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Scratching the surface: treatment of scabies

Siobhan Hicks
RGN BSc
Nurse Practitioner
Tower Hamlets PCT
London
Lecturer in public health and primary care
City University
London

Scabies is the commonest skin infestation, with an estimated 300 million cases worldwide.(1) It is easily missed or misdiagnosed and can affect people of any age, but it is mostly seen in children, young adults, immobilised elderly people and those who are immunologically compromised.

It is more common in urban areas, where there is overcrowding and close body contact; it is more dominant in women than men and more prevalent in winter than summer.(2) It causes misery and embarrassment from severe itching and sleepless nights.
 
Scabies is an intensely itchy contagious rash caused by the mite Sarcoptes scabiei - a tiny eight-legged parasite, smaller than a pinhead (see Figure 1). After mating the adult male remains on the skin surface, but the female mite tunnels into the stratum corneum of the skin and can travel at a rate of 3mm per day; this is unnoticed at first, although her eggs are laid within 30 minutes of burrowing, and she proceeds to lay approximately 2-3 eggs per day, which take about 10 days to reach maturity.(3)

[[NIP28_fig1_20]]

The itch frequently starts one month after exposure to the mites - the absorption of mite excrement into the skin capillaries generates a hypersensitivity reaction that is very often mistaken for eczema.

How is it spread?
The infestation is caused by close and intimate contact with other people who have scabies; most cases are probably caught from holding hands with an infected person, this being the most common site to be first affected, otherwise sleeping in the same bed and sexual contact are the most common ways of passing the mite.(4)

The mite can survive for 36-72 hours off the human body at room temperature. However, it is thought to be rare to pick it up from objects such as clothing or bedding.(5)

Occasionally outbreaks occur in nurseries or residential homes where people are in close contact. Crusted or Norwegian scabies affects people with a poor immune system. Crusts form that are full of mites, but there is little or no itching. This form of scabies is highly contagious within residential care and would require specialist management advice.(6) It is impossible to catch scabies from pets, cutlery, toilet seats or swimming pools. It also has no respect for socioeconomic status and can infest people who do wash and those who do not!

Signs and symptoms
The main symptom of scabies is a severe persistent itchy rash with many small papules. It is also very common for several people in same family to become infected.(2)
 
The rash is usually symmetrical in distribution, and there may be excoriation, dermatitis and secondary infection with vesicles and pustules caused by scratching. Skin infected with bacteria becomes red, hot and tender (see Figure 2). The initial rash often starts on the hands and spreads to wrists, and then elbows, underneath the arms, abdomen, breasts, genitals and buttocks.(6) Papules or nodules resulting from itching may affect the genital area; the head and neck are usually spared, except in infants.

[[NIP28_fig2_22]]

The mites themselves show up mainly where they burrow - characteristic grey/white burrows (short elevated serpiginous [s-shaped] tracks in the superficial epidermis) - and are often obscured just under the skin surface and are seen on finger webs, sides of the fingers, wrists and elbows.

The severe and persistent itch is often worse at night and after a hot bath - this is very often the presenting complaint, indicating hypersensitivity has developed. In subsequent attacks the incubation period can be as little as 24 hours due to previous sensitivity.

Diagnosis
Diagnosis is made by taking a thorough subjective and objective history,(7) ensuring confidentiality, privacy and nonjudgemental communication. By eliciting the patient's story, one should assess the need for further sexual health screening, as other sexually transmitted infections may be present, although the exact risk is not clear, and a full sexual health screen should be considered for those who are sexually active. Child protection concerns should also be considered; refer to the local child protection team if appropriate.(8)

The clinical appearance of scabies is usually typical but should be considered in the differential diagnosis of any itchy rash affecting the flexures, especially as scabies is often mistaken for eczema. Identification of the mite can be confirmed in a variety of ways. Some clinicians scrape the skin at the newest or least disturbed burrow sites to expose the mites and eggs.(9) The mite may then be extracted with a sharp needle (not a venepuncture needle as the mite will disappear down the bore shaft) and be seen with the use of a magnifying glass. Alternatively, skin scrapings may be sent to the lab.

