Scabies is a common dermatological problem managed by general physicians, family physicians, and dermatologists. Scabies manifests itself in various forms in different individuals. The treatment of scabies has undergone developments with the evolution of new treatment strategies and antiectoparasitic drugs. The advent of drugs such as permethrin and ivermectin has revolutionised treatment. Management of scabies not only includes prescribing the drugs but also educating patients properly about the method of application of the drug. Treatment failures may result for various reasons and must be managed appropriately.
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Scabies has presumably afflicted humanity since antiquity.1 It is caused by the mite Sarcoptes scabiei var hominis, an obligate human parasite measuring about 300–400 microns.2 The mite burrows a tunnel in the stratum corneum and completes its life cycle there.3 Treatment of “the itch” has undergone various changes from the days of Celsus when sulphur mixed with liquid pitch was used in treatment. Many drugs, particularly insecticides, were used to treat scabies in the 20th century. However, most of them had an innate toxicity. More recently, reports of resistance to various existing drugs, the severe course of the disease, and adverse drug reactions have prompted the development of new treatment strategies and antiectoparasitic drugs for optimal management.4
All races, ages, and social levels are susceptible to scabies and the disease has a worldwide distribution.5 In 1941, Freidman noted “the Acarus scabiei[Sarcoptes scabiei] is notorious for its lack of respect for person, age, sex or race. Whether it is in the epidermis of an emperor or a slave, a centenarian or a nursling, it makes itself perfectly at home with undiscriminating impudence and equal obnoxiousness”.5 There are about 300 million cases of scabies in the world each year.6 The burden of the disease is highest in developing countries where the disease is endemic, and in certain Indian villages, the prevalence is about 100%.7 In the developed world, scabies occurs as an epidemic in hospitals, nursing homes, and long term care facilities.8
MODE OF TRANSMISSION
Scabies is usually contracted from another person by direct skin-to-skin contact.9 It can also be transmitted by sexual contact.
The incubation period of scabies is three weeks. In cases of reinfestation, symptoms develop in 1–3 days.3 Patients with scabies complain of itching, which is most severe at night. The most characteristic lesion is the burrow, the linear tunnel in which the mite lives. Burrows occur as wavy, dirty appearing line in the interdigital web spaces (see fig 1).10 Other skin manifestations include papules, blisters, nodules, and eczematous changes. The skin lesions commonly involve web spaces, flexor surface of wrists, axillae, waist, feet, and ankles. Different types of scabies have been described based on the clinical features such as nodular scabies, bullous scabies, and crusted scabies.3,8,11
The papular pustular, pruritic eruption or rash of scabies is caused by burrowing and the release of toxic or antigenic secretions of the mite. Itching takes approximately four to six weeks to develop and is thought to be due to sensitisation.
Atypical lesions such as vesicles, pustules, and nodules are seen in children.3
Crusted or Norwegian scabies occurs commonly in the immunocompromised (AIDS patients, organ transplant recipients) and in those who are mentally or physically handicapped (for example, patients with leprosy or paraplegia). It is characterised by psoriasiform or warty lesions accompanied by nail hyperkeratosis and patients complain of minimal itching.5 The average number of mites in these cases is two million; it is highly contagious and may be the source of epidemics.12
Scabies is one of the easiest and yet most difficult diagnoses in dermatology.14 Epidemiological history, the occurrence of itching, distribution of the lesions, and pruritus form the basis of the diagnosis.9 Definitive diagnosis is made by microscopic identification of mites, eggs, or mite faeces (scybala) from skin scrapings or epiluminescence microscopy. The scabies mite can be picked from the end of the burrow but this requires skill and practice. Skin scrapings from the burrow or the papule are useful in diagnosis of scabies. At first, a drop of mineral oil is placed on a sterile scalpel blade and five to six suspicious lesions are scraped. The scrapings and oil are placed on a glass slide and covered with a cover slip for examination. The presence of any stage of the mite is diagnostic. Though this procedure is simple, a skilled technician or dermatologist is required to obtain good results.
In many countries where scabies is endemic these facilities do not exist and hence diagnosis is made based on the history and clinical features. Further, if there is a clinical suspicion the diagnosis is scabies until proven otherwise.15
The treatment of scabies is as important as making a correct diagnosis. The mainstay of treatment is topical scabicidal agents. Certain principles have to be followed to treat patients successfully and are summarised in box 1.
