Scabies Dr. Ghassan Salah
Scabies Scabies is a skin infestation caused by a mite
Causes: The female scabies mite (Sarcoptes scabei ) burrows through the stratum corneum of the skin and lays its eggs just above the boundary between the epidermis and dermis. Sites of burrowing are mainly the finger webs and wrists but can also be the palms of the hands, soles of the feet, external genitalia of both sexes and women s breasts. Mite burrows can sometimes be identified as slightly raised greyish pencil lines, but they are not easy to spot.
Symptoms The principal symptom of infection is severe itching, caused by mite faeces or saliva containing watersoluble glycopeptides, which eventually provoke an allergic response, although this takes several weeks to develop. The areas of itching are not necessarily the same as the sites of infection but can be diffuse and widespread, and they are often distributed symmetrically on both sides of the body. Symptoms are often as much a manifestation of damage done to the skin and secondary infection caused by scratching as of the infestation itself, and can be confused with excoriated eczema.
Transmission Skin contact for several minutes is necessary for transmission of infection from one person to another and often occurs through holding hands. Individuals may unwittingly spread infection for several weeks before symptoms develop, and so treatment of all close contacts is necessary once infestation has been identified. The scabies mite cannot survive for long outside the human body, and infection is not transferred through bedclothes or clothing.
Diagnosis Diagnosis of a scabies infestation usually is made based upon the customary appearance and distribution of the rash and the presence of burrows. Whenever possible, the diagnosis of scabies should be confirmed by identifying the mite or mite eggs or fecal matter (scybala). This can be done by carefully removing the mite from the end of its burrow using the tip of a needle or by obtaining a skin scraping to examine under a microscope for mites, eggs, or mite fecal matter (scybala). However, a person can still be infested even if mites, eggs, or fecal matter cannot be found.
Treatment Scabicidal preparations that are available without prescription contain one of the following: 1. Permethrin 2. Malathion 3. Benzyl benzoate 4. Crotamiton.
Treatment Antipruritic topical preparations, including those containing calamine and crotamiton, and systemic antihistamines can be used to treat the itching. topical permethrin appears to be the most effective treatment. Benzyl benzoate has also been found to be effective, but permethrin produced less skin irritation and fewer eczematous reactions than benzyl benzoate. The British National Formulary recommends permethrin as the first-choice treatment, and malathion if permethrin is inappropriate.
Malathion Mode of action Malathion is an organophosphorus compound. It is a potent cholinesterase inhibitor, preventing the breakdown of acetylcholine and interfering with neuromuscular transmission. Malathion is poorly absorbed through human skin, and it is also much more efficiently detoxified by human metabolic processes than by those of insects.
Malathion Contraindications, cautions and side-effects There are no contraindications to the use of malathion, apart from known sensitivity. Preparations are not contraindicated in pregnant or breastfeeding women, although manufacturers recommend caution. Malathion preparations may coloured or bleached hair. The only reported side-effect is very rare skin irritation.
Malathion Administration The lotion preparations licensed for the treatment of scabies are the same as those for head lice, but the method of administration is different. Products are licensed for use without prescription from the age of 6 months. The products may also be used in pregnancy, but treatment under medical supervision is advisable.
Malathion Administration The lotion should be applied to cool, dry, clean skin using the hand, cotton-wool, a small sponge or an 8 cm paintbrush. The lotion should be applied to the entire body surface, from the soles of the feet to the hairline. The lotion should be reapplied to the hands if they are washed after application.
Malathion Administration Traditional advice that the head and neck do not need to be treated is incorrect, as mites can be present on the face and ears, particularly in elderly people and young children; missing out these areas can therefore lead to treatment failure.
Malathion Administration About 100 ml of malathion lotion is needed for a single application for an average adult. Mites are usually killed within minutes, but the lotion should be left on for 24 hours to ensure complete eradication. Two applications 1 week apart are now recommended for both permethrin and malathion. Itching may persist for up to 2 3 weeks until the allergenic mite material is cleared from the skin, and should not be regarded as a sign of treatment failure. Patients should therefore be reassured and symptomatic relief offered, if necessary.
Malathion Administration Treatment failure may have occurred if itching has not ceased after 3 weeks, or if new areas of itching continue to appear 7 10 days after treatment. In these situations, the patient should be referred to their doctor for confirmation of the diagnosis, in which case a second application of scabicide may be advised. If treatment fails for a second time, the patient should be referred to a dermatologist
Permethrin Mode of action Natural pyrethrum, extracted from pyrethrum flowers (Chrysanthemum cinerariaefolium, Compositae), has been used as a horticultural pesticide for many years. Pyrethroids are rapidly absorbed across the insect cuticle and exert their action on the sodium channels of louse nerve axons, causing initial excitement and then paralysis.
Permethrin Contraindications, and side-effects The manufacturers recommend that permethrin cream is avoided in pregnancy and during cautions breastfeeding. Administration The product is presented as a 5% cream. (The 1% cream rinse is licensed for head lice; it is not effective for, and should not be used for, scabies.) It is not licensed for use without prescription in children under 2 years of age. Treatment under medical supervision is advised for patients over 70 years and for pregnant women.
Permethrin Administration Application is as for malathion, but the preparation need only be left on the skin for 8 12 hours before being washed off. For a single application of the cream, between 7.5 g for a 2-year-old child and 60 g for a large adult is required.
Benzyl benzoate Benzyl Benzoate Application BP 2009, a 25% emulsion, was at one time the first-choice treatment for scabies, but it can be unpleasant to use and has now been superseded by more effective products. At least two, and sometimes three, consecutive applications of benzyl benzoate, left on for 24 hours each, may be necessary to eradicate mite infestations. In addition, benzyl benzoate has an unpleasant smell, is irritant, can cause itching, burning and stinging, and may cause skin rashes. It should not be used on patients with skin excoriated through scratching. For children, it has been suggested that the emulsion can be diluted to reduce adverse effects, but this also reduces efficacy.
Crotamiton Crotamiton has antipruritic and weak scabicidal activity. However, up to five 24-hour applications at daily intervals are necessary for complete eradication of infections. Crotamiton is recommended for controlling residual itching after treatment with a more effective scabicide. It appears to have a relatively long duration of activity of 6 10 hours, requiring application only two or three times a day. One trial found it to be less effective than permethrin in reducing pruritus
Ivermectin Is an oral medication shown by many clinical studies to be effective in eradicating scabies, often in a single dose. It is the treatment of choice for crusted scabies, and is often used in combination with a topical agent. It has not been tested on infants and is not recommended for children under 6 years of age. Topical ivermectin preparations have been found to be effective for scabies in adults, and are attractive due to their low cost, ease of preparation, and low toxicity.
Product selection points Permethrin appears to be the most effective scabicide. Unlike the case with head lice, all close contacts of a person infected with scabies should be treated. Residual itch following treatment is not necessarily a sign of treatment failure.
Product selection points First choice for eradication of infection permethrin cream; second choice malathion lotion. For treatment of residual pruritus a systemic antihistamine, with additional application of calamine lotion or crotamiton cream or lotion, if desired.
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