Scabies (from Latin, from scabere ‘to scratch’)

Also called as seven years itch. It is caused by scab mite or itch mite (Sarcoptes scabiei). The female mite burrows into the stratum corneum (superficial layer of skin) and lays eggs in these burrows, which when hatch cause intense itching and further migration of the mite from one part of the body to another.

A delayed type IV hypersensitivity reaction to the mites, their eggs, or scybala (packets of feces) occurs approximately 30 days after infestation. The presence of the eggs produces a massive allergic response that, in turn, produces more itching.

 

 

 

life cycle

The burrowing is carried out using the mouth parts and special cutting surfaces on the front legs. While these are being used, the mite anchors itself with suckers on its feet. Eggs are laid in small numbers as the mite burrows, and, as these hatch, six-legged larvae climb out on to the skin and search for hair follicles, where they feed and moult (discard old cuticles to grow). In the hair follicles, the larvae show the first nymphal stages, with eight legs.

In the nymphal stages, the creature feeds and moults, and if male, gives rise to the adult. In the case of females, another moult occurs before adulthood. The female has more moults than a male, so takes longer — 17 days compared to 9 to 11 days for a male — to reach adulthood. The female is about twice the size of the male.he eggs are laid by the female at a rate of about two to three eggs a day for about two months.

The scabies mite Sarcoptes scabiei var. hominis goes through four stages in its lifecycle: egg, larva, nymph, and adult.

Upon infesting a human host, the adult female burrows into the stratum corneum (outermost layer of skin), where she deposits two or three eggs per day. These oval eggs are 0.1–0.15 mm (0.0039–0.0059 in) long and hatch as larvae in three to four days. A female can lay up to 30 eggs, then dies at end of a burrow. Upon hatching, the six-legged larvae migrate to the skin surface and then burrow into molting pouches, usually into hair follicles, where vesicles form (these are shorter and smaller than the adult burrows). After three to four days, the larvae molt, turning into eight-legged nymphs. This form molts a second time into slightly larger nymphs, before a final molt into adult mites. Adult mites then mate when the male penetrates the molting pouch of the female. Mating occurs only once, as that one event leaves the female fertile for the rest of her life (one to two months). The impregnated female then leaves the molting pouch in search of a suitable location for a permanent burrow. Once a site is found, the female creates her characteristic S-shaped burrow, laying eggs in the process. The female will continue lengthening her burrow and laying eggs for the duration of her life.

 

Structure of Mites

Adult scabies mites are spherical, eyeless mites with four pairs of legs (two pairs in front and two pairs behind). They are recognizable by their oval, ventrally flattened and dorsally convex tortoise-like bodies and multiple cuticular spines.Females are 0.3–0.45 mm (0.012–0.018 in) long and 0.25–0.35 mm (0.0098–0.0138 in) wide, and males are just over half that size.

Spread

The mites are transmitted by skin contact with carriers, and they very easily spread by skin to skin contact or with contact to clothes of infected persons like sharing of towels or bed.

 

Treatment

Treatment of scabies is application of topical scabicides like permethrin, benzyl benzoate, gammaxene, ivermectin etc. 

Person is advised to cover the entire body from neck down to feet in the morning after a hot shower and leave the lotion over the body and let it dry. Let the dried lotion remain overnight or till the next bath and repeat the same procedure for 3 days.

Wash all of the personal clothes like towels, bedsheets, blankets, underclothes in hot water to prevent reinfestations. Washing of all recently worn clothing, underwear, pajamas, used sheets, pillowcases, and towels in very hot water or dry-cleaned.

 

Topical lotions

 

Permethrin

The synthetic pyrethroid 5% permethrin -  is a first-line acaricide.  It has an excellent record for safety and low toxicity. Its efficacy as an ovicide remains unresolved, and therefore a second treatment after 1 week to kill residual hatched eggs is prudent. It applications should be repeated at end of 1st week and 2nd week after first application.

 

Benzyl benzoate - used at a concentration of 25% in adults, and 10% in children. It advantages include: its high efficacy, especially at 25%; that no resistance has been reported; and its low cost. Side-effect is that there is sometimes skin irritation, thereby limiting its tolerance. It should be left on for 24 hours and used on repeated consecutive days (minimum 3 days). 25% benzyl benzoate is an extremely rapidly acting acaricide in vitro, with death of mites occurring within 30 min.

 

Crotamiton -  (old drug) used at a concentration of 10% has a wide margin of safety and is suitable for infants. Although crotamiton has good acaricidal properties in vitro,  clinical efficacy is variable, with multiple applications advised.

 

Lindane (1% Gammaxene)

1% Lindane ([gamma] benzene hexachloride), an organochloride insecticide and formerly the treatment mainstay, has been withdrawn from many regions worldwide due to concerns regarding neurotoxicity. adverse events (including ataxia, tremors, seizures) to inappropriate or excessive application. Because of the neurotoxicity associated with 1% Gammaxene, it is not recommended in children and in adults it is used only as second line drug. 

 

Oral Ivermectin

It is most commonly administered at a weight-based dose of 200 μg/kg.  The principal indication for oral ivermectin therapy has been for the treatment of the most severe form of infection, namely crusted scabies. ( ivermectin is not approved for use in children below 15 kg, or in pregnant or lactating women).

Medicated soaps

NEKO soap - has Trichlorocarbanilide (Triclocarban) is a poweful germicide. It has got good anti-bacterial properties and mild scabicidal use. It prevents secondary infections of scabies lesions due to itching.

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