Scabies and lice

F A Campbell, A Drummond, D T Roberts

HK Pract 2003;25:214-221

Summary

Infestations with scabies or lice is very common problem worldwide. Both conditions can be treated successfully with a variety of topical and systemic agents once the diagnosis is made. The primary care physician will treat the vast majority of cases and must provide clear and concise advice on treatment to the patients and any close contacts. This will help to reduce treatment failure rates and developing resistance to pediculicides now recognized globally.

摘要

疥瘡和虱病是世界各國的常見病，一旦確診無論局部或是全身治療都很有效。基層醫生治療絕大多數病人， 必須向病人及有密切接觸者提供清晰準確的指引。這樣既可降低治療失敗率，又可減少全球日漸嚴重的對滅虱藥的抗藥性。

Scabies

Aetiology and epidemiology

The mite Sarcoptes scabiei var hominis, a member of the suborder Astigmata and in turn a member of the order Acari, causes human scabies. Zoophilic strains of the scabies mite may infest humans but the problem is short lived as the mite cannot complete its life cycle in the human host. The General Practitioner will diagnose and prescribe treatment for the majority of cases; only those that cause diagnostic difficulty or are recalcitrant to treatment are referred to a dermatologist.

The prevalence of the disease is unknown but hundreds of millions of people are infested worldwide each year. The prevalence rises during times of war, when people are institutionalised or live in overcrowded and poor conditions.¹

Spread is by direct, person to person contact. The mite can remain alive for as many as 3 days away from its human host on floors and furniture but there is little evidence to support a significant role for indirect spread.²

The female mite measures approximately 0.4 x 0.3mm with the male being smaller at 0.2 x 0.15mm (Figure 1). The female excavates a small burrow for mating after which the male dies. The fertilised female then enlarges the burrow and begins laying eggs, which number approximately 40-50 during her lifespan of 4-6 weeks. The larvae emerge from the eggs after 3-4 days and ascend from the burrow to the skin surface where they dig short burrows in which they transform into nymphs and then adult males and females (Figure 2). The average number of female mites on each individual patient is about 12, unless the patients have crusted (Norwegian) scabies.³

Figure 1: Scabies mite and eggs

Figure 2: Life cycle of scabies mite

Clinical features

Itch is the commonest clinical manifestation of scabies and typically worsens at night or when the patient is warm. It is due to an allergic response to the mite or its products and often takes 4-6 weeks to develop following initial exposure. Re-infection of a previously cured patient will produce a reaction within a few days.³ A widespread eruption of inflammatory papules appears concurrent with the itch and again is likely to represent an allergic immune response (Figure 3).

Figure 3: Clinical photograph of scabies eruption

Figure 4: Clinical photograph of scabies burrow

The pathognomonic lesion of scabies is the burrow (Figure 4). This is a slightly raised, brownish, tortuous lesion with minimal scale at the entry point and a tiny vesicle at the distal end, adjacent to which is the female mite. The commonest sites are the wrists, hands (notably web spaces and sides of fingers), feet (especially the instep) and male genitalia. Other common sites include the axillae, umbilicus, buttocks and nipples. Head and neck involvement is unusual except in babies and young children. The pruritic papules that develop are predominantly found around the axillae, across the abdomen, buttocks and thighs. In babies and infants vesicular or vesiculo-pustular lesions of the hands and feet are very suggestive of the diagnosis. Bullous lesions resembling bullous pemphigoid have been described in adults.⁴

Secondary features may confuse the clinical picture and lead to diagnostic difficulties. Eczematous changes are frequent and can be widespread and severe. The inappropriate use of topical steroids may produce so-called "scabies incognito". Secondary bacterial infection may produce a folliculitis or impetigo.

Persistent nodular scabies results from a chronic allergic reaction to the mite or its products. This is a clinical variant and presents as infiltrated papules or nodules in patients successfully treated for scabies. The nodules are usually few in number, extremely pruritic and typically involve the groin and axillary regions and male genitalia. The histology of a nodule can simulate lymphoma with a pleomorphic dermal infiltrate composed of T-lymphocytes, plasma cells, eosinophils, histiocytes and reticulum cells.⁵ This can lead to diagnostic confusion if nodular scabies has not been considered. Treatment with intralesional corticosteroid injections may be required.

Diagnosis

A typical history in patients presenting with a pruritic eruption is highly suggestive of the diagnosis. More than one affected member of a household is almost certain evidence of the diagnosis. Absolute confirmation can only be made by finding a burrow and demonstrating the presence of the mite, eggs or egg fragments. The burrow can be gently scraped with a blunt scalpel, the material placed in a drop of 10% potassium hydroxide or mineral oil on a microscope slide and the presence of mites, eggs or fragments will confirm the diagnosis.

