Scabies

The scabies mite (Sarcoptes scabiei var hominis), which causes infection in humans, is a creamy white human parasite. It belongs to the family Sarcoptidae, which has 3 subfamilies. Sarcoptes belongs to the subfamily Sarcoptinae.
The female mite burrows into the stratum corneum at a rate of 0.5 to 5 mm per day and lays eggs after copulation. In 3-4 days, the eggs hatch into larvae that leave the burrow and mature on the skin surface. The life cycle is about 14-21 days. The lifespan of a female mite is 4-6 weeks during which she lays about 40-50 eggs.
The mite avoids areas that have a high density of sebaceous glands. There are about 12 mites on average in an individual with classical scabies. Outside the human host, mites can survive for about 24-36 hours, but this time is longer at low temperatures. The ability to infest a host decreases with increasing time spent outside the host.

Transmission occurs by direct skin-to-skin contact. The minimum time required for skin-to-skin transmission is 5 min: a quick handshake is almost never enough to infect, as are many time-limited contacts such as a hug.
Scabies mites are able to survive in the environment, outside the human body, for 24-36 h under normal environmental conditions (21°C and 40-80% relative humidity); during this time, they remain susceptible to infestation.
Indirect transmission (via clothing, bedding and other fomites) is possible; however, this phenomenon has been difficult to prove experimentally. Early studies have shown that indirect transmission is unlikely to play a significant role, except perhaps in cases of crustose scabies where the host is severely infected.

More than 200 million people are affected globally, with a particularly high prevalence in poorer tropical regions. Overall, the prevalence of scabies is highest in the Pacific and Latin American regions and is substantially higher in children aged 1-4 years than in adolescents or adults.
In contrast, regions with a low overall scabies burden such as North America and Western Europe show a more even distribution of scabies prevalence across all age groups.
In developed countries in the Western Hemisphere, scabies outbreaks are a particular problem in institutions, including nursing homes, schools, military camps and prisons. In Europe, there is a growing population of refugees seeking asylum, often displaced from areas of Africa or the Middle East due to conflict. These are vulnerable populations, and individuals are at risk of contracting a number of important infectious diseases, in addition to scabies, which often coexist.

Scabies mite infestation causes a rash with intense itching consisting of papules, nodules, and vesicles. This is mostly the result of host hypersensitivity, although the direct effect of mite invasion contributes.
For this reason, the incubation period before symptoms occur is 3 to 6 weeks in case of primary infestation, but only 1-2 days in case of reinfestation.
Sensitization to mite antigens has been demonstrated up to 1 month after primary infestation, and indeed it can take up to 6 weeks before signs and symptoms of hypersensitivity resolve.
Typical distribution of signs of infestation include areas between the fingers, wrists, armpits, groin, buttocks, genitals, and breasts in women. In infants and young children, the palms of the hands, soles of the feet, and head (face, neck, and scalp) are most commonly involved. Mites seem to avoid areas with a high density of hair follicles.
Although effective treatments exist, people living in regions where the pathogen is endemic are susceptible to reinfestation. This can occur rapidly even when household contacts are treated.
Chronic infestations occur with severe eczematous skin changes, and so-called "scabies nodules" may be observed particularly on the male genitalia and breasts. The predominant symptom of scabies infection is severe and persistent itching that can be highly debilitating and stigmatizing.
Patients typically describe the itching as being more intense during the night, and this is associated with sleep disturbances and reduced ability to concentrate.
In a small number of cases, hyper-infestation may occur, leading to crusted scabies, where the host may be invaded by millions of mites. This is in contrast to classical scabies where the host has an average of 10-15 mites.
Crustose scabies often, though not exclusively, occurs in the context of immunosuppression, such as in those with advanced HIV infection or neoplasia, and in the elderly. Pathogenic factors, such as virility of the scabies mite, are not thought to play a role.

Diagnosis is challenging and often delayed. The diagnosis of scabies is largely made on clinical grounds. The description of an intensely itchy rash, often worse at night, is supportive, and a history of contact with known cases is often present. Examination may reveal skin lesions in a typical distribution, and serpiginous burrows, also called burrows, in which mites hide, may be visible.
Other noninvasive techniques have been used, including videodermatoscopy and confocal reflectance microscopy, which provide a more detailed inspection of the mite. Parasitological confirmation can be obtained by lightly scraping the skin to remove the mite, which can then be placed on a slide and observed under a microscope.

A range of effective treatments for scabies is available. However, prescription practice varies widely among countries and is largely based on factors such as availability and cost of treatment and physician preference. Two of the most commonly used treatments for scabies are topical permethrin (a synthetic insecticide) and oral ivermectin (an antibiotic with broad-spectrum activity against nematodes and arthropods). Both have comparable efficacy and are generally very well tolerated.5% permethrin cream is the first-line topical therapy in the United Kingdom and the United States. Permethrin is adulticidal and ovicidal against the scabies mite and is therefore highly effective after a single application (although the prescribed regimen often involves two applications). Adverse effects occur rarely and are limited to local skin reactions including erythema, burning, and itching.Many other topical treatments have been used to treat scabies. Sulfur compounds can be effective, with preparations containing 5-10% sulfur in kerosene widely used in Africa and South America, although they are unpleasant to use and can cause skin irritation.Ivermectin is effective as an oral treatment against scabies. In the absence of ovicidal activity, a second dose of oral therapy is essential after 14 days the first to ensure that newly hatched mites are killed.Standard treatment, with 2 doses 2 weeks apart, results in a cure rate approaching 100%, comparable to that of topical permethrin at 5%.Oral ivermectin has been commercially available for years; it was first approved for the treatment of scabies in France in 2001, where it is licensed for the treatment of outbreaks in residential homes. In recent years it has gained approval in Australia, New Zealand, Japan, Germany, and the Netherlands.

To prevent infection in the home, it is important to wash all clothing, linens and towels that could be contaminated with high-temperature washes (above 60°C) in order to bring the scabies mite to death.
What cannot be washed at home can be sealed in a plastic bag for about 2 weeks.
Cohabitants in close contact with an infected person should generally carry out scabies treatment and change linens frequently until the treatment is over.
There is no prophylaxis through vaccination to date; funding will be needed to support increased scabies research. Priority areas include the development of robust diagnostic tests for scabies and better treatment and control strategies, particularly given the emerging threat of drug resistance.