Box 1: Principles of treatment of scabies
Establish your diagnosis.
Choose an appropriate medication.
Treat the whole body from neck to toes in adults and head and face in babies.
Treat all the contacts.
Give a detail verbal and written prescription.
Treat secondary infection if present.
Avoid over treatment.
Have a follow up at one and four weeks after treatment.
Launder clothing and bedding after completing treatment.
Patients should be properly instructed about the method of using the scabicide; this information can also be given as a pamphlet. A typical instruction sheet is shown in box 2, and it can be modified to suit the drug.
Box 2: Instructions to patients
Instructions for the treatment of scabies
Scabies is caused by itch mite and it can be easily cured if the following instructions are followed carefully:
Start with a warm bath and dry thoroughly afterwards.
The medication provided should be rubbed into the skin. All parts of the body from chin downwards, whether involved or uninvolved should be treated.
Treatment is best done at night before going to bed.
Avoid touching your mouth or eyes with your hands.
Change your underclothing and sheets the next day and launder them.
You may itch for few days but do not repeat the treatment.
Everyone in the house should be treated at the same time.
Report to your doctor after one week.
Modified from Alexander’s Arthropods and Skin2
Various treatment modalities have been used since time immemorial but the search for an ideal scabicide is ongoing. An ideal scabicide should be effective against adult and egg, easily applicable, non-sensitising, non-irritating, non-toxic, and economical2; it should also be applicable in all ages. As yet, no drug can be considered an ideal scabicide.
Scabicidal drugs can be broadly divided into topical agents and oral agents. The various topical agents that are used in the treatment of scabies are summarised in box 3 (drugs that are not used now are not listed).
Box 3: Antiscabietic drugs
Permethrin 5% cream.
Lindane (gamma benzene hexachloride) 1% lotion or cream.
Benzyl benzoate 10% and 25% lotion or emulsion.
Malathion 0.5% lotion.
Monosulfiram 25% lotion.
Crotamiton 10% cream.
Precipitated sulphur 2%–10% ointment.
Esdepallethrine 0.63% aerosol.
Ivermectin 0.8% lotion.
The topical and oral agents that are used in both in developed and developing countries are discussed below.
Sulphur is the oldest antiscabietic in use. Celsus used sulphur mixed with liquid pitch for management of scabies as early as 25 AD.1,9 Sulphur is used as an ointment (2%–10%) and usually 6% ointment is preferred. The technique is very simple: after a preliminary bath, the sulphur ointment is applied and thoroughly rubbed into the skin over the whole body for two or three consecutive nights.16 Patients should apply the ointment personally, as it ensures that their hands will be well impregnated. Ointments are more useful than any other preparation.2,17
Topical sulphur ointment is messy, malodourous, stains clothing, and in a hot and humid climate may lead to irritant dermatitis.9 It has the advantage of being cheap and may be the only choice in areas of the world where the need for mass therapy or economy dictates the choice of scabicide.7
Sulphur should be used only in situations where adults cannot tolerate lindane, permethrin, or ivermectin as it is inferior to all these agents.4 Sulphur is recommended as a safe alternative for the treatment of scabies in infants, children, and pregnant women.3
Benzyl benzoate, an ester of benzoic acid and benzyl alcohol is obtained from balsam of Peru and Tolu. Benzyl benzoate is neurotoxic to the mites. It is used as a 25% emulsion and the contact period is 24 hours. Benzyl benzoate should be applied below the neck three times within 24 hours without an intervening bath.4,9 In young adults or children, the dosage can be reduced to 12.5%. Benzyl benzoate is very effective when used correctly. If not properly applied, it may lead to treatment failure. Moreover, it can also cause irritant dermatitis on the face and scrotum. Repeated usage may lead to allergic dermatitis. It is forbidden in pregnant and lactating women, infants, and young children less than 2 years of age.4 Because of the side effects and the availability of less toxic agents, this scabicide had fallen into disrepute.4 However, recent studies have found it to be effective in the management of permethrin resistant crusted scabies18 and in combination with ivermectin in patients with relapses after a single treatment with ivermectin.19 In developing countries where the resources are limited, it is used in the management of scabies as a cheaper alternative.
