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Scabies (from Latin: scabere, "to scratch"), also known colloquially as the seven-year itch, is a contagious skin infection caused by the mite Sarcoptes scabiei. The mite is a tiny, and usually not directly visible, parasite which burrows under the host's skin, causing intense allergic itching. The infection in animals other than humans is caused by a different but related mite species, and is called sarcoptic mange.
Scabies is classified by the World Health Organization as a water-related disease. The disease may be transmitted from objects, but is most often transmitted by direct skin-to-skin contact, with a higher risk with prolonged contact. Initial infections require four to six weeks to become symptomatic. Reinfection, however, may manifest symptoms within as few as 24 hours. Because the symptoms are allergic, their delay in onset is often mirrored by a significant delay in relief after the parasites have been eradicated. Crusted scabies, formerly known as Norwegian scabies, is a more severe form of the infection often associated with immunosuppression.
The characteristic symptoms of a scabies infection include intense itching and superficial burrows. The burrow tracks are often linear, to the point that a neat "line" of four or more closely placed and equally developed mosquito-like "bites" is almost diagnostic of the disease.
In the classic scenario, the itch is made worse by warmth, and is usually experienced as being worse at night, possibly because there are fewer distractions. As a symptom, it is less common in the elderly.
The superficial burrows of scabies usually occur in the area of the hands, feet, wrists, elbows, back, buttocks, and external genitals. Except in infants and the immunosuppressed, infection generally does not occur in the skin of the face or scalp. The burrows are created by excavation of the adult mite in the epidermis.
In most people, the trails of the burrowing mites are linear or s-shaped tracks in the skin often accompanied by rows of small, pimple-like mosquito or insect bites. These signs are often found in crevices of the body, such as on the webs of fingers and toes, around the genital area, and under the breasts of women.
Symptoms typically appear two to six weeks after infestation for individuals never before exposed to scabies. For those having been previously exposed, the symptoms can appear within several days after infestation. However, it is not unknown for symptoms to appear after several months or years. Acropustulosis, or blisters and pustules on the palms and soles of the feet, are characteristic symptoms of scabies in infants.
The elderly and people with an impaired immune system, such as HIV, cancer, or those on immunosuppressive medications, are susceptible to crusted scabies (formerly called Norwegian scabies). On those with weaker immune systems, the host becomes a more fertile breeding ground for the mites, which spread over the host's body, except the face. Sufferers of crusted scabies exhibit scaly rashes, slight itching, and thick crusts of skin that contain thousands of mites. Such areas make eradication of mites particularly difficult, as the crusts protect the mites from topical miticides, necessitating prolonged treatment of these areas.
In the 18th century, Italian biologist Diacinto Cestoni (1637–1718) described the mite now called Sarcoptes scabiei, variety hominis, as the cause of 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.
Sarcoptes scabiei mites are microscopic, but sometimes are visible as pinpoints of white. Pregnant females tunnel into the dead, outermost layer (stratum corneum) of a host's 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 host's skin. Males roam on top of the skin, occasionally burrowing into the skin. In general, few mites usually occur on a healthy hygienic person infested with noncrusted scabies; about 11 females in burrows can be found on such a person.
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. Immediate antibody-mediated allergic reactions (wheals) have been elicited in infected persons, but not in healthy persons; immediate hypersensitivity of this type is thought to explain the observed far more rapid allergic skin response to reinfection seen in persons having been previously infected (especially having been infected within the previous year or two). Because the host develops the symptoms as a reaction to the mites' presence over time, usually a four– to six-week incubation period after the onset of infestation is found. As noted, those previously infected with scabies and cured may exhibit the symptoms of a new infection in a much shorter period, as little as one to four days.
Scabies is contagious and can be spread by scratching an infected area, thereby picking up the mites under the fingernails, or through physical contact with a scabies-infected person for a prolonged period of time. Scabies is usually transmitted by direct skin-to-skin contact. It can also be spread through contact with other objects, such as clothing, bedding, furniture, or surfaces with which a person infected with scabies might have come in contact. Scabies mites can survive without a human host for 24 to 36 hours. As with lice, scabies can be transmitted through sexual intercourse even if a latex condom is used, because it is transmitted from skin-to-skin at sites other than sex organs.
The symptoms are caused by an allergic reaction of the host's body to mite proteins, though exactly which proteins remains a topic of study. The mite proteins are also present from the gut, in mite feces, which are deposited under the skin. The allergic reaction is both of the delayed (cell-mediated) and immediate (antibody-mediated) type, and involves IgE (antibodies, it is presumed, mediate the very rapid symptoms on reinfection). The allergy-type symptoms (itching) continue for some days, and even several weeks, after all mites are killed. New lesions may appear for a few days after mites are eradicated. Nodular lesions from scabies may continue to be symptomatic for weeks after the mites have been killed.
Scabies may be diagnosed clinically in geographical areas where it is common when diffuse itching presents along with either lesions in two typical spots or there is itchiness of another household member. The classical sign of scabies is the burrows made by the mites within the skin. To detect the burrow, the suspected area is rubbed with ink from a fountain pen or a topical tetracycline solution, which glows under a special light. The skin is then wiped with an alcohol pad. If the person is infected with scabies, the characteristic zigzag or S pattern of the burrow will appear across the skin; however, interpreting this test may be difficult, as the burrows are scarce and may be obscured by scratch marks. A definitive diagnosis is made by finding either the scabies mites or their eggs and fecal pellets. Searches for these signs involve either scraping a suspected area, mounting the sample in potassium hydroxide and examining it under a microscope, or using dermoscopy to examine the skin directly.
