Scabies

Scabies is a parasitic infestation of the skin caused by Sarcoptes scabiei, a mite that has accompanied humans for at least 2,500 years. It is possibly mentioned in the Bible and also by Aristotle, but it is the Roman physician Celsus who first described it and gave it its present name.

CAUSES

The scabies mite (Sarcoptes scabiei var hominis), which causes infection in humans, is a creamy-white, human parasite. It belongs to the Sarcoptidae family, which is divided into 3 subfamilies. Sarcoptes belong to the subfamily Sarcoptinae.                                                              

The female mite burrows into the stratum corneum of the skin at a rate of 0.5 to 5 mm per day and lays eggs after mating. In 3-4 days the eggs hatch into larvae that leave the burrow and mature on the surface of the skin. The life cycle is approximately 14-21 days. The lifespan of a female mite is 4-6 weeks during which she can lay approximately 40-50 eggs.

The mite avoids areas that have a high density of sebaceous glands. There are usually an average of 12 mites in an individual with classic scabies. Away from the human host, the mites can survive for around 24-36 hours, but the time period is longer in lower temperatures. The ability to infest a host decreases as the time away from the host increases.

TRANSMISSION

Transmission occurs through direct skin-to-skin contact. The minimum time required for skin-to-skin transmission is 5 minutes: a quick handshake is almost never enough time to pass the mite into others, as is short, limited contact, such as a hug.

Scabies mites are able to survive outside the human body for 24-36 hours. under normal environmental conditions (21°C and 40-80% relative humidity); during this period, they can still infest humans.

Indirect transmission (via clothing, bedding and other fomites) is possible; however, this has been difficult to prove experimentally. Early studies have shown that indirect transmission is unlikely to play a significant role, except perhaps in the case of crustose scabies, where the host is severely infected.

GEOGRAPHICAL DISTRIBUTION

More than 200 million people are affected globally, with a particularly high prevalence in more disadvantaged tropical regions. Overall, the prevalence of scabies is higher 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, such as nursing homes, schools, military camps and prisons. In Europe, there is a growing population of asylum seekers, many of whom have been displaced due to conflict in areas of Africa or the Middle East. These are vulnerable populations, and individuals are more at risk of contracting a number of significant infectious diseases, in addition to scabies, which often coexist.

SYMPTOMS

Scabies mite infestation causes a rash with intense itching, papules, nodules, and vesicles. Although this is also an effect of the mite invasion, this reaction is mostly due to the host’s hypersensitivity.

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.

Sensitisation to mite antigens has been demonstrated up to 1 month after primary infestation and, in fact it can take up to 6 weeks for the signs and symptoms of hypersensitivity to resolve.

The typical distribution of the signs of infestation include the 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 affected. 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 also occur rapidly, even when household contacts are also treated.

Chronic infestations occur with severe eczematous skin alterations, and so-called "scabies nodules" may be observed, particularly on male genitalia and breasts. The predominant symptom of scabies infection is severe and persistent itching that can be highly debilitating and embarrassing.

Patients typically describe itching as more intense at night, and this is associated with sleep disturbances and reduced ability to concentrate.

In a small number of cases, hyper-infestation can occur, leading to crustose scabies, where the host may be invaded by millions of mites. This is in contrast to classical scabies in which the host has an average of 10-15 mites.

Crustose scabies often, though not exclusively, occurs in the context of immunosuppression, such as individuals with advanced HIV infection or neoplasia, and the elderly. Pathogenic factors, such as the virility of the scabies mite, are not believed to play a role.

DIAGNOSIS

Diagnosis is challenging and often delayed. The diagnosis of scabies is largely made on clinical grounds. The description of an intensely itchy rash, which is often worse overnight, is a supportive feature, and a history of contact with known cases is often present. Examination may reveal skin lesions in a typical formation, and intraepidermal tunnels, also called scabies burrows, where the mites hide, may also be visible.

Other non-invasive techniques have been used, including videodermatoscopy and reflectance confocal microscopy, which allow for 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.

TREATMENT

Effective treatments for scabies are available. However, prescription practices vary widely between countries and is largely based on factors, such as availability, the cost of treatment, and the physician’s preference.

Two of the most commonly used treatments for scabies are topical permethrin (a synthetic insecticide) and oral ivermectin (an antiparasitic medication with broad-spectrum activity against nematodes and arthropods). Both are comparably effective and are generally very well tolerated.

Permethrin 5% cream is the first-line topical treatment in the United Kingdom and the United States. Permethrin is adulticidal and ovicidal against the scabies mite and is therefore highly effective, even after a single application (although the prescribed regimen often involves two applications). Adverse effects occur rarely and are limited to local skin reactions, which can include erythema, burning, and itching.

Many other topical treatments have been used to treat scabies. Sulphur compounds can be effective, with preparations containing 5-10% sulphur in kerosene widely used in Africa and South America. However, 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 14 days after the first dose, to ensure complete eradication of any newly hatched mites.

The standard treatment, with 2 doses, 2 weeks apart, results in a cure rate approaching 100%, comparable with that of topical permethrin 5%.

Oral ivermectin has been commercially available for years; it was first approved for the treatment of scabies in France in 2001, where it has been approved for the treatment of outbreaks in residential homes. In recent years it has gained approval in Australia, New Zealand, Japan, Germany and the Netherlands.

PREVENTION

To prevent infection in the home, it is important to wash all potentially contaminated clothing, sheets and towels at a high-temperature (over 60°C) in order to kill the scabies mites.

Any items that cannot be washed at home can be sealed in a plastic bag for approximately 2 weeks.

People living in close contact with an infected individual should also take anti-scabies treatment and change their towels and bedding frequently until the treatment is over.

There is currently no vaccine; funding will be needed to support further research into scabies. 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.

 

Bibliography

David J. Chandlera Lucinda C. Fuller. A Review of Scabies: An Infestation More than Skin Deep. Dermatology. DOI: 10.1159/000495290. 2018 S. Karger AG, Basel;

Lucia Romani, Andrew C Steer, Margot J Whitfeld, John M Kaldor. Prevalence of scabies and impetigo worldwide: a systematic review. JLancet Infect Dis 2015. S1473-3099(15)00132-2. June 16, 2015;

Philippa J. May1, Steven Y. C. Tong2,3, Andrew C. Steer4,5, Bart J. Currie3,6, Ross M. Andrews3,7, Jonathan R. Carapetis8,9,10 and Asha C. Bowen. Treatment, prevention and public health management of impetigo, scabies, crusted scabies and fungal skin infections in endemic populations: a systematic review. Tropical Medicine and International Health. Volume 24 no 3 pp 280–293 march 2019