Crotamiton (crotonyl-N-ethyl-o-toluidine) is used as 10% cream or lotion. The success rate varies between 50% and 70%. The best results have been obtained when applied twice daily for five consecutive days after bathing and changing clothes.20,21 However, much stress has been put on its antipruritic properties but recent studies have not revealed any specific antipruritic effects.12 Some authors do not recommend crotamiton because of the lack of efficacy and toxicity data.5
The chemical name of monosulfiram is tetraethyl thiuram monosulphide. Percival first used it to treat human scabies in 1942.22 Monosulfiram emulsion is applied all over the body after a bath,9 and it should be rubbed in well once a day on two or three consecutive days. Monosulfiram is chemically related to antabuse and hence alcoholic beverages should be avoided during or soon after treatment.2 Soaps containing monosulfiram have been used in the past as a prophylactic measure in infected communities.9
Malathion is an organophosphate insecticide that irreversibly blocks the enzyme acetylcholinesterase. Malathion is not recommended nowadays for treatment of human ectoparasitic infestations because of the potential for severe adverse affects.4
Lindane, also known as gamma benzene hexachloride, is an insecticide. Wooldridge first used it to treat scabies in 1948.23 It acts on the central nervous system (CNS) of insects and leads to increased excitability, convulsions, and death. Lindane is absorbed through all portals of entry including the lung mucosa, intestinal mucosa, and other mucous membranes and it is distributed to all body compartments with the highest concentration in lipid-rich tissue and the skin. It is metabolised and excreted in urine and faeces.4
A single six hour application is effective in treatment of scabies. Some authors recommend a repeat application after one week.9,12 Lindane 1% cream or lotion has been found to be very effective in the treatment. It is non-irritating and ease of application has made it a popular treatment. Its disadvantage is that it can cause CNS toxicity and rare cases of CNS toxicity, convulsions, and death have been reported. However, all these reports are in children or infants with overexposure or an altered skin barrier (which increases lindane absorption).9 Accidental ingestion can lead to lindane poisoning. The clinical signs of CNS toxicity after lindane poisoning include headache, nausea, dizziness, vomiting, restlessness, tremors, disorientation, weakness, twitching of eyelids, convulsions, respiratory failure, coma, and death.24,25 There is some evidence that lindane may affect the course of haematological abnormalities such as aplastic anaemia, thrombocytopenia, and pancytopenia.26 To reduce the incidence of failure and toxicity, the “do’s and don’ts” relating to the use of lindane are described in table 1.
Despite the problems, the benefits outweigh the risk.9 It is a cheap and effective alternative to permethrin in many developing countries where scabies is widely prevalent. Rare reports of resistance to lindane exist.27
Permethrin is a synthetic pyrethoid and potent insecticide.4 Permethrin is very effective against mites with a low mammalian toxicity. Permethrin is absorbed cutaneously only in small amounts, rapidly metabolised by skin esterases, and excreted in urine. Permethrin 5% dermal creams are applied overnight once a week for two weeks to the entire body, including the head in infants. The contact period is about eight hours. It is the latest and most effective treatment for scabies.4,12 Permethrin can be safely used in young children. It has virtually no allergic side effects and cosmetically it is highly acceptable. Several studies have shown that permethrin has a higher clearance rate than lindane and crotamiton.28 The limiting factor in the use of permethrin is its cost as it is the most expensive of all the topical scabicides.4
The various topical antiscabietic agents and their side effects are summarised in table 2.
Oral antiscabietic agent
Ivermectin, the 22, 23 dihydro derivative of avermectin B1 is similar to macrolides, but without any antimicrobial action. It acts via the suppression of conduction of nerve impulses in the nerve-muscle synapses of insects by stimulation of gamma amino butyric acid from presynaptic nerve endings and enhancement of binding to postsynaptic receptors. Scabies is treated with ivermectin 0.2 mg/kg in a single dose.29,30 The clinical efficacy is good with good clearing of skin lesions and a marked decrease in pruritus.31 It is rapidly absorbed and excreted through the faeces. The toxic effect of ivermectin after a single dose for scabies appears to be insignificant. It is relatively safe with side effects such as headache, pruritus, pains in the joints and muscles, fever, maculopapular rash, and lymphadenopathy, which were observed in patients with filariasis. Whether these symptoms are directly related to drug action or secondary to destruction of the filariae remains unclear.4 Ivermectin is contraindicated in patients with an allergy to ivermectin and CNS disorders. It is also not indicated during pregnancy, lactation, and in children less than 5 years of age. It is very effective, safe to use, cheap, and convenient. Ivermectin has been found to be useful in patients with a high mite burden such as crusted scabies in addition to keratolytics.29,30,32 Ivermectin promises to be the drug of the future. Ivermectin lotion has also been used to treat scabies.33
Allethrin I, widely used as an insect repellent, was effective when used as a spray in scabies. It is neither irritant nor sensitiser. Thiabendazole 5% cream has been tried in treatment of resistant scabies.
