Crusted scabies is a more severe form of the disease. It typically only occurs in those with a poor immune system and people may have millions of mites, making them much more contagious. In these cases, spread of infection may occur during brief contact or by contaminated objects. The mite is very small and usually not directly visible. Diagnosis is based on the signs and symptoms.A number of medications are available to treat those infected, such as ivermectin or permethrin, crotamiton, and lindane creams. Sexual contacts within the last month and people who live in the same house should also be treated at the same time. Bedding and clothing used in the last three days should be washed in hot water and dried in a hot dryer. As the mite does not live for more than three days away from human skin, more washing is not needed. Symptoms may continue for two to four weeks following treatment. If after this time symptoms continue, retreatment may be needed.Scabies is one of the three most common skin disorders in children, along with ringworm and bacterial skin infections. As of 2015, it affects about 204 million people (2.8% of the world population). It is equally common in both sexes. The young and the old are more commonly affected. It also occurs more commonly in the developing world and tropical climates. The word scabies is from Latin: scabere, to scratch. Other animals do not spread human scabies.
Sarcoptes is a genus of skin parasites and part of the larger family of mites collectively known as scab mites. These organisms have eight legs as adults, and are placed in the same phylogenetic class (Arachnida) as spiders and ticks.S. scabiei mites are under 0.5 mm in size, but are sometimes visible as pinpoints of white. Gravid females tunnel into the dead, outermost layer (stratum corneum) of a hosts skin and deposit eggs in the shallow burrows. The eggs hatch into larvae in three to ten days. These young mites move about on the skin and molt into a "nymphal" stage, before maturing as adults, which live three to four weeks in the hosts skin. Males roam on top of the skin, occasionally burrowing into the skin. In general, the total number of adult mites infesting a healthy hygienic person with noncrusted scabies is small, about 11 females in burrows, on average.The movement of mites within and on the skin produces an intense itch, which has the characteristics of a delayed cell-mediated inflammatory response to allergens. IgE antibodies are present in the serum and the site of infection, which react to multiple protein allergens in the body of the mite. Some of these cross-react to allergens from house dust mites.