Treating scabies infestations in children and adults

Scabies is a skin condition caused by a burrowing mite infestation that usually presents as an itchy papular rash. This article describes how to recognise a scabies infestation and examines the efficacy and suitability of the various treatment options.

Scabies is a common and often extremely itchy skin condition, caused by infestation with the burrowing mite Sarcoptes Scabiei. Adult mites are very small – females are about 0.3mm by 0.4mm, and males are 0.2mm by 0.15mm. This makes them too small to be seen with the naked eye. The adult female burrows into the upper part of the epidermis. The adult male moves between burrows to find an unfertilised female. There are around 12 mites on the skin in a typical infestation, although this number can be greatly increased in cases of crusted scabies, or where the scratching response is reduced.1

The male mite dies soon after mating, and the fertilised female burrows deeper into the epidermis, laying her eggs along the burrow. She may lay 40–50 eggs in total. From the eggs hatch six-legged larvae, which escape from the burrow vertically, and make their own short burrows in which to go through two more moults into eight-legged nymphs and then again into adults.2 It takes 10–15 days for an egg to develop into an adult mite.

The prevalence of scabies is not accurately reported, but the rate of GP consultations for scabies in the UK is thought to be around 100 per 100,000 population per month.1

Transmission of classical scabies is by close person-to-person skin contact, and it is commonly spread through prolonged hand holding and bed sharing. The mites move slowly, at about 2.5cm a minute on skin, but they may survive for up to three days away from the host. In their burrows, they are protected, and survive hot baths and scrubbing of the skin. They cannot jump or fly.

The itch is caused by a delayed hypersensitivity reaction to the mite antigens (from faeces, eggs, saliva and moulted body parts). This may take two to six weeks to develop after initial infestation, but is faster in subsequent episodes.1 Scabies is infectious before it becomes symptomatic in primary infections.

Crusted scabies (also known as hyperkeratotic, Norwegian or atypical scabies) is a condition where the number of mites on the skin is hugely increased, to many millions. In this situation, as one might expect, transmission is greatly increased, and is not just from direct contact. In crusted scabies, the mites are easily passed on through contact with towels, clothing and furniture. Crusted scabies is more commonly found in people with reduced immune function (eg HIV infection, malnutrition, or taking immunosuppressive drugs), people with a reduced ability to scratch (eg spinal injury, physical incapacity, or inability to feel the itch), and people with learning difficulties, dementia or Down syndrome.3 Around 40% of people with crusted scabies have no identifiable risk factor.4 In crusted scabies, hyperkeratotic warty crusts typically affect hands and feet, but can affect any body part. Secondary infection is common because of fissuring of the skin, and this may cause widespread lymphadenopathy and a raised eosinophil count on a blood test.

Scabies commonly affects multiple individuals in institutions such as hospitals and nursing homes, particularly if there is an index case with crusted scabies. Healthy people exposed to individuals with crusted scabies are likely to develop classical scabies. It is important to realise that residents in care homes may have scabies and be asymptomatic, particularly if they have dementia, and so all residents and staff in care homes with an index case of scabies need to be carefully examined.5

Common complications of scabies are exacerbations of eczema and psoriasis, and secondary bacterial infection from scratching. In crusted scabies, possibly at least in part because of the immunosuppression that predisposes individuals to this type, secondary infections may be serious or even fatal.2 Some people develop persistent delusional parasitosis, and demand repeated retreatment, even in the absence of any new burrows.

Animal scabies

Rarely, humans can contract scabies from dogs or cats. In this situation, the infestation is restricted to the area of contact, and burrows are not found. The animal mites do not reproduce in human skin, so this infestation is self-limiting and does not require any treatment of the human2 – but the animal needs to be treated, of course.

Diagnosing scabies

Classical scabies
Patients usually present with an itchy papular rash. The itch is often more severe at night, may start on the hands, and the rash is usually widespread (beyond the area the mites are actually found). At the end of a burrow, a small vesicle is commonly seen, and there may be nodules around the elbows, axillae and penis. Burrows (seen by naked eye as fine curved or wavy lines of about 5mm length) may be identified in areas of thicker skin, such as the hands and feet, axillae, buttocks, around the umbilicus, and around the areolar area in women. However, they are often damaged by scratching and are difficult to find if there is excoriation or secondary eczematisation. Papules on the soles of the feet of babies, and on the shaft of the penis, should be regarded as highly suggestive of possible scabies. Babies may also have burrows on the scalp and trunk, which is unusual in adults. The elderly commonly have burrows on the palms, soles and trunk.

