Scabies

Scabies is a transmissible ectoparasite skin infection characterized by superficial burrows, intense pruritus (itching) and secondary infection. The word scabies comes from the Latin word for "scratch" (scabere).

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Etiology

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 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 (e.g. bed partners, schoolmates, daycare), and thus is sometimes classed as a sexually transmitted disease. Spread by clothing, bedding, or towels is a less significant risk, though possible.

Onset

It takes approximately 4-6 weeks to develop symptoms after initial infestation. Therefore, a person may have been contagious for at least a month before being diagnosed. This means that person might have passed scabies to anyone at that time with whom they had close contact. Someone who sleeps 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, thus the 4-6 week "incubation" period. There are usually relatively few mites on a normal, healthy person — 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, however, they can and do occasionally burrow. Both males and females surface at times, especially at night. They can be washed or scratched off (however scratching should be done with a washcloth to avoid cutting the skin as this can lead to infection), which, although not a cure, helps to keep the total population low. Also, humans create antibodies to the scabies mites which do kill some of them.

Signs, symptoms, and diagnosis

A delayed hypersensitivity (allergic) response resulting in a papular eruption (red, elevated area on skin) often occurs 30-40 days after infestation. While there may be hundreds of papules, fewer than 10 burrows are typically found. The burrow appears as a fine, wavy and slightly scaly line a few millimeters to one centimeter long. A tiny mite (0.3 to 0.9 mm) may sometimes be seen at the end of the burrow. Most burrows occur in the webs of fingers, flexing surfaces of the wrists, around elbows and armpits, the areolae of the breasts in females and on genitals of males, along the belt line, and on the lower buttocks. The face usually does not become involved in adults.

The rash may become secondarily infected; scratching the rash may break the skin and make secondary infection more likely. In persons with severely reduced immunity, such as those with HIV infection, or people being treated with immunosuppressive drugs like steroids, a widespread rash with thick scaling may result. This variety of scabies is called Norwegian scabies.

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 colour 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.

Generally diagnosis is made by finding burrows, which often may be difficult because they are scarce, 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.

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.

When a suspected burrow is found, diagnosis may be confirmed by microscopy of surface scrapings, which are 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.

Scabies in animals

 Many domestic animals have their own species of Sarcoptes mites, and all can transiently affect humans.^[1][2] The most frequently diagnosed form is Sarcoptic mange in dogs. In dogs and other animals, scabies produces severe itching and secondary skin infections. Affected animals often lose weight and become unthrifty.

Compromised immune systems

People with compromised immune systems may not develop antibodies to the mites and may develop crusted Norwegian scabies. In this case, many form scabs or develop very red skin especially in the elderly and the mentally handicapped where white or gray crusted areas develop with little itching and little or no red bumps and mite population numbers soar to hundreds, thousands, or millions in AIDS patients^{[citation needed]}. These cases require additional treatment options to ensure a complete kill. Ivermectin is the treatment of choice in these patients combined with any other topical treatment.

Treatment

Topical Medications

• Permethrin^[3]: Another pesticide, lacks carcinogenic and teratogenic testing in humans although animal tests showed no signs of carcinogenic or teratogenic effects. Toxicity may resemble allergic reactions. [1]
• Eurax (USP Crotamiton ) [2]
• Lindane: (Kwellada). For use with patients where permethrin has failed or is contraindicated. [3]

Lindane is FDA approved as safe and effective when used as directed for the second-line treatment for both scabies and lice. Serious side effects are rare and have almost always resulted from product misuse.^{[4] [5]} Lindane is registered for use in 50 countries, with restricted-use status in 33 of these countries.^{[5] [6]} The latter includes the U.S. and Canada, which support public health uses of pharmaceutical lindane but no longer allow agricultural applications.^{[5] [7]} Lindane should be washed off with warm, and not hot, water to avoid absorption through the skin.^[8] Five to –10% sulfur ointments are considered historical.

• 10% sulfur ointment: Can be used in pregnant women and infants under two months of age. It is available over-the-counter.

Oral Medication

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. [4]

Preventing reinfection

All family and close contacts should be treated at the same time, even if asymptomatic. Cleaning of environment should occur simultaneously, as there is a risk of reinfection. Without a host, scabies mites can on average survive up to 48-72 hours away from human skin. [5] (In cases of Crusted Scabies, mites can survive up to 7 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:

• Vacuuming floors, carpets, and rugs.
• Disinfecting floor and bathroom surfaces by mopping.
• Daily washing of recently worn clothes, towels and bedding in hot water and drying in a hot dryer.
• Hot drying pillows for 30 minutes.
• Overnight freezing, in a plastic bag: stuffed animals, brushes, combs, shoes, coats, gloves, hats, robes, wetsuits, etc.
• Quarantine in a plastic bag for two weeks: things that cannot be washed, hot dried, frozen or drycleaned.
• Drycleaning: things that cannot be washed, hot dried or frozen or quarantined.

Treatment for pets

Pets and humans get infected with different types of mites, however this particular species of mite is zoonotic; it survives on both humans and animals.

Itchiness during treatment

Options to combat itchiness include antihistamines such as cetirizine. Prescription: Doxepin (Sinequan - oral or Zonalon - topical).

References

• The Merck Manual of Diagnosis and Therapy, 17th edition, 1999
• Clinician's Pocket Reference, 9th edition, 2002
• Taber's Cyclopedic Medical Dictionary, 17th edition, 1993
• United States Centers for Disease Control and Protection
• World Health Organization Essential Medicines Library
• Information on Scabies Treatments and Symptoms
• American Social Health Association
• Chosidow O. "Scabies". New Engl J Med 354 (16): 1718–1727.
• MedlinePlus Drug Information: Lindane

Numbered references

1. ^ Chakrabarti A (1985). "Some epidemiological aspects of animal scabies in human population". Int J Zoonoses 12 (1): 39–52. PMID 4055268.
2. ^ Ulmer A, Schanz S, Röcken M, Fierlbeck G (2007). "A papulovesicular rash in a farmer and his wife". Clin Infect Dis 45 (3): 395–96. PMID 17599314.
3. ^ The topical medication of choice is 5% permethrin because it is safe for all age groups.Scheinfeld NS (2004). "Controlling scabies in institutional settings: a review of medications, treatment models, and implementation.". Amer J Clin Dermatol 5 (1): 31-7.
4. ^ U.S. Food and Drug Administration (FDA). Lindane Post Marketing Safety Review. Posted 2003. Available at: http://www.fda.gov/cder/drug/infopage/lindane/lindaneaeredacted.pdf.
5. ^ ^{a b c} http://www.fda.gov/cder/foi/label/2003/006309lotionlbl.pdf.
6. ^ Commission for Environmental Cooperation. North American Regional Action Plan (NARAP) on lindane and other hexachlorocyclohexane (HCH) isomers. November 30, 2006.
7. ^ U.S. EPA. Assessment of lindane and other hexachlorocyclohexane isomers. February 8, 2006
8. ^ Medication Guide Lindane Lotion USP, 1%. Updated March 28, 2003. Available at: http://www.fda.gov/cder/drug/infopage/lindane/lindaneLotionGuide.htm.

See also

Lindane

Crab louse

Head louse