The presence of inflammatory papules or nodules, sometimes mounted by burrows, on the male genitalia can provide an important diagnostic sign if no burrows are found elsewhere.

Treatment

The standard treatment for scabies involves the topical application of acaricides.^3,5 Sulphur has been used for many centuries, and 10% sulphur in yellow soft paraffin (2.5% in infants) is generally safe and effective. However, there are now several scabicides available and the choice of therapy will depend on the age of the patient and the extent of secondary eczema.

Malathion

This should be left on the skin for 24 hours, and reapplied after a few days. It is available in an aqueous base as well as a lotion in an alcohol base. The aqueous preparations are less irritant to excoriated skin. It is not recommended for infants less than 6 months of age.

Permethrin

The 5% dermal cream is applied only once and can be washed off after 8-12 hours. It is often used to treat infants as it can be applied to the scalp and face in infants from 2 months of age.

Benzyl benzoate

A natural ingredient of balsam of Peru and Tolu, is now synthesised. It should remain on the skin for 24 hours and various regimens are suggested. Most involve 2 or 3 applications, either within 24 hours, on successive days or weeks. It is irritant to the skin and patients should be warned about over use.

Gamma benzene hexachloride (Lindane)

A 12-24 hour single application is recommended but a 6 hour one is probably equally effective. There are reports of adverse neurological effects with this treatment, although recent reports suggest it is safe if used correctly. This treatment is no longer available in the UK and should probably be avoided in children less than 2 years of age and pregnant women.

Monosulfiram

A 25% solution in industrial methylated spirits diluted with 2-3 parts of water forms an emulsion that can be applied once daily for 2-3 days. Alcohol should be avoided during or shortly after use. This is no longer available in the UK.

Ivermectin

This is structurally similar to a macrolide antibiotic but does not have any antibacterial activity. However, it does have activity against ecto- and endoparasites and is used extensively in veterinary medicine. In humans it is primarily used to treat filarial disease, principally onchocerciasis. It is not licensed for use in human scabies but has been shown to be a highly effective treatment, especially in the crusted variety. There has been some concern about its side effect profile because of a report of excess deaths among a group of elderly people who received the treatment.⁶ This evidence has been challenged and not substantiated in other publications.

Proper compliance with treatment is crucial for successful treatment of scabies. It is important that patients do not wash their hands after application of their therapy and so treatment before bedtime is least disruptive. If hands are washed the treatment requires to be reapplied to this area. Treatment of the head and neck area is generally not required unless there is clinical evidence of involvement, most common in infants and young children. It is also important to remind patients to pay particular attention to the nails, especially in crusted scabies, as the mites may persist subungually. All members of a family and close physical contacts must be treated simultaneously, whether symptomatic or not. A second application is recommended 7 days later to increase treatment efficacy. A change of clothing and bedding is recommended after treatment and old clothing, towels and bedding require to be laundered. Patients should be warned that itching might persist for a few days after treatment but will generally resolve within 2-4 weeks. A topical antipruritic such as crotamiton with hydrocortisone can be prescribed to cover this period. Secondary infection is best treated with a systemic antibiotic and if eczematisation is severe then a non-irritant scabicide should be used. The treatment of scabies is difficult in pregnancy with avoidance the only totally safe option. However, treatment is required in all cases of confirmed scabies. The risk to the fetus is likely to be minimal, as there is no definite reported evidence of teratogenicity. The use of scabicides with regard to breastfeeding is also difficult with very little written information available. Once again avoidance is best but for all cases it is suggested that feeding is stopped for seven days after completion of treatment to allow for metabolism of any absorbed scabicide.⁵

Crusted scabies (Norwegian scabies)

This is an infection with Sarcoptes scabiei var hominis in which the number of mites present is enormous, and may number millions. In patients with crusted scabies itch, and therefore scratching, is minimal or absent and this may partly account for the vast mite population present. A modified host immune response to the mite is also likely. Crusted scabies may be difficult to diagnose as it can mimic hyperkeratotic eczema, psoriasis, Darier's disease and contact dermatitis. The diagnosis is readily confirmed by taking skin scrapings as these are usually teeming with mites and eggs. Treatment is with ordinary topical scabicides but they may need to be applied on several occasions. Oral ivermectin is a useful treatment when topical therapy fails.

