Scabies is a transmissible ectoparasite skin infection caused by tiny mites of the species Sarcoptes scabiei, with the disease characterized by intense itching (pruritus), a rash, and secondary infection, as well as superficial burrows where mites tunnel under the skin of their hosts. The human scabies mite, Sarcoptes scabiei var. hominis, is responsible for human infections. Domestic and wild animals also may be infected by various varieties of Sarcoptes mites, such as S. scabiei var. canis, which is a parasite of dogs and foxes. Varieties affecting domestic animals may be transferred to humans (Chakrabarti 1985; Ulmer et al. 2007; Mumcuoglu and Rufli 1979). This article will be about infection in humans, unless otherwise stated.
Scabies is most prevalent among people in overcrowded conditions and conditions of poor hygiene (Carson-DeWitt 2002). This is a preventable disease and one tied to issues of personal responsibility, including good hygiene. There are readily available treatments for this condition, as well as means to prevent reinfection tied to cleaning of one's environment and personal items, such as clothing. Ultimately, this is a disease that could be significantly reduced, if not exterminated, through individuals taking special precautions to prevent the spread of these mites, which cannot survive long outside of the human skin.
The word scabies comes from the Latin word for "scratch" (scabere).
In humans, scabies is caused by the tiny mite Sarcoptes scabiei, variety hominis. Scabies was one of the earliest human diseases with a known cause (Arlian 1989). The mite was first described and illustrated in 1689, by the Italians Diancinto Cestoni and Giovanni Cosimo Bonomo in a now-famous letter to Francesco Redi (Arlian 1989). However, only some 200 years later, was it generally accepted that scabies was a parasitic disease (Arlian 1989).
S. scabiei var. hominis infection produces intense, itchy skin rashes when the impregnated female mite tunnels into the skin's outermost layer, the stratum corneum, and deposits eggs in the burrow. Upon hatching, the larvae move to the surface of the skin, molt into a "nymphal" stage, mature into adult mites, and mate, and repeat the life cycle either within the original host or its next host (Carson-DeWitt 2002; Jacques 2008). 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 produces a massive allergic response which, in turn, produces more itching.
Scabies is transmitted readily, often throughout an entire household, by skin-to-skin contact with an infected person (for example, bed partners, schoolmates, daycare). Spread by clothing, bedding, or towels is a less significant risk, though possible.
In individuals with strong immune systems and those that scratch (which apparently, often inadvertently, removes some of the mites), infections commonly involve no more than 15 mites altogether, with an average of 11 adult females in an affected human host (Jacques 2008; Carson-DeWitt 2002). However, in those physically infirm, or otherwise immunocompromized, such as the elderly, there may be an infection with a more severe variant called crusted scabies or Norwegian scabies, with a human host infected by thousands to more than two million adult females (Jacques 2008; Carson-DeWitt 2002). This type of infection involves thickened, crusty areas all over the body (Carson-DeWitt 2002).
Various varieties of Sarcoptes scabiei may infect wild and domestic animals. S. scabiei var. canis is parasitic on dogs, foxes, and wolves, causing sarcoptic mange, and it can also infect cats and humans to a lesser extent. S. scabiei var. wombati affects wombats. S. scabiei var. cuniculi is the rabbit scabies mite. S. scabiei var. bovis infects cattle.
S. scabiei mites typically are adapted to their preferred host or hosts, and they may be unable to complete their life cycle in other, dissimilar hosts. Nonetheless, often they can transfer to other hosts, including humans, causing opportunistic infections. Mumcuoglu and Rufli (1979) report a case where S. scabiei var. bovis infected a farmer. They further reported that this variety penetrates the human skin, but does not form burrows, which is in contrast to typical S. scabiei infections of preferred hosts, such as with variety hominis affecting humans. S. scabiei var. wombati likewise only burrows in a short way in opportunistic infections of humans.
De Geer, 1778
Sarcoptes scabiei is a tiny mite, a parasitic arthropod (phylum Arthropoda) belonging to the class Arachnida, subclass Acarina (or Acari), a taxon that contains ticks as well, the order astigmata, and the family Sarcoptidae.
As members of the class Arachnida, mites would be expected to have a segmented body with the segments organized into two tagmata: A prosoma (cephalothorax) and an opisthosoma (abdomen). However, only the faintest traces of primary segmentation remain in mites, the prosoma and opisthosoma are insensibly fused, and a region of flexible cuticle (the cirumcapitular furrow) separates the chelicerae and pedipalps from the rest of the body. This anterior body region is called the capitulum or gnathosoma and is also found in the Ricinulei. The remainder of the body is called the idiosoma and is unique to mites.
The sarcoptes are part of the larger family of mites collectively known as “scab mites;” they are also related to the scab mite “psoroptes,” also a mite that infests the skin of domestic animals. Sarcoptic mange affects domestic animals and similar infestations in domestic fowls causes the disease known as “scaly leg.”
S. scabiei is only about 0.3 to 0.4 millimeters (.012 to .016 inches) long as an adult (Jacques 2008; Carson-DeWitt 2002). The type that infects humans is Sarcoptes scabiei, variety hominis. Scabies are microscopic, although sometimes they are visible as a pinpoint of white.
The life cycle of mites includes four stages: Egg, larva, nymph, and adult. The life cycle of Sarcoptes scabiei, variety hominis involves the female burrowing under the lower stratum corneum of the skin and laying eggs in small numbers along the line of the burrow (Arlian 1989; Carson-DeWitt 2002). The six-legged larvae hatch in about three to ten days and move toward the surface of the skin, where they will make new burrows and mature. Specifically, the larvae search for hair follicles, where they feed and molt (discard old cuticles to grow), and mature into adult mites. It is in the hair follicles that the larvae show the first nymphal stage, with eight legs. After reaching maturity and being fertilized, the adult female mite burrows into the skin, of either the same host or a new host, and then lays its eggs, completing the life cycle. The life cycle is about three to four weeks in total (Jacques 2008).
