What is scabies?

Scabies is a pruritic (itchy), contagious disorder caused by itch mite Sarcoptes scabiei, which is a common problem worldwide in distribution, especially in poor unhygienic living conditions.

The human itch mite, Sarcoptes scabiei:

The gravid female human itch mite measures approximately 0.2 mm in length. They burrow superficially beneath the epidermis (stratum corneum) and deposit three or fewer eggs per day. The nymph hatch from eggs and mature in approximately 2 weeks and then emerge as adults to the surface of the skin. On the surface of the skin they mate and invade the skin of the same (reinvade) or another host. Sometimes newly fertilized female mites may be transferred from person to person by intimate contact.

What is the global problem of scabies?

Scabies infest approximately 300 million people worldwide. It is estimated that in the United States, scabies accounts for up to 5% of visits to dermatologists. In the developing and poor countries the incidence of scabies is much more common in compare to developed industrialized countries due to poor housing, poor personal hygiene, overcrowding etc.

How scabies is transmitted?

Scabies can be transmitted by close and intimate person to person contact and is transmission is facilitated by overcrowding, poor hygiene, and multiple sexual partners. If mites do not get a host, they generally die within a day and due to this transmission of scabies through sharing of contaminated bedding or clothing is infrequent. Outbreak of scabies can occur in nursing homes, mental institutions, hospitals, military barracks, schools, hostels and other places where there is overcrowding with poor hygiene level.

What is the pathogenesis of scabies?

The female mites bore into the stratum corneum of the epidermis of skin and for burrows. Burrows of itch mites (Sarcoptes scabiei) become surrounded by eosinophils, lymphocytes, and histiocytes. Later on a generalized hypersensitivity rash may develop. The numbers of mites are generally less than 15 per person due to scratching and immunity that develops against mites.

What are the signs and symptoms of scabies?

Generally an initial infestation by itch mite remains asymptomatic for up to 6 weeks but a reinfestation can produce hypersensitivity reaction without delay. Itching and rash are the common symptoms of scabies and are due to the deposition of excreta of the itch mite in the burrows. The itching of scabies may be intense and worsens at night and after a hot shower. There are excoriative lesions which are commonly seen in the elbow, on the volar wrists, between the fingers and also on the penis. Papules and vesicles which are small and are symmetrically distributed on skin folds under the breasts and around the navel, axillae, belt line, buttocks, upper thighs, and scrotum as well as in the elbow, on the volar wrists and between the fingers. The papules and vesicles may be accompanied by eczematous plaques, pustules, or nodules. Generally the face, scalp, neck, palms, and soles are not affected by scabies, but in infants these parts may also be affected.

Typical burrows may be possible to find as they are few in number and may be obscured by excoriations. The burrows look like dark wavy lines in the epidermis and can be up to 15 cm in length.

What is Norwegian scabies?

Norwegian scabies or crusted scabies is a serious form of scabies with infestation of thousands mites (hyperinfestation) where normally less than 15 mites are seen in usual scabies. Norwegian scabies or crusted scabies generally occurs due to glucocorticoid use, immunodeficiency, and neurologic and psychiatric illnesses (that limit itching and scratching).

Norwegian scabies or crusted scabies may resemble psoriasis if it is widespread with erythema (redness of skin), thick crusts, and scaling. In Norwegian scabies or crusted scabies burrows may be difficult to find, and there may be no itching. Norwegian scabies or crusted types of scabies are highly contagious and are generally responsible for outbreaks of classic scabies in hospitals.

How scabies is diagnosed?

Scabies can be diagnosed on clinical grounds. Scabies should be considered as a probable diagnosis in patients with itching and symmetric polymorphic skin lesions, especially in characteristic locations of scabies, with a history of household contact with a case of scabies. The burrows should be tried to uncover and unroofed with a sterile needle or scalpel blade, and scrapings taken for microscopic examination for the mite, its eggs, and its fecal pellets for diagnosis. Biopsy of the lesion can be used for diagnosis which shows eosinophils, lymphocytes, and histiocytes infiltration. Microscopic examination of clear adhesive tape lifted from lesions may also be sometimes diagnostic. If mites or mite products can not be identified, than the diagnosis of scabies can be made on clinical ground and treated accordingly.

How scabies is treated?

Scabicides (drugs which kills itch mites or Sarcoptes scabiei) should be applied thinly but thoroughly behind the ears and from the neck down to toes after bathing and are removed with soap and water 8 hours later. Scabicides should not be applied on the face or eyes. 5% permethrin cream is commonly used which is less toxic than 1% lindane preparations (which were commonly used previously) and also effective against lindane-tolerant infestations.

Norwegian scabies should be treated with more than one scabicide to the whole body including scalp, face, and ears, after pre-application of a keratolytic agent (which soften the skin) like 6% salicylic acid. If single scabicide is used it may require repeated application.

The only orally active scabicide is ivermectin which is useful in all forms of scabies including Norwegian scabies. The dose of ivermectin is 200 mcg/kg (microgram per kg) body weights as a single dose. Norwegian scabies may require two doses of ivermectin separated by an interval of 1 to 2 weeks. Ivermectin is not approved by the United States Food and Drug Administration (USFDA) for use in scabies, although it is used for scabies in many countries.

After treatment of scabies with proper scabicide, it becomes non infectious within a day, but itching and rash may persist for weeks or months, which are due to hypersensitivity to the dead mites and their excreted as well as secreted products. Treatment with topical scabicide may cause contact dermatitis, especially if re-treated. Antihistamines, salicylates, and calamine lotion can relieve itching during treatment. Use of topical glucocorticoids can help in relieving itching that may present for long time after effective treatment of scabies.

How to prevent scabies infestation?

To prevent scabies infestation and reinfestations, bedding and clothing should be washed and/or dried on high heat or heat-pressed, and close contacts (all family members should be treated simultaneously for best result), even if asymptomatic, should be treated simultaneously. Maintenance of personal hygiene is very important in preventing scabies. Overcrowding and unhygienic living conditions as far as possible should be avoided.