Institutional outbreaks of scabies

These generally affect patients and medical personnel in hospitals and residential and nursing homes. An undiagnosed case of crusted scabies may be the source of an outbreak of the common type of scabies. In this setting it is important to examine all patients and members of staff for any cases of severe or crusted scabies as they may require several applications of topical treatment and isolation until cured. All medical staff, nursing staff, patients and their families require treatment with a scabicide, even if they are asymptomatic. It is important to review any individuals that are still symptomatic 4 weeks after treatment to determine if they still have evidence of scabies as they are a potential source of re-infection. Such outbreaks are difficult to manage because of the potentially enormous number of contacts. Oral ivermectin clearly has a place in such a setting.

Lice infection (Pediculosis)

Aetiology

Humans may be parasitised by two species of lice, Pediculus humanus and Pthirus pubis, which are obligate blood-sucking ectoparasites of mammals. Both belong to the suborder Anoplura, in turn a member of the order Phthiraptera. The subspecies humanus is further subdivided into P. humanus capitis, the head louse, and P. humanus corporis, the clothing or body louse.

The true prevalence is unknown but there are millions of cases of pediculosis worldwide each year.^7,8 Transmission is usually directly between individuals but may occur indirectly by contact with brushes, combs or clothing.⁷

Head lice (Pediculosis capitis)

The head louse, the commonest type, is 1-4mm long and has three pairs of legs ending in powerful claws adapted for clinging to hair. The female, which is slightly larger than the male, has a lifespan of approximately 40 days during which she will lay 300 eggs at a rate of 7-10 per day. The eggs are cemented to the hair shaft by a glandular secretion and positioned close to the scalp surface where the temperature is suitable for incubation. Eggs are oval and flesh-coloured but following emergence of the louse nymph 8 days later the egg case or "nit" appears white (Figure 5). As the hair grows the "nit" becomes more distant from the scalp. Its continuing presence after treatment is not a sign of active infestation nor treatment failure. The louse nymph reaches maturity after 10 days and the cycle continues but lice will die within 24 hours if separated from the host.

Figure 5: Empty egg case or "nit"

Figure 6: Head lice with gross secondary infection

Clinical features

Head lice are most common in children aged 3-12 years and all socio-economic groups are affected. Infestation is not significantly influenced by hair length or by frequent brushing or shampooing.⁹ Many cases are asymptomatic and patients remain unaware that they have lice infestation until pruritus develops as a result of sensitisation to louse saliva, which may take up to 3 months following initial infection. When itch develops scratching can lead to secondary bacterial infection and associated reactive occipito-cervical lymphadenopathy (Figure 6). Itchy papules may be evident at the nape of the neck and occasionally there may be a generalised itchy rash. In neglected cases secondary infection may cause severe impetigo that may cause matting of the hair.

Diagnosis

The diagnosis can be made by dry-combing with a close-set tooth comb, wet-combing following application of conditioner or direct scalp inspection by parting the hair.

All methods aim to promote visualisation of a live louse or viable egg. Dry and wet-combing - though more time-consuming - are thought to be more efficient.¹⁰

In cultures where daily hair brushing is customary, infested individuals are unlikely to host more than a dozen live lice. In areas where less frequent grooming is the norm, there may be a hundred or more.

Treatment

Many trials on the treatment of lice were conducted before the recognition of resistance to pediculicides and hence review of these does not afford the clinician useful guidelines.¹¹ The best approach to treatment is probably a combination of physical and chemical methods with wet-combing being preferable to dry. The hair is washed and a generous amount of conditioner applied prior to combing from the roots outwards, allowing easy visualisation of lice. Four wet-combings in two weeks are thought to suffice although studies suggest that physical treatment remains inferior to chemical.¹²

Three insecticides are currently available for the treatment of headlice but none are 100% ovicidal. These are malathion, the pyrethroids permethrin and phenothrin, and carbaryl. All are neurotoxic but young eggs do not have a nervous system and may survive a single treatment. Two treatments should therefore be given 7 days apart.

Aqueous preparations are preferable to alcohol-based ones since they are non-irritant and less likely to cause allergic problems in atopics. Standard treatments include malathion 0.5% in an alcohol or aqueous basis, permethrin cream 1% in an alcohol basis and phenothrin 0.2-0.5% in a choice of bases. Malathion has the added advantage of being adsorbed on to keratin conferring a residual protective effect against reinfection for 6 weeks.

In general, the following rules apply:

Carbaryl can be used in resistant cases but should be regarded as an unlikely but potential carcinogen in the light of animal studies.⁷ The use of oral co-trimoxazole, cited as a useful alternative in recalcitrant cases,¹³ should be discouraged due to the risk of severe allergic reactions.