The female has more molts than a male and, therefore, takes longer to reach adulthood—seventeen days for the female versus nine to eleven days for a male. The female is about twice the size of the male. Males tend to roam on top of the skin, although they can and do occasionally burrow. Both males and females surface at times, especially at night.
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 produces a massive allergic response which, in turn, produces more itching.
The adult female usually burrows into the hands or wrists; however, other parts of the body may also be affected, and lays its eggs. The burrowing is carried out using the mouthparts and special cutting surfaces on the front legs. While these are being used, Sarcoptes scabiei anchors itself with suckers on its feet.
Although the life cycle is short, individual patients are seldom found to have more than about a dozen mites on them. Even so, this number can cause agonizing itching, in particular at night, and severe damage to the skin often comes as a result of scratching, particularly by the introduction of infective bacteria, which may lead to impetigo or eczema.
The conditions most favorable to Sarcoptes scabiei are found on parts of the human body, such as the hands and wrists, where there is frequent skin to skin contact between persons, as the mites are transmitted by skin contact with carriers, and they very easily spread.
The mites can live away from human skin for only about three days (Carson-DeWitt 2002; CDC 2008).
Symptoms develop approximately four to six weeks after initial infestation. Therefore, infected persons may have been contagious for at least a month before being diagnosed and may have passed scabies to anyone with whom they had close contact. Persons who sleep in the same room with a person with scabies has a high possibility of having scabies as well, although they may not show symptoms.
The symptoms are caused by an allergic reaction that the body develops over time to the mites and their by-products under the skin, such as the feces; thus the four to six week "incubation" period.
Diagnosis is usually achieved by observing the characteristic burrows of the mites (Carson-DeWitt 2002). A tiny mite may sometimes be seen at the end of a burrow. Burrows are located most commonly on the sides of fingers, between the fingers, on top of the wrists, around the armpits and elbows, around the nipples in women's breasts, in the genitalia of men, around the waist (beltline), and the lower part of the buttocks (Carson-DeWitt 2002). Babies may have burrows on the palms of their hands, soles of their feet, and faces (Carson-DeWitt 2002). The face usually does not become involved in adults.
When a suspected burrow is found, diagnosis may be confirmed by microscopy of surface scrapings. The pearly bump at the end of a burrow can be removed and examined microscopically, possibly identifying the mite itself (Carson-DeWitt 2002). Skin scrapings may be placed on a slide in glycerol, mineral oil, or immersion in oil and covered with a coverslip. Avoiding potassium hydroxide is necessary because it may dissolve fecal pellets. Positive diagnosis is made when the mite, ova, or fecal pellets are found.
Finding burrows may be difficult because they are scarce, or because they are obscured by scratch marks, or by secondary dermatitis (unrelated skin irritation). If burrows are not found in the primary areas known to be affected, the entire skin surface of the body should be examined.
Scabies is frequently misdiagnosed as intense pruritus (itching of healthy skin) before papular eruptions form. Upon initial pruritus, the burrows appear as small, barely noticeable bumps on the hands and may be slightly shiny and dark in color rather than red. Initially, the itching may not exactly correlate to the location of these bumps. As the infestation progresses, these bumps become more red in color.
The suspicious area can be rubbed with ink from a fountain pen or alternately a topical tetracycline solution which will glow under a special light. The surface is then wiped off with an alcohol pad; if the person is infected with scabies, the characteristic zigzag or S pattern of the burrow across the skin will appear.
The rash may become secondarily infected as scratching the rash may break the skin and make such infection more likely.
People with compromised a immune system may develop crusted Norwegian scabies. In this case, they may be infected with huge numbers of mites—thousands or even millions. Examples of cases in which individuals do not scratch (and thus inadvertently remove mites) or have weakened immune systems include: the physically infirm; those where other diseases that decrease the sensation in the skin; those with leukemia or diabetes; those taking medication that lowers the immune response; and those patients with AIDS that have a lower immune response (Carson-DeWitt 2002). Likewise, in people being treated with immunosuppressive drugs like steroids, the elderly, or those mentally handicapped, a widespread rash with thick scaling may result. These cases require additional treatment options to ensure a complete kill. Ivermectin is a single oral treatment of choice in these patients combined with any other topical treatment.
Some impact is made simply by the patient scratching or washing. Although not a cure, this helps to keep the total population low. (However scratching should be done with a washcloth to avoid cutting the skin, as this can lead to infection.), Also, humans create antibodies to the scabies mites, which do kill some of them.
Treatment options include various medications.
Medications typically are applied from head to foot (avoiding the eyes and mouth) and left on overnight (Jacques 2008).
A single dose of ivermectin has been reported to cure scabies. In 1999, a small scale test comparing topically applied Lindane to orally administered ivermectin found no statistically-significant differences between the two treatments (AAFP 2000).
All family and close contacts should be treated at the same time, even if asymptomatic. Cleaning of the environment should occur simultaneously, as there is a risk of reinfection. Since the scabies mites can, on average, survive only up to 48-72 hours away from human skin (CDC 2008), shoes or other articles that cannot be washed can be sealed in a plastic bag for week or longer (Jacques 2008). (In cases of Crusted Scabies, mites can survive up to seven days.) Therefore, it is recommended to wash all material (such as clothes, bedding, and towels) that has been in contact with all infested persons in the last three days.
Cleaning the environment should include:
Options to combat itchiness include antihistamines such as cetirizine. Prescription: Doxepin (Sinequan; oral or Zonalon; topical) or Hydroxine.
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