Scabies

Sarcoptes scabiei is an obligate human parasite. It is a skin mite that is about 0.35 mm long. The female mite tunnels into the epidermis, and deposits eggs along the burrow. The larvae hatch in a few days and create new burrows. Development from egg to adult takes about 10-15 days. Mites die after 4-6 weeks. An average host harbours 10 mites, but this may be greatly increased in immunocompromised patients.

• Scabies epidemics usually occur in institutional settings or in socially deprived groups. The more crowded the living conditions, the higher the prevalence of scabies in the population.¹
• Scabies has a cyclical rise in incidence roughly every 20 years in the UK. Reported cases have been rising in the UK since 1991.²
• Outbreaks may occur in schools, and residential or nursing homes.

Risk factors

• More prevalent in urban areas, winter months, and in children and young adults (but all ages can be affected).
• Infections with human T-cell leukaemia/lymphoma virus I (HTLV-I) and HIV are associated with scabies.³
• Classical scabies is transmitted only via direct skin contact. Crusted scabies can also be transmitted via bedding, towels, clothes and furniture.
• Skin contact times have to be significant to transfer mites.

• In primary infestation, signs and symptoms only develop after 3-4 weeks. Symptoms reappear within a few hours if the person is re-infested.
• Typically presents with a papular, intensely pruritic eruption usually involving the interdigital spaces and flexural creases.
  
  
  
  
  The diagnosis can be made when a burrow is detected at a typical site and the lesion causes severe itching.
• The rash is symmetrical, usually consisting of small, red papules, particularly obvious on the inside of the thighs, the axillae, the periumbilical region, the buttocks, and the genitals. Vesicles or nodules may also occur.
• Scabies often causes severe itching. Scratching may further promote secondary infection. Pruritus tends to be worse during the nights and after a hot bath or shower. It usually develops 2-6 weeks after the initial infestation and appears at the same time as a rash.
• Burrows appear as fine, wavy, greyish, dark or silvery lines, 2-15 mm long, with a minute spec (the mite) at the closed end.
  
  
  
  
  
  They are most often seen in the interdigital web spaces of the hands, flexor surfaces of the wrists and elbows, axillae, ankles, feet, buttock areas, male genitalia and periareolar area in women. Skin that is too thick is relatively resistant. It therefore tends to spare the soles of the feet and upper back region in adults.
• Young children, the elderly and immunocompromised may also have face, neck, ear and scalp involvement.
• Even a single burrow is pathognomonic but burrows are often obliterated by bathing, scratching, crusts, or superinfection.
• Papules are small and erythematous. They can be sparse, or numerous and close-set. Over time papules can change into vesicles and bullae.
  
  
  
  
  
  The face and neck are often affected in infants and children.
• Papules and vesicles frequently develop into excoriations, eczema, secondary infections, and crusts.
• Nodules may develop, especially at the elbows, anterior axillary folds, penis, and scrotum. These are firm, dull red or brownish and may persist for months. They do not necessarily indicate active infestation.

• Crusted scabies is a hyper-infestation with thousands of mites present in exfoliating scales as a result of the host's insufficient immune response. This usually occurs in patients with underlying immunosuppression (e.g. HIV infection, leukaemia or lymphoma), elderly patients and those with decreased peripheral sensation.
• Hyperkeratotic crusted lesions typically affect the hands, feet, nails, scalp and ears, but all areas of skin including the scalp and trunk may be involved.
• Crusted scabies is a hyperkeratotic skin disease resembling psoriasis.
• May present with generalised lymphadenopathy and be associated with eosinophilia.
• Often becomes secondarily infected.
• This form of scabies is very contagious and is far more difficult to eradicate than classical scabies.

• Misdiagnosis is common and other pruritic skin disorders should be considered.
• Insect bites.
• Other dermatological conditions, including atopic eczema, tinea, contact dermatitis, lichen planus, or dermatitis herpetiformis.
• Scabies may also mimic systemic lupus erythematosus, bullous pemphigoid, urticaria pigmentosa and seborrhoeic dermatitis.
• Crusted scabies may resemble psoriasis.

• The diagnosis is clinical but can be confirmed by taking a skin scraping from an affected area, placing the material on a glass slide with a drop of 10 % potassium hydroxide and seeing an adult mite, egg or eggshell under light microscopy.
• The sensitivity of skin scrapings is low but it is very specific and a mite or eggs seen in the microscope is diagnostic.⁴

• All close contacts should be treated simultaneously. It is important that all contacts apply treatment on the same day to reduce the risk of re-infestation from an untreated contact.
• The primary method of treatment for scabies is by topical application of a parasiticidal preparation overnight to the whole body from head to toe.
• Apply treatment to the whole body, including the scalp, neck, face, and ears, and especially between the fingers and toes and under the nails. Treatment should not be applied after a hot bath (increases systemic absorption and removes the drug from its treatment site). If the hands are washed, the liquid or cream must be reapplied. Two applications of treatment are used 7 days apart:
  • First-line: permethrin 5% dermal cream has been the most widely studied. It has been shown to be more effective than crotamiton or lindane,³ and possibly also more effective than a single dose of ivermectin. Patients with crusted 'Norwegian' scabies may require two or three applications on consecutive days to ensure that enough penetrates the hyperkeratotic areas to kill all the mites.
  • Second-line: malathion 0.5% aqueous liquid. Malathion has only been studied in non-controlled trials.
  • Crotamiton 10% cream or lotion: less effective than permethrin and is rarely used because of its poor efficacy. It may help to relieve the itch caused by scabies.
  • Benzyl benzoate 25% emulsion is less effective than malathion or permethrin. The emulsion can irritate the skin, eyes and mucosal surfaces and so is now much less often used.³
  • Oral ivermectin (as a single oral dose of 200 micrograms/kg) is available on a named-patient basis as an adjunct to topical treatment for crusted (Norwegian) scabies. This treatment is usually initiated on specialist advice.
• Clothes, towels, and bed linen should be machine-washed (at 50 degrees Celsius or above) to prevent re-infestation and transmission. Items that cannot be washed can be kept in plastic bags for at least 72 hours to contain the mites until they die.
• Outbreaks of scabies in a residential or nursing home should be referred to a Consultant in Communicable Disease Control. All residents, staff, and their families are treated simultaneously on an agreed treatment date.
• The risk of transmission of scabies is low in schools, although outbreaks do sometimes occur. Children can return to school after the first application of treatment has been completed.
• Treatment of itch
  • Antihistamines are of little help in treating pruritus. A sedative oral antihistamine at night may help with sleeping and so reduce scratching.
  • Crotamiton cream or lotion has soothing qualities and may help to relieve the itch caused by scabies.
  • Low-dose steroid creams or simple emollients and moisturisers stored in the fridge may also ease discomfort.
• If correctly applied treatment is ineffective, give a course of a different parasiticidal preparation. This reduces the development of resistance to the medication.
• Secondary bacterial infections should be treated with antibiotics if significant.

• Scabies can trigger the onset of common conditions such as eczema and non-specific pruritus.
• Secondary infection: scabies may be associated with secondary bacterial infection, especially with staphylococci or streptococci. Scabies is a risk factor for acute post-streptococcal glomerulonephritis.¹
• Those whose immunity is highly sensitive can present with extensive dermatitis or even erythroderma.

• Scabies persists indefinitely unless treated.
• Itching persists for up to 3 weeks after successful treatment.