Understanding the causes and scabies
Scabies, known colloquially as the seven-year itch, is a contagious skin infection that occurs among humans and other animals. It has been classified by the WHO as a water-related disease. It is caused by a tiny and usually not directly visible parasite, the mite Sarcoptes scabiei, which burrows under the host's skin, causing intense allergic itching. The infection in animals (caused by different but related mite species) is called sarcoptic mange.
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 little 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 show as linear or s-shaped tracks in the skin, often accompanied by what appear as 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 On those with a weaker immune system, 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 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, there are usually few mites on a healthy hygienic person infested with non-crusted scabies; approximately eleven 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, there is usually a four– to six-week incubation period after the onset of infestation. 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 physical 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.
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.
Mass treatment programs that use topical permethrin or oral ivermectin have been effective in reducing the prevalence of scabies in a number of populations. There is no vaccine 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. Asymptomatic infection is relatively common. 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.