Similarly, oral Ivermectin cited by one group as being effective for resistant cases should be used with caution and only in selected cases.¹⁴

Following treatment, affected patients should have clean clothes, bedding and towels and all household members should be examined. The advantage of systematic therapy for unaffected contacts has not been demonstrated.¹

When recommended therapy "fails", the physician should consider the possibility of misdiagnosis of active infection, non-compliance with treatment, re-infestation or resistance to the pediculicide.

Body lice (Pediculosis corporis)

Body lice infestation is now much less common and in developed countries it is largely confined to vagrants¹⁰ and to areas of poverty and overcrowding where clothing and bedding are not regularly changed or laundered.

Morphologically the body louse is very similar to the head louse but it is capable of transmitting diseases such as trench fever and Rickettsial typhus. Trench fever, caused by Bartonella quintana, was originally recognised in World War One and affects the homeless and residents of refugee camps. Typically patients have non-specific malaise, fever and myalgia but more severe cases may develop endocarditis.

Although suspected as the vector of trench fever for many years, PCR detection has now confirmed that B.quintana is the culprit.⁷

Clinical features

The presenting symptom is pruritus as a result of secondary sensitisation to salivary antigens. Tolerance develops with chronicity of infestation so that itch may disappear completely. Frequently there are numerous excoriations which become secondarily infected and ultimately hyperpigmentation is a feature.

Diagnosis

The natural habitat of the body louse is clothing but it emerges to feed on the host. Close inspection will reveal live lice in clothing seams and the fibres of those seams lying closest to the body usually harbour numerous egg cases.

Treatment

Treatment is directed at the clothing rather than the host and if disposal of the garments is not feasible the lice and eggs can be killed by laundering at 60 degrees celsius or higher. Alternatively clothing exposed to the dry heat of a tumble-drier will be similarly "sterilised". Where very large numbers are involved such as in prisoner-of-war or refugee camps, hand-pumped malathion-charged dust-guns may be used.

Pubic lice (Phthiriasis pubis)

The pubic or "crab" louse is morphologically distinct from head and body lice and has a squat body with pincer claws on its second and third pairs of legs. These are adapted for gripping hair of a certain density, favouring the pubic area and axillae as well as trunk and limbs. Scalp hair is generally too dense but lice can exist at scalp margins, in the eyebrows, eyelashes and beard area. Infection is acquired by close physical contact, usually sexual and other sexually-transmitted diseases may co-exist.

Clinical features

The presenting symptom is usually of itch, most troublesome at night when the lice become active and move around. Otherwise there are very few signs of the disease apart from the lice themselves which are often seen by the patient and can readily be visualised by the doctor, sometimes with the help of a magnifying glass. Children may have eyelash and scalp involvement due to close physical contact with a parent and therefore an isolated episode may not be indicative of sexual abuse.

Diagnosis

Careful inspection reveals lice gripping hair close to the skin surface and rust-coloured louse faeces on the surrounding skin. There may be eggs cemented to the hair. All potentially affected hair-bearing areas should be examined.

Treatment

Malathion, permethrin and phenothrin are all effective against crab lice as is carbaryl but in the UK carbaryl remains unlicensed for this indication. The treatment of choice is 0.5% malathion which is more effective than the pyrethroids.¹⁵ The aqueous form should be applied to all areas of the body and left to dry naturally before removal by washing 12 hours later. Alcohol-based preparations should be avoided in view of their tendency to irritate scrotal skin particularly. Treatment should be repeated 7-10 days later to kill lice emerging from surviving eggs. Close contacts should follow similar Eyelashes should not be tampered with physically by tweezing or combing and pediculicides are not licensed for this area. White soft paraffin suffocates the lice by blocking their spiracles and should be applied three times daily for 2-3 weeks.

Key messages

1. The treatment of scabies may be delayed when widespread eczematous changes and secondary infection lead to diagnostic difficulties.
2. Two or more affected members of a household is almost certain evidence of a diagnosis of scabies.
3. Crusted scabies can mimic other skin conditions but is readily diagnosed on skin scrapings.
4. Clear instructions and proper compliance with treatment for scabies and lice are crucial for treatment success.
5. Symptoms may persist for up to four weeks after successful treatment and alternative causes should not be sought prior to this time.
6. Resistance to pediculicides is worldwide but ongoing surveillance, limitation of spread of infection and appropriate treatment should serve to reduce this problem.