Finally, the ink test helps show the burrows of scabies. Indian ink or a washable felt-tip marker is painted over the suspected burrow; leave for a few minutes and wash off with an alcohol swab; any burrow will be immediately apparent.(9)

Treatment options
Patients should be given clear and accurate written advice on their condition. Poor results are often due to poor education. Infected patients and their contacts should be treated at the same time regardless of whether they have symptoms or not. This involves all members of the household, as well as all skin-to-skin contacts within the last three months.(9)

Malathion and permethrin
Two products are licensed for use in the Nurse Prescribers' Formulary: malathion 0.5% aqueous lotion (Derbac M, Quellada M) and permethrin 5% cream (Lyclear dermal cream). These applications are then washed off after 8-12 hours if using permethrin and after 24 hours if using malathion. This may be applied twice, repeated after one week.
 
A Cochrane review supports permethrin as treatment of choice even though it is more expensive than malathion.(6,10) Alcoholic lotions are not recommended on excoriated skin or for asthmatics, and an adult will need 30g of cream or 100ml lotion to cover the whole body. There is a lack of evidence regarding safety of use in pregnancy and breastfeeding, but the limited evidence available suggests permethrin 5% cream is the medication of choice.(8)  However, referral for medical management is encouraged as, in this instance, it should be used with caution and only if clearly needed.(6) The NPF also states that permethrin is only used in under-2s on doctor's advice, and similarly with malathion for babies less than six months.(6)

Benzyl benzoate
Benzyl benzoate was the traditional treatment for scabies. However, it is an irritant, requiring three treatments one day apart, and the scabies mite may be resistant to it.

Oral ivermectin
Oral ivermectin may be given in an outbreak of Norwegian scabies; this is an unlicensed medicine.(9)

Crotamiton
Crotamiton (Eurax 10%) may be applied to soothe the itch, and oral antihistamines such as chlorpheniramine may be useful if itching is a problem at night, particularly for children, and calamine lotion may also help.(6,10)
 
Application
Patients should not have a hot bath before treatment, because this may increase the absorption of the lotion (ascaricide) into the blood and therefore remove it from the site of action on the skin. The lotion can be applied with a small paintbrush or cotton wool ball to cool, dry skin, paying particular attention to the webs of the fingers and toes; the lotion should be allowed to dry before redressing, and babies should wear mittens to stop them licking the lotion off after application. Treatment must be reapplied if any area is washed within the 12-24-hour period.

The lotion should be applied to cool, dry skin on the whole body from the neck down. In 2 year olds and younger, immunologically compromised patients and older people, and in treatment that has failed, application should be extended to head and neck, ears, face and scalp.(5)
 
Children should stay off school until the first application has been treated.
 
Bedding and clothing should be washed at 50(˚)C or above after the first application. Items that cannot be washed can be kept in plastic bags for 72 hours to contain the mites until they die.(4)

Treatment failure is usually due to inadequate application of scabicide, or reinfection, which is unfortunately quite common. However, the patient should be reminded that the itch is an allergic reaction and will continue for at least 3-4 weeks and may get worse; this does not mean treatment has failed. It may be helpful for the patient to be reviewed 2-4 weeks following treatment to assess their progress and offer support.

[[NIP28_pp_24]]

References

  1. Stein DH. Scabies and pediculosis. Curr Opin Ped 1991;3:660-6.
  2. British Association of Dermatologists. Scabies patient information. Available from URL: http://www.bad.org.uk
  3. Society of Sexual Health Advisers. Scabies. Available from URL: http://www.ssha.info/public/infections/scabies.asp
  4. Patient.co.uk. Scabies. Available at URL: http://www.patient.co.uk
  5. Treatment of scabies. Drugs and Therapeutics Bulletin 2002;40(6):43-6. Available from URL: http://www.gpnotebook.co.uk/simplepage.cfm?ID=-362086387&linkID=6459&coo...
  6. NPC. Management of scabies and threadworms. Nurse Prescribing Bulletin 1999;1(3) Available from URL: http://www.npc.co.uk/nurse_prescribing/pdfs/scabies&threadvol1no3%20.pdf
  7. Bates B. A guide to physical examination and history taking. 6th ed. Philadelphia: Lippincott; 1995.
  8. British Association for Sexual Health and HIV, and Clinical Effectiveness Group. National guidelines for consultations requiring sexual history taking. CEG. Available from URL: http://www.bashh.org/guidelines/draft/draft_sex_hist_291105.pdf
  9. Family Practice Notebook. Scabies. Available from URL: http://www.fpnotebook.com
  10. Walker GJA, Johnstone PW. Interventions for treating scabies. The Cochrane Library. (1) Oxford: Update Software; 1999.