Scabies is an infestation of the skin caused by the mite, Acarus scabiei. Most of the cases of scabies in man are caused by the human variety of mite. Occasionally, however, the mite responsible for scabies of the dog, cat, cattle, sheep, horses, or other animals can also infest the human skin when it is called animal scabies.
The infestation is transmitted by close skin to skin contact as by sitting and sleeping together.
Wars, natural calamities, social or religious congregations where large numbers of people live together in camps especially under unhygienic conditions, provide good opportunities for the mite to spread and cause epidemics of scabies.
An interchange of clothes as well might result in scabies transmission.
On being transmitted to a new host, the pregnant female mites burrow into the stratum corneum and lay eggs at the rate of two to three eggs per day for 30 days; into the tunnels created during their passage.
After three to four days these eggs hatch into larvae which during the next nine to twelve days undergo two or three moultings and then mature into adult males and females. After copulation, the males die, while the pregnant females burrow into the stratum corneum again to repeat the cycle. During these cycles, some of the pregnant females can be transmitted to other individuals also and result in spread of the infestation.
The first symptoms of the disease may not appear till the patient develops hypersensitivity to the secretions of the mite or its feces (Scybala). This may take a month or even longer, but in individuals with a previous history of scabies, the symptoms may appear even within a few hours. All individuals are susceptible to scabies though it occurs more easily in children and infants. One attack of scabies does not confer any protection from the next attack.
Clinically, the disease is characterized by severe itching which is worse at night, and diffusely scattered, papular and papulovesicular lesions which may appear on all parts of the body, except the face. The lesions are specially located in the interdigital clefts of the hands, wrists, elbows, axillae, lower part of abdomen, buttocks, thighs, ankles, shaft of the penis and scrotum in the male and breasts in the female.
On the scrotum, the lesions may form nodules which tend to persist for two to four weeks even after scabies has been treated. Secondary infection with pyogenic organisms is common and leads to pustules, crusts, fever, and lymphadenopathy. When scabies occurs in cleaner individuals, the lesions may occur only on the trunk, while the hands may not show any lesions. In infants, eczematisation is very common and it leads to exudation and crusting especially on the wrists and ankles.
Burrows are the characteristic lesions of scabies and consists of a two to three mm long tortuous tunnel in the stratum corneum commonly seen in the interdigital areas and wrists in adults and palms and soles in infants.
The papular and papulo-vesicular lesions represent hypersensitivity to the mite, while the burrows represent the tunnels made by the mite in the stratum corneum.
It is common to find other individuals, especially children staying in the same house, also showing similar manifestations.
The diagnosis of scabies can be easily made on the basis of clinical characteristics and in fact, any severely itchy, diffusely scattered papular rash should be considered scabies and given a therapeutic trial with anti-scabies treatment.
The diagnosis can be confirmed by demonstrating the mite from the tip of a fresh burrow.
Benzyl benzoate, 25 % emulsion, 3 applications over one and a half days was the treatment earlier. Benzyl benzoate can cause severe irritation on the skin especially the scrotum, groins, and axillae.
Now, 1% gamma benzene hexachloride is preferred over benzyl benzoate because it has no irritant potential.
Gamma benzene hexachloride is to be avoided in children less than 2 years in age and also in pregnant women because percutaneous absorption of this drug can sometimes lead to central nervous system toxicity.
It is very essential to look for other cases of scabies in the household including the pets, or among the friends, and treat them in the same manner simultaneously. An untreated case can re-infest all the other family members.
One course with either of these drugs is sufficient to cure all cases of scabies, but the patient may continue to have some itching during the next few days as the papules take some time to heal. Calamine lotion and anti-itching drugs can help during this period.
If papules continue to appear during the next week, the same course of treatment can be repeated after an interval of seven days.
In eczematisation, it is better to use ten percent precipitated sulfur in calamine lotion for local applications twice a day for one to two weeks. The patient can take baths, but the medicine should be applied after the bath.
Crotorax ointment or lotion containing 10 percent crotamiton can also be applied all over the body including the face, twice a day for one to two weeks and is also safe for children.
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