Secondary eczematisation and bacterial infection are commonly seen. This has occasionally been associated with development of glomerulonephritis.2

The diagnosis is easiest to confirm with dermoscopy, where the mite may often be seen in a burrow (see Figure 1). Burrows can also be detected with an ink test, where ink is dropped onto the skin near a suspected burrow, then wiped away with an alcohol swab. Any ink on the surface is removed, but the ink that has gone into the burrow by capillary action will remain, so the burrow appears as a black line. An alternative, but more difficult, test is to try to scrape a mite from the end of a burrow onto a microscope slide and examine it for mite body parts.

Figure 1. Scabies mite in a burrow on dermoscopy

Crusted scabies
Patients with crusted scabies may or may not be itchy. They may have a widespread papular rash, but they may also have pale hyperkeratotic crusts, typically on the hands and feet, which may have fissures within them. The crusts may be extensive over the body, or limited to one area. These crusts are teeming with mites, and can be readily seen with dermoscopy or on a microscope slide of the crusts.

Management

NICE recommends that the whole household, close contacts and sexual contacts should all be treated simultaneously with a topical insecticide, whether they are symptomatic or not.1 Pregnant and breastfeeding women and babies over two months of age should all be treated. Specialist advice may be needed for very young babies. Children who have been diagnosed with scabies should stay off school or nursery until the first treatment has been completed.

In institutions, public health may need to be informed to help trace all possible contacts, and care homes should have a scabies protocol in case of an outbreak.

Topical permethrin cream is the first-line treatment for scabies.6 The insecticide should be applied to the whole body, including the face, neck and scalp, paying particular attention to areas between the fingers and toes, and under the nails. It should be applied to cool, dry skin (not straight after a hot bath)7 and allowed to dry on the skin before dressing. It should be left on the skin for 8–12 hours before washing off. If the hands are washed within eight hours of treatment, then the insecticide should be reapplied. Breastfeeding women should wash the insecticide off the nipple to feed, then reapply it afterwards. The treatment should be repeated a week later. The recommendation to repeat treatment a week later is different to the summary of product characteristics (SPC), which indicates that a single treatment is usually sufficient. The SPC states that approximately 90% will be cured after a single application, and treatment only needs to be repeated if there are no signs of the original lesions healing or if new lesions have appeared. However, expert consensus is that all patients should be advised to repeat the treatment after a week.1

It should be pointed out to patients that the head, neck and scalp should be treated in all cases, even though the patient information leaflet suggests that the head only needs to be treated in children and the elderly.7

The clothing, towels and bedlinen should be washed at 50oC on the day of the first treatment.1 Delicate items that cannot be washed at this temperature may be placed in a plastic bag for three to seven days, during which time any mites will die without a host to feed on. Ironing with a hot iron, dry cleaning or tumble drying on a hot setting for 10–30 minutes are alternatives.8

Malathion aqueous liquid is a second-line treatment for scabies. It is applied in the same way as permethrin cream, but the contact time is 24 hours before washing it off. It is useful for people who cannot tolerate permethrin cream, but is generally second line because the cure rate is slightly lower than for permethrin. Malathion has a 70–80% cure rate in trials.1 The aqueous preparation is preferred because the alcohol preparation stings on broken skin and is flammable.

Oral ivermectin is not licensed in the UK, but may be used to treat scabies that has not responded to permethrin alone, or in cases of crusted scabies. It is available on a named-patient basis only. Trials of its efficacy have to date been small, but it appears to have similar efficacy to topical permethrin when used as a single dose, or as two doses in classical scabies.3 It should be taken with food.9

In the treatment of crusted scabies, oral ivermectin is often used alongside topical permethrin cream. One suggested regimen is topical permethrin cream once daily for seven days then twice weekly until cure, plus oral ivermectin (200µg/kg) on days 1, 2, 8, 9 and 15, and days 22 and 29 in severe cases.9 Small case studies have shown ivermectin to be effective in the majority of cases.3 Ivermectin has not been demonstrated to be safe in children weighing under 15kg. The commonest side-effect is transient exacerbation of pruritus at the beginning of treatment. Other side-effects may include irritation, headache, nausea, pustular rash, cellulitis, abdominal pain and transient diarrhoea.3

Topical sulfur creams are widely used to treat scabies in the USA and Canada, and studies do show them to be efficacious.10 The US Centers for Disease Control and Prevention (CDC) list topical sulfur 5%–10% as a first-line treatment for scabies, including for babies under two months of age.11 Although not listed on the Drug Tariff, and not licensed in the UK for this use, topical preparations of sulfur are widely available from internet sources, and patients may sometimes self-treat with them in the UK.

Table 1 outlines the main treatment options for scabies and their properties.

