Scabies
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Scabies is a transmissible ectoparasite skin infection characterized by superficial burrows, intense pruritus (itching) and secondary infection. The word 'scabies' is Latin for 'itch'.
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Etiology
Caused by the mite Sarcoptes scabiei, variety hominis, 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 motion of the mite in and on the skin 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 prolonged skin-to-skin contact with an infected person (e.g. bed partners), and thus is sometimes classed as a sexually transmitted disease. Spread by clothing, bedding or towels is a less significant risk, though possible.
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, less 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 - 0.4mm) 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 intitially as intense pruritis (itching of healthy skin) may occur 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.
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.
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Treatment
Topical (surface) medications are often effective and must be applied thoroughly to all skin from the face down, especially to areas known to be primarily affected (skin folds, etc.). Medication should remain on for more than 12 hours, and preferably 24, and then washed off.
Although the mites may be rapidly killed, improvement is sometimes slow and residual inflammation may take some time to finally subside. The topical medication of choice is 5% Permethrin because it is safe for all age groups. Lindane (hexachlorocyclohexane) creams or lotions are considered historical treatments, and should be avoided because they have been shown to have neurotoxic effects in children and infants. Similarly, 5–10% sulfur ointments are considered historical.
A single dose of Ivermectin (dosing: 200μg/kg) has been reported to cure, but is an off-label use, and thus considered experimental. Additional topical treatments include 10% crotamiton (except to eyes, nose, mouth), 25% benzyl benzoate cream or lotion; permethrins offer a simpler, one-application treatment which may be applied with in a 5% cream that remains on overnight or for 8-14 hours.
Tea tree oil is also known to be a homeopathic remedy.
A person can be reinfected with scabies. Without a host, Scabies mites survive for a few days. Therefore it is recommended, after treatment, to wash all material (such as clothes and bedding) that has been in prolonged contact with the infested in the last four days.
To prevent reinfestation, all social contacts and members of the family, even if not infested, should be treated similarly and most importantly at the same time.
Approximately 300 million cases of infestation with scabies occur worldwide annually.
Scabies also occurs in dogs; see Mange. Note: Although the mange mites are not able to complete their life cycle on humans, they will cause quite a bit of itching before they finally die.
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 (http://www.cdc.gov/ncidod/dpd/parasites/scabies/factsht_scabies.htm)
- World Health Organization Essential Medicines Library (http://mednet3.who.int/eml/disease_factsheet.asp?diseaseId=193&disgroup=SEXUALLY%20TRANSMITTED%20INFECTIONS&DisGP=1)
- American Social Health Association (http://www.ashastd.org/stdfaqs/scabies.html)da:Fnat