Scabies, also known as sarcoptic mange and colloquially known as the itch, is a contagious ectoparasitic skin infection characterized by superficial burrows and intense pruritus (itching). It is caused by the mite Sarcoptes scabiei. The word scabies itself is derived from the Latin word (scabere) for "scratch".
 
Signs and symptoms
 
The characteristic symptoms of scabies infection include superficial burrows, intense pruritus (itching), a generalized rash and secondary infection. Acropustulosis, or blisters and pustules on the palms and soles of the feet, are characteristic symptoms of scabies in infants.
 
S-shaped tracks in the skin are often accompanied by small, insect-type bites called nodules that may look like pimples. These burrows and nodules are often located in the crevices of the body, such as the webs of fingers, toes, feet, buttocks, elbows, waist area, genital area and axilla, and under the breasts in women.
 
The intense itching and rash characteristic of scabies infection is caused by an allergic reaction of the body to the burrowed microscopic scabies mites. The rash can be found over much of the body, especially in immunocompromised people (HIV positive or elderly); the associated itching is often most prevalent at night.
 
Secondary infection of impetigo, a Streptococci or Staphylococci bacterial skin infection, may occur after scratching. Cellulitis may also occur, resulting in localized swelling, redness and fever (DermNet).
In immuno-compromised, malnourished, elderly or institutionalized individuals, infestation can cause a more severe form of scabies known as crusted scabies or Norwegian scabies. This syndrome is characterized by a scaly rash, slight itching and thickened crusts of skin containing thousands of mites. Norwegian scabies is the form of scabies that is hardest to treat.
 
In individuals never before exposed to scabies, the onset of clinical signs and symptoms is 4–6 weeks after infestation. Some people may not realize that they have it for years; in previously exposed individuals, onset can be as soon as 2–4 days after infestation.
 
Cause
 
Scabies is highly contagious and can be spread by scratching, picking up the mites under the fingernails and simply touching another person's skin. They can also be spread onto other objects like keyboards, toilets, clothing, towels, bedding, furniture, and anything else onto which the mite may be rubbed off, especially if a person is heavily infested. The parasite can survive up to 14 days away from a host, but often do not survive longer than two or three days away from human skin. Scabies is caused by the mite Sarcoptes scabiei, variety hominis, as shown by the Italian biologist Diacinto Cestoni in the 18th century. It produces intense, itchy skin rashes when the impregnated female tunnels into the stratum corneum of the skin and deposits eggs in the burrow. The larvae, which hatch in 3–10 days, move about on the skin, molt into a "nymphal" stage, and then mature into adult mites. The adult mites live 3–4 weeks in the host's skin.
 
The action of the mites moving within the skin and on the skin itself produces an intense itch which may resemble an allergic reaction in appearance. The presence of the eggs does not, in fact, produce more itching; this conjecture is only myth. It is rather the feces of the mites which cause the allergic reaction.
 
Scabies can be transmitted readily throughout an entire household by skin-to-skin contact with an infected person (e.g. bed partners, schoolmates, daycare). It can be spread by clothing, bedding or towels. Washing clothing in very hot water and dry on high heat will help prevent the transmission. Alternatively, permethrin sprays can be used for items that cannot be laundered.
 
The symptoms of itching and rash are caused by an allergic reaction that the human body develops over time to the mites and their by-products under the skin. As such, there is usually a 2-6 week incubation period between infestation and presentation of symptoms. However, in individuals with prior exposure to scabies, the incubation period is much shorter: as little as 1–4 days.
 
There are usually relatively few mites on a normal, healthy person (who is infested with scabies) — about 11 females in burrows. Scabies are microscopic although sometimes they are visible as a pinpoint of white. The females burrow into the skin and lay eggs there. Males roam on top of the skin but can also occasionally burrow.