(click table for full-size image)
Table 1
. Treatment options for scabies and their properties

Treatment failure

Patients often expect the itch to stop immediately following treatment, and they need to be warned that the itch may continue for four to six weeks after successful treatment. This may be due to scabies antigens remaining in the epidermis until it has been fully shed. Some patient information may suggest that the itch resolves sooner than this, and patients may need reassurance that the itch will resolve eventually. Itch may be treated with emollients, including emollients containing crotamiton, together with topical steroids and/or antihistamines, but these are often ineffective. The role of antihistamines is for night time sedation to help break the itch-scratch cycle, rather than relief of pruritus per se. Chlorphenamine is often used off licence for this indication. Hydroxyzine is licensed for use in pruritus, but should not be used in patients with a prolonged QT interval or risk factors for a prolonged QT interval, including the elderly.12

Nodules on the genitals or axillae may persist for many months and do not indicate treatment failure.8 They may require intralesional steroid injections to resolve.13

True treatment failure may be identified by the appearance of new burrows at any point after treatment, or very persistent symptoms (over six weeks) that are not due to other causes. Treatment failure may have several causes: the insecticide may have been applied incorrectly or inadequately, or parts may have been missed; the patient may have been reinfected by untreated contacts, or contacts who have not been treated simultaneously; the scabies mite may have been resistant to the insecticide; the original diagnosis may have been incorrect; there may have been sensitisation or contact dermatitis to the insecticide, or a flare of endogenous eczema; or the patient may have ongoing psychogenic itch, which can be associated with delusional parasitosis.

Future treatments

A new oral treatment for scabies is currently being developed. Moxidectin is a veterinary compound, used to treat sarcoptic mange, and is currently being considered for regulatory submission for the treatment of onchocerciasis in humans, having undergone phase 1 and 2 trials. If successful, this may pave the way for trials of its use in human scabies.14

References

1. NICE Clinical Knowledge Summaries. Scabies. Last revised May 2016. Available from: https://cks.nice.org.uk/scabies [Accessed 21 February 2017]
2. McCarthy JS, et al. Scabies: more than just an irritation. Postgrad Med J 2004;80:382–7. Available from: http://pmj.bmj.com/content/80/945/382
3. National Institute for Health and Care Excellence. Difficult-to-treat scabies: oral ivermectin. ESUOM29. March 2014. Available from: https://www.nice.org.uk/advice/esuom29
4. Roberts LJ, et al. Crusted scabies: clinical and immunological findings in seventy-eight patients and a review of the literature. J Infect 2005;50(5):375–81.
5. Cassell J, et al. A prospective study of scabies outbreaks in ten residential care facilities for the elderly in South-East England, 2014–15. Public Health England Annual Conference 2016, 13-14 September 2016, University of Warwick.
6. Strong M, Johnstone P. Interventions for treating scabies. Cochrane Database Syst Rev 2007;3:CD000320. Available from: http://www.cochrane.org/CD000320/INFECTN_interventions-for-treating-scabies
7. British National Formulary. Scabies. February 2017. Available from: https://www.evidence.nhs.uk/formulary/bnf/current/13-skin/1310-anti-infective-skin-preparations/13104-parasiticidal-preparations/scabies [Accessed 21 February 2017]
8. Patient. Scabies patient information leaflet. Available from: https://patient.info/health/scabies-leaflet [Accessed 21 February 2017]
9. Centers for Disease Control and Prevention. 2015 Sexually Transmitted Diseases Treatment Guidelines. Ectoparasitic infections. Available from: https://www.cdc.gov/std/tg2015/ectoparasitic.htm [Accessed 21 February 2017]
10. Sharquie KE, et al. Treatment of scabies using 8% and 10% topical sulfur ointment in different regimens of application. J Drugs Dermatol 2012;11(3):357–64.
11. Centers for Disease Control and Prevention. Parasites – scabies. Medications. Available from: https://www.cdc.gov/parasites/scabies/health_professionals/meds.html [Accessed 21 February 2017]
12. Medicines and Healthcare products Regulatory Agency. Hydroxyzine (Atarax, Ucerax): risk of QT interval prolongation and Torsade de Pointes. April 2015. Available from: https://www.gov.uk/drug-safety-update/hydroxyzine-atarax-ucerax-risk-of-qt-interval-prolongation-and-torsade-de-pointes [Accessed 21 February 2017]
13. Patient. Professional Reference. Scabies. Available from: https://patient.info/doctor/scabies-pro [Accessed 21 February 2017]
14. Mounsey JS, et al. Prospects for moxidectin as a new oral treatment for human scabies. PLoS Negl Trop Dis 2016;10(3):e0004389.

Declaration of interests

Dr Whybrew has been paid a speaker fee by Dermal and Leo Pharma for presentation of her own noncommercial material.

Dr Chin Whybrew is a GP partner, Stoke Road Surgery, Bishops Cleeve, Cheltenham, tutor for Cardiff Diploma in Practical Dermatology and executive committee member of the Primary Care Dermatology Society (PCDS)

Treating scabies infestations in children and adults

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