Endoparasitic mites

Sarcoptes scabiei (scabies mites)

The scabies mite, Sarcoptes scabiei, is the cause of scabies in humans and other mammals. If man is infested by any of the forms of the scabies mite which normally parasitize animals the infestation usually disappears after some time. This is because each one of the different scabies mite populations are relatively host specific.

The scabies mite is whitish to nearly transparent, almost round, and due to its size (1/3mm) hardly visible to the naked eye (Figure 23.5). The female scabies mite makes ducts in the epidermis in which the eggs are laid. The eggs hatch after about one week. After the larval and nymphal instars the adult stage is reached. The adult mites will mate on the skin surface (Gordon and Lavoipierre 1972). After copulation, the female burrows in the skin,

Pictures Scabies Infection Knees
Figure 23.5 Female scabies mite Sarcoptes scabiei with two eggs. Natural size c. 0. 3 mm. (Drawing by Inga Thomasson after Smith (1973).)

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particularly where it is thin. Particularly favoured sites are the skin between the fingers, the underside of the wrists, the elbows and knees, navel, breasts, shoulders, buttocks, scrotum, and penis. The face and scalp are not usually infested, except in children. The length of the ducts varies between a few millimetres to several centimetres. The number of ducts and adult females can sometimes exceed 100. The average number of ducts per infected person may be about 10-15. The itching, which is an immunological response to mites and their faeces, is particularly serious during night-time. In newly infested persons the itching begins not until about a month after the initial infestation. The itching will often indirectly cause secondary bacterial infections of the skin with purulent ulcers. Infested persons will develop an extensive rash with erythema and follicular papules that can cover areas where no mites can be found (Alexander 1984; Varma 1993). Scabies-infected individuals with immunological defects, for example, HIV-infected people, can develop a very serious and very contagious type of scabies, denoted as crusted or Norwegian scabies (after a Norwegian scientist who first described the condition). In persons with crusted scabies the mite population increases to extremely high abundances (>1000 mites per person).

The diagnosis is preferably made by using a thin needle to catch a (female) mite in a burrow in the skin. A captured mite can thereafter be identified in the microscope. By pouring ink onto supposedly scabies-infested skin and then washing away surplus ink from the skin a positive scabies diagnosis is made by finding ink remaining in the mite ducts.

Human infestation with S. scabiei has a world-wide distribution but appears to be most prevalent in relatively poor subtropical and tropical areas. Persons of all ages are affected, although prevalence rates appear to be the highest in children. Thus, investigations of pre-school children in the tropics have revealed point prevalences between 30 and 80%.

Transmission of scabies mites is presumably mainly by prolonged close personal contact, for example, by holding hands or by resting or sleeping in the same bed as a scabies-infested person. However, it is likely that the mites can also be transmitted via clothes, towels, bed linen, etc. Since the mites are most active during the night, transmission is also most likely to occur during this time. Control of scabies infestation is primarily by acaricidal (chemical) treatment of the infested skin. In similarity to the situation in louse infestations, the choice of scabicide to be recommended depends partly on the resistance status of the mite population in the geographical area concerned. The resistance status can change within a few months in a particular area depending on the types of acaricides that have been used there. Thus, it is not possible to give any general recommendation here about the choice of acaricide to be used. Rather, it is necessary to get up-to-date information about which acaricide is recommended in a particular area. The most commonly used anti-scabies products contain as the active ingredient(s) benzyl-benzoate (with disulfiram), crotamiton (V-ethyl-O-crotontoluide), lindane or malathion. Benzyl-benzoate is not recommended for usage on children. Lindane is one of the environmentally harmful persistent organic pollutants (POPs) the use of which should be abandoned (Jaenson 1996).

When scabies has been detected in a family, all its members and other close contacts should be treated. The treatment with a suitable acaricide, usually obtainable in the local pharmacy, should be preceded by thoroughly washing the body. The duration of the treatment, that is, the time that the active substance shall remain on the skin depends on the type of acaricide and how it is formulated. After the acaricidal treatment, the body should be thoroughly washed with soap and water. Clothes, towels, bed linen, caps, etc. should now be washed or, if this is not possible, an alternative is to place, for example, gloves, shoes, etc., to air for 5-7 days.

Epidemiolgy Cyclospora
Figure 23.6 The hair follicle mite Demodex folliculorum. Natural size c.8mm. (Drawing by Inga Thomasson after Smart (1948).)
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