Incidence and epidemiology
The incidence of scabies in developed countries show cyclical fluctuation. It affects all races and social classes world-wide. The interval between the end of one epidemic to the beginning of another is about 10-15 years. Scabies may occur in any age but it is most common in children and young adults. The overall sex incidence is equal. Overcrowding associated with poverty and poor hygiene in under-developed countries encourages the spread of scabies. Transmission is by close physical contact like sharing of a bed. Studies have demonstrated that indirect spread by clothing and bedding is not important. Although scabies is common in school-age children, transmission in schools is unlikely. Outbreaks occur in hospitals, old-age homes and other institutions. Outside the host, free adult mite and eggs can survive 36 hours and 10 days respectively.
Human scabies is caused by the mite Sarcoptes Scabiei var.hominis. The adult female mite is 0.4 mm long while the male mite is 0.2 mm long. Copulation occurs in a small burrow excavated by the female mite. The fertilized female enlarges the burrow and begins egg-laying. About 50 eggs are laid by each female mite during its life span of 4 to 6 weeks. The mite shows a preference for certain sites to burrow. They tend to avoid area with a high density of pilosebaceous follicles. Infested patients harbour an average of 11 mites.
Symptom occurs 3 to 4 weeks after the acquire of the infection. This latency period may not occur if an individual has had a previous infestation. Itchiness is the most obvious symptom of scabies. It is worst at night time when the patient is warm.
The pathognomonic sign of scabies is a burrow; it is a short, wavy, dirty-appearing line crossing skin lines. They may occur on the wrists, the borders of the hands, the sides of the fingers and the finger web-spaces , the feet particularly the instep and in male the genitalia and nodules on scrotum. Burrows are uncommon on the trunk in adult but they may be found in elderly and infants. Pruritic papules which accompany hypersensitivity reaction occur around axillae, peri-areolar regions, peri-umbilical regions, buttock and thighs. The fact that patients develop symptoms much more rapidly when reinfested supports the claim that the lesions of scabies are the result of a hypersensitivity reaction. Generally in adults, the lesions do not occur above the neck-line. Secondary change like eczematous change frequently give confusion to the clinical picture. Inappropriate use of topical steroid may change the clinical picture to mimic other dermatoses.
The diagnosis is suspected when burrows are found or when a patient has typical symptoms with characteristic lesions and distribution, however absolute confirmation can only be made by the discovery of the mites in microscopic examination. A burrow is gently scraped off the skin with a blunt scalpel, and the material placed in a drop of mineral oil on a microscopic slide. Oil mounting of the specimen sharpens the microscopic image and does not kill the mites which may be present (as potassium hydroxide would). Presence of mites, eggs or fragments of egg-shells confirms the diagnosis . Other diagnostic tests include needle extraction of mite, epidermal shave biopsy and punch biopsy.
It is important that all members of the household and all close contacts should be treated simultaneously. Elderly members of the family often resent for treatment but they can be asymptomatic reservoirs of infection.
Treatment must be given on two consecutive nights but not for longer. Anti-scabies preparations are primary irritants which will eventually cause eczema, patients should be warned about over-use. The patient should first take a bath, and this is followed by a brisk toweling to open the hydrated burrows. Hot bath increases the percutaneous absorption of the drug and may cause toxicity. Benzyl benzoate employed as a 25% emulsion should remain on the skin for 24 hours. The emulsion is most conveniently applied with a 2" paint brush applied to the whole body from the neck down including the genitalia and the soles of the feet. This anointing is repeated on the following morning. On the following evening the patient should take a bath again and has the bed-linen and clothing changed which are then laundered in the usual way.
After the scabicidal treatment, pruritus may persist for a further 2 weeks. A topical antipruritic such as crotamiton cream may be applied on residual itchy areas. Postscabetic eczema can be treated with topic steroid.
Secondary infection should be treated with a systemic antibiotic. If eczematisation is severe, a non-irritant scabicide, preferably in an aqueous base, should be used.
Treatment of neonates - 6.25% benzyl benzoate emulsion may be used. Other alternatives include 10% crotamiton cream (applied nightly for 2 nights and washed off after the second application), 10% sulphur in petrolatum (applied nightly for 3 nights and washed off 24 hours after the last application) and 5% permethrin cream.
Other drugs used:
1) 1% gamma benzene hexachloride (Lindane) - a single application wash off after 12-24 hours is usually recommended. It is not recommended to be used in young children, pregnant and nursing women, and those with neurological diseases.
2) malathion - malathion 0.5% in aqueous base has been used as scabicide. It should be left on the skin for 24 hours. The advantage over BBE is that it is much less stinging and acceptable.
3) permethrin - 5% dermal cream employed as a single application, wash off 8-12 hours. It is of low toxicity, and a single application which is removed in 8-10 hours is adequate. The disadvantage is that it is much more expensive than BBE or malathion.
4) monosulfiram - 25% solution diluted with 2-3 parts of water to be applied daily for 2 or 3 days.
5) crotamiton - 10% crotamiton cream is not highly effective and should not be a first line treatment for scabies. It is at best an adjunctive treatment for post-treatment pruritus and an alternative for BBE in infants and pregnant ladies.
6) topical sulphur e.g. 10% sulphur in petrolatum.