduces and lays eggs.

Human scabies is caused by a different mite than that of other animals, such as dogs. As such, scabies is seldomly acquired through contact with household animals. Usually they survive for only a short period on another host.

This infection is very common as it is easily spread from person to person through touch or through contact with infected clothing materials. Due to the high contagiousness of this condition, it is common to find multiple individuals affected in the same household.

Over 300 million cases of scabies are reported each year, worldwide.

While not life-threatening, scabies is usually very symptomatic, presenting with intense itch and skin lesions. In particularly susceptible individuals (HIV-infection, immunodeficiencies, taking immunosuppressant drugs) scabies can manifest as an exuberant and more difficult to treat form, named crusted scabies, or, as it is popularly known, Norwegian scabies.

Many of the presenting complains are not directly caused by the mite, but by the immune response the patient mounts against the parasite. As such, the clinical manifestations of this disease are diverse and variable from patient to patient. Furthermore, resolution of the symptoms may take a while after successful eradication of the parasite.

3 - SIGNS AND SYMPTOMS

Scabies can present with a wide range of symptoms. Most individuals present with intense itch (more severe during nighttime) and lesions on the skin. Most of these lesions are small red bumps (papules) that can be excoriated from scratching. These lesions are most frequently found on the interdigital webs, the umbilical area, axillae, genital area, areolar area, buttocks, elbows, wrists and ankles. Scabiotic lesions almost always spare the head and face.

In the genital area, particularly the scrotum in men, these lesions often present as small pinkish nodules. However, in 7-10% of cases, particularly in small children, nodules may be the dominating feature of scabies. Children may also present with vesicles and small boils.

In the elderly, the back area is frequently affected and usually excoriations in the areas accessible to scratching are easily noted.

In crusted scabies, thick plaques and crusts are found, generally with a widespread distribution, but hands and feet are typically severely affected.

The hallmark lesions of scabies are scabiotic burrows, tunnel-like structures in the skin produced by the mite. These lesions present macroscopically as thin grayish lines that may range from 1-10 mm long. Although very characteristic, these lesions are usually hard to find due to the extensive excoriation secondary to patient scratching.

4 - WHO GETS AND CAUSES

Virtually anyone can get scabies, as this disease is easily transmitted from skin-to-skin contact or infested clothing. There is still significant stigma towards patients with scabies, however it must be stressed that scabies is not exclusive to people with poor hygiene, as the mite is not killed from bathing.

Due to the means of transmission, people who live/work in crowded spaces are more susceptible to this disease. Likewise, occupational activities that involve direct contact with other people or their clothes carry a higher risk of infection. It comes as no surprise that outbreaks of scabies are relatively common in kindergartens, schools, nursing homes and hospitals, where these predisposing factors are found. Scabies can also be transmitted sexually, due to direct skin-to-skin contact.

5 - DIAGNOSIS AND TREATMENT

The diagnosis is almost always made clinically, through the characteristic symptoms, lesions and epidemiological information of contact with other people with similar complaints.

However, scales from affected skin can be easily collected and looked under a microscope to identify the mites and/or its eggs. This provides diagnostic confirmation when clinical data alone are insufficient to establish the diagnosis.

Treatment of scabies involves two parts: eradicate the causing mite and alleviate symptoms.

To eradicate the mite, many options are available. Most frequently prescribed regimens consist of topical therapy. For more severe, recalcitrant cases and when a large number of individuals are to carry the treatment at the same time (eg. Outbreaks in nursing homes), oral treatment should be considered. Crusted scabies is a more severe form, and such usually requires concomitant treatment with a topical drug and oral ivermectin.

Table 1 - Most frequent therapeutic regimens

Drug

Contraindications

Special populations

Posology

Benzyl benzoate

Pregnant and breastfeeding women

Children <30 months old

May be used in children >30 months old

Apply on whole skin under the chin level once daily for 3 days. Repeat after one week

Permethrin 5%

Pregnant and breastfeeding women

May be used in children >2 months old

Apply on whole skin, and wash after 8-14h. May be repeated after one week.

Sulphur 6-10% (in Petrolatum ointment)

None

May be used without age restrictions

Apply every 24h for 3 consecutive days.

Oral Ivermectin (0.2mg/kg)

Pregnant and breastfeeding women

Not to be used in children <15kg

Take once. Repeat dosage after 2 weeks if necessary.

To control the itch, oral anti-histamines and emollients may provide some relief.

Cohabitants should be offered topical treatment even if asymptomatic as they may be asymptomatic hosts for the mite and provide a source for reinfestation.

6 - TIPS FOR MANAGING

All cohabitants should be treated simultaneously to eradicate the mite in your household.

As infected clothes may harbor the mite, all clothing that had contact with the affected patient (eg. everyday clothes, pajamas, underwear, bed linen, pillow cases, etc…) should be washed at 60ºC for at least half an hour, prior to ironing. All clothes that cannot undergo this treatment should be sealed in a bag for at least 1 weeks in order to kill the mite.

School children should be evicted of attendance before the treatment is completed in order to prevent outbreaks. Your medical doctor will inform you when it is safe to resume school attendance.

Even after complete eradication of the mite, pruritus and skin lesions may persist for 2-4 weeks. This does not mean treatment failure. Maintain supportive care with anti-histamines and calming creams until complete resolution of your symptoms.

If new lesions develop after treatment, seek the counsel of your doctor. 

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