Although many drugs are used for treating scabies, recommendations for the management of scabies from the Centers for Disease Control in Atlanta include only lindane, permethrin, and ivermectin (see table 3).34
Among the various array of drugs available, the most suitable for the individual patient can be decided based on the factors2 shown in box 4.
Box 4: Factors deciding suitability of patient for various drugs
Age of the patient.
Pregnancy and lactation status if women.
Efficacy of the drug.
Extent of eczematisation.
Potential toxicity of the agent.
In general, the drug of choice for scabies is permethrin followed by lindane and benzyl benzoate. In most patients, permethrin 5% cream can be used safely except in pregnant and lactating women where it should not be used. Ivermectin is now emerging as an effective oral drug and can be used safely in adults. It is the agent of choice in Norwegian scabies and scabies epidemics in institutions.3 Lindane and benzyl benzoate still hold the sway in developing world where permethrin is beyond the reach of the poorest of the poor.
MANAGEMENT OF SPECIAL FORMS OF SCABIES
Scabies in infants
Scabies in children
Permethrin 5% cream is the most effective and safe modality of treatment in children. Benzyl benzoate can be diluted to 12.5% emulsion and used topically. Benzyl benzoate is a cheap alternative to permethrin in developing countries.9
Scabies in pregnant and lactating women
In pregnant women and lactating women, 6% sulphur precipitate is recommended. Ivermectin, permethrin, and lindane are contraindicated.4
Nodular scabies is a chronic form of scabies characterised by nodules on the covered parts of the body particularly on the male genitalia, groin, and axillary region. This type of scabies is treated with antiscabitics followed by intralesional steroids.12
Crusted scabies needs prolonged and persistent treatment. Oral ivermectin is effective but multiple doses plus topical agents may be required.29,30,35 The hyperkeratosis is treated with a keratolytic agent (5%–10% salicylic acid in petrolatum). Nails are cut short and brushed with a scabicidal agent. A cure is obtained after a mean treatment of three weeks.3,8
PROBLEMS ENCOUNTERED IN TREATMENT
Certain problems are encountered in the treatment of scabies that are peculiar to scabies: persistent itching and resistance to the scabicidal.
Itching usually persists for about one week after treatment but if it is present for longer then it should be evaluated. The cause of persistent itching and its treatment are shown in table 4.
Causes of treatment failure
The treatment of scabies can fail because of various reasons.
(1) Improper application
The drug should be applied from the neck downwards all over the body. The most common mistake is that the drug is applied only to the affected areas, which leads to a relapse of the disease.2
(2) Inadequate application
The drug dispensed should be used and it should not be diluted. When drugs like lindane are diluted, their efficacy is reduced.2
Resistance has been reported with drugs such as lindane, permethrin, and crotamiton.36,37 Resistance to lindane has been reported in El Salvador, Peru, Panama, New Zealand, Egypt, and from 18 different locations in United States.5 Resistance to permethrin is very rare and only isolated reports exist. In such areas, the combination of lindane and benzyl benzoate or permethrin can be used. Resistance should be considered only if all the other causes for treatment failure are ruled out.
Treatment of scabies not only includes a scabicidal but also symptomatic treatment in the form of antipruritics such as antihistaminics is essential to relieve itching. Moreover, if secondary bacterial infection is present it should be appropriately treated with antibiotics.
Sarcoptes scabiei is a highly parasitic blind insect, handicapped by its parasitism, as it cannot survive outside the human body. However, despite all its handicaps it survives and thrives probably because of failure on the part of human beings to understand its biology. Hence, in this review the stress has not only been on the recent advances in treatment but also on the practical aspects.
Scabies is a common dermatological problem that can be managed effectively if physicians possess an appropriate knowledge of the available drugs and instructs patients appropriately. It requires a willing physician and a cooperative patient to successfully treat this disease.