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Scabies

Introduction

Scabies, also known as the seven-year itch, is a contagious skin infestation caused by the mite Sarcoptes scabiei var. hominis. This ectoparasite burrows into the skin, causing intense itching and a pimple-like rash. The mite is very small, often visible only as a pinpoint of white. Scabies is a common condition worldwide, affecting about 204 million people, or 2.8% of the global population as of 2015.

Signs and Symptoms

Itching

In scabies, itching is often worse at night and can be exacerbated by warmth. This symptom is less common in the elderly.

Rash

The rash from scabies is typically characterised by superficial burrows in areas like the finger webs, feet, ventral wrists, elbows, back, buttocks, and external genitals. In infants and immunosuppressed individuals, the face and scalp can also be affected. The burrows appear as linear or S-shaped tracks in the skin, often accompanied by pimple-like lesions.

Commonly involved sites of rashes of scabies
Commonly involved sites of rashes of scabies

Crusted Scabies

Crusted scabies, also known as Norwegian scabies, is a severe form of the disease that affects those with weakened immune systems. It is characterised by scaly rashes, slight itching, and thick crusts of skin that contain large numbers of mites.

Crusted scabies in a person with AIDS
Crusted scabies in a person with AIDS

Cause

Scabies Mite

The Sarcoptes scabiei mite is responsible for scabies. Adult mites are under 0.5 mm in size, with females burrowing into the skin to lay eggs. The eggs hatch into larvae, which mature into adults. The movement of mites within and on the skin produces an intense itch due to an allergic reaction to the mites' proteins.

Lifecycle of scabies
Lifecycle of scabies

Transmission

Scabies is primarily spread through prolonged physical contact with an infected person. This can occur during sexual intercourse or other forms of close skin-to-skin contact. Less commonly, it can be transmitted through sharing clothes, towels, and bedding. Healthcare workers are at risk due to extended contact with patients.

Diagnosis

Diagnosis is based on detecting the mite, its eggs, or faecal pellets. This can be done by scraping a suspected area and examining it under a microscope or using dermoscopy. Classic signs include the burrows made by mites in the skin.

A photomicrograph of an itch mite (_S. scabiei_)
A photomicrograph of an itch mite (S. scabiei)

Treatment

Effective treatments for scabies include permethrin, ivermectin, and other medications. Treatment should involve the entire household and close contacts to prevent reinfestation. Bedding, clothing, and towels used in the last three days should be washed in hot water and dried in a hot dryer.

Permethrin

Permethrin is the most effective treatment, applied from the neck down and left on the skin for eight to 14 hours before being washed off. A second application is often recommended as a precaution.

Ivermectin

Oral ivermectin is effective, often in a single dose. It is the treatment of choice for crusted scabies and can be used in combination with a topical agent.

Other Treatments

Other treatments include lindane, benzyl benzoate, crotamiton, malathion, and sulphur preparations. Each has its own advantages and limitations, with some being more suitable for children or those with specific health concerns.

Day 4
Day 4
Day 8 (treatment begins)
Day 8 (treatment begins)
Day 12 (under treatment)
Day 12 (under treatment)
Healed
Healed

Prevention and Management

Mass-treatment programmes using topical permethrin or oral ivermectin have been effective in reducing scabies prevalence. Community-wide control strategies are often required in endemic areas. Cleaning of the environment is essential in cases of crusted scabies to prevent transmission.

Epidemiology

Scabies is one of the most common skin disorders in children, with an estimated 300 million cases occurring annually worldwide. It affects all ages, races, and socioeconomic classes, being more prevalent in crowded and unhygienic living conditions.

Wax figurine of a man with Norwegian scabies
Wax figurine of a man with Norwegian scabies

Scabies in Animals

Scabies can also affect domestic and wild animals, causing severe itching and secondary skin infections. The most frequently diagnosed form in animals is sarcoptic mange, particularly in dogs and cats.

A street dog in Bali, Indonesia, with sarcoptic mange
A street dog in Bali, Indonesia, with sarcoptic mange

Society and Culture

The International Alliance for the Control of Scabies was initiated in 2012 to address the global health implications of scabies. The World Health Organisation (WHO) has included scabies on its list of neglected tropical diseases.

Public health worker Stefania Lanzia using a soft toy scabies mite to publicise the condition in a 2016 campaign
Public health worker Stefania Lanzia using a soft toy scabies mite to publicise the condition in a 2016 campaign

Self-assessment MCQs (single best answer)

What is the primary causative agent of scabies?



Which symptom of scabies is typically worse at night?



Which form of scabies is characterised by thick crusts of skin?



What is a common method for diagnosing scabies?



Which medication is most commonly used as a topical treatment for scabies?



What is the primary mode of transmission for scabies?



Scabies is most commonly found in which type of living conditions?



Which population is at increased risk for crusted scabies?



How can scabies be prevented in a household where one member is infected?



Which organisation included scabies on its list of neglected tropical diseases?



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