Symptoms of early scabies infestation mirror other skin diseases, including dermatitis, syphilis, various urticaria-related syndromes, allergic reactions, and other ectoparasites such as lice and fleas.
Mass treatment programs that use topical permethrin or oral ivermectin have been effective in reducing the prevalence of scabies in a number of populations. No vaccine is available for scabies. The simultaneous treatment of all close contacts is recommended, even if they show no symptoms of infection (asymptomatic), to reduce rates of recurrence. Since mites can only survive for two to three days without a host, objects in the environment pose little risk of transmission except in the case of crusted scabies, thus cleaning is of little importance. Rooms used by those with crusted scabies require thorough cleaning.
A number of medications are effective in treating scabies; however, treatment must often involve the entire household or community to prevent reinfection. Options to improve itchiness include antihistamines.
Permethrin is the most effective treatment for scabies, and is the treatment of choice. It is applied from the neck down, usually before bedtime, and left on for about eight to 14 hours, then showered off in the morning. One application is normally sufficient for mild infections. For moderate to severe cases, another dose is applied seven to 14 days later. Permethrin causes slight irritation of the skin, but the sensation is tolerable. The medication, however, is the most costly of topical treatments.
Ivermectin is an oral medication shown by many clinical studies to be effective in eradicating scabies, often in a single dose. It is the treatment of choice for crusted scabies, and is often used in combination with a topical agent. It has not been tested on infants and is not recommended for children under six years of age.
Topical ivermectin preparations have been found to be effective for scabies in adults, and are attractive due to their low cost, ease of preparation, and low toxicity. It has also been useful for sarcoptic mange (the veterinary analog of human scabies).
Other treatments include lindane, benzyl benzoate, crotamiton, malathion, and sulfur preparations. Lindane is effective, but concerns over potential neurotoxicity has limited its availability in many countries. It is approved in the United States for use as a second-line treatment. Sulfur ointments or benzyl benzoate are often used in the developing world due to their low cost; 10% sulfur solutions have been shown to be effective, and sulfur ointments are typically used for at least a week. Crotamiton has been found to be less effective than permethrin in limited studies. Crotamiton or a sulfur preparation is often recommended instead of permethrin for children, due to concerns over dermal absorption of permethrin.
Scabies is endemic in many developing countries, where it tends to be particularly problematic in rural and remote areas. In such settings community wide control strategies are required to reduce the rate of disease, as treatment of only individuals is ineffective due to the high rate of reinfection. Large-scale mass drug administration strategies may be required where coordinated interventions aim to treat whole communities in one concerted effort. Although such strategies have shown to be able to reduce the burden of scabies in these kinds of communities, debate remains about the best strategy to adopt, including the choice of drug.
The resources required to implement such large-scale interventions in a cost-effective and sustainable way are significant. Furthermore, since endemic scabies is largely restricted to poor and remote areas, it is a public health issue that has not attracted much attention from policy makers and international donors.
Scabies is one of the three most common skin disorders in children, along with tinea and pyoderma. As of 2010 it affects approximately 100 million people (1.5% of the population) and is equally common in both genders. The mites are distributed around the world and equally infect all ages, races, and socioeconomic classes in different climates. Scabies is more often seen in crowded areas with unhygienic living conditions. Globally as of 2009, an estimated 300 million cases of scabies occur each year, although various parties claim the figure is either over- or underestimated. About 1–10% of the global population is estimated to be infected with scabies, but in certain populations, the infection rate may be as high as 50–80%.
Scabies is an ancient disease. Archeological evidence from Egypt and the Middle East suggests scabies was present as early as 494 BC. The first recorded reference to scabies is believed to be from the Bible (Leviticus, the third book of Moses) circa 1200 BC. Later, in the fourth century BC, the ancient Greek philosopher Aristotle reported on "lice" that "escape from little pimples if they are pricked"; scholars believe this was actually a reference to scabies.[who?]
Nevertheless, Greek physician Celsus is credited with naming the disease "scabies" and describing its characteristic features. The parasitic etiology of scabies was later documented by the Italian physician Giovanni Cosimo Bonomo (1663–99 AD) in his famous 1687 letter, "Observations concerning the fleshworms of the human body". With this discovery, scabies became one of the first diseases with a known cause.
The International Alliance for the Control of Scabies was started in 2012, and brings together over 70 researchers, clinicians and public health experts from more than 15 different countries. It has managed to bring the global health implications of scabies to the attention of the World Health Organization. Consequently, the WHO has included scabies on its official list of neglected tropical diseases and other neglected conditions.
Scabies may occur in a number of domestic and wild animals; the mites that cause these infestations are of different subspecies. These subspecies can infest animals or humans that are not their usual hosts, but such infections do not last long. Scabies-infected animals suffer severe itching and secondary skin infections. They often lose weight and become frail.
The most frequently diagnosed form of scabies in domestic animals is sarcoptic mange, which is found on dogs. This disease is caused by the subspecies Sarcoptes scabiei canis. Scabies-infected domestic fowl suffer what is known as "scaly leg". Domestic animals that have gone feral and have no veterinary care are frequently afflicted with scabies and a host of other ailments. Nondomestic animals have also been observed to suffer from scabies. Gorillas, for instance, are known to be susceptible to infection via contact with items used by humans.
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