Scabies

Human scabies is a parasitic infestation caused by Sarcoptes scabiei var hominis. The microscopic mite burrows into the skin and lays eggs, eventually triggering a host immune response that leads to intense itching and rash.

Scabies occurs worldwide. However, it is the most vulnerable groups - young children and the elderly in resource-poor communities - who are especially susceptible to scabies and the secondary complications of infestation. The highest rates of infestation occur in countries with hot, tropical climates, especially in communities where overcrowding and poverty coexist, and where there is limited access to treatment. The most serious complications of the disease are rheumatic heart disease, sepsis, and kidney disease.

Countries in the Western Pacific Region where the populations (nearly approximately 26.6 million) are at high risk include American Samoa, Cambodia, Cook Islands, Fiji, French Polynesia, Guam, Kiribati, Marshall Islands, Federated States of Micronesia, Papua New Guinea, Samoa, Solomon Islands, Tuvalu and Vanuatu. Populations in some areas (for instance indigenous populations in Australia) of other countries are also at a lower risk. These countries include Australia, Japan, New Zealand, and Brunei. The highest burden is Papua New Guinea where all the provinces are endemic. The disease is believed to be prevalent in other countries in the Western Pacific Region (such as Lao People's Democratic Republic) but the lack of proper mapping makes it challenging to understand the scale of the prevalence in these countries. As such, the major challenge facing scabies is the availability and quality of data on disease burden estimates. Sufficient evidence is available showing that mass drug administration with ivermectin can significantly suppress the burden of the disease to very low levels.

Symptoms of scabies usually begin 4–6 weeks after infestation. Sometimes there are visible signs before symptoms begin.

Symptoms of scabies include:

• Severe itch, often worse at night;
• Itchy lines (linear burrows) and bumps (papules) on the fingers, wrists, arms, legs and belt area;
• Enflamed bumps on male genitalia and female breasts; and
• Larger rash in infants and small children, including on the palms, soles of the feet, ankles, and scalp.

Most individuals are infected with 10–15 mites.

People with suppressed immune systems, including people living with HIV, may develop crusted (Norwegian) scabies. This severe infection can have thousands or millions of mites and causes dry, scaley areas on the skin. It often does not cause itch. Crusted scabies spread very easily and can cause secondary infections. It is life-threatening.

Scabies mites burrow into the top layer of skin, where the adult female lays eggs. The eggs hatch in 3–4 days and develop into adult mites in 1–2 weeks. After 4–6 weeks the patient develops an allergic reaction to the presence of mite proteins and faeces in the scabies burrow, causing intense itch and rash.

Mite effects on immunity, as well as the direct effects of scratching, can lead to inoculation of the skin with bacteria, leading to the development of impetigo (skin sores), especially in the tropics. Impetigo may become complicated by deeper skin infection such as abscesses or serious invasive diseases, including septicemia.

Scabies can be treated with topical creams or oral medication in more severe cases. Itchiness often gets worse for 1–2 weeks after treatment starts.

Topical treatments that are applied to the whole body include:

• 5% permethrin cream
• 0.5% malathion in aqueous base
• 10–25% benzyl benzoate emulsion
• 5–10% sulphur ointment.

Ivermectin taken orally is also highly effective, but it should not be taken by pregnant women or children who weigh less than 15 kg.

Treatments do not kill the parasite’s eggs, and treatment should be repeated to kill newly hatched mites. People do not experience symptoms in the early stages of infestation. To reduce spread, all people in the household should be treated, even if they do not have symptoms.

Other treatments may be needed to treat the complications of scabies. Antiseptics or antibiotics are used to treat bacterial skin infections or impetigo.

Patients with crusted scabies are highly infectious and a source of reinfection to the rest of the community. Patients with crusted scabies need intense treatment with both topical and oral medications.