Scabies

Scabies is a highly contagious ectoparasite skin infection characterized by superficial burrows, intense pruritus (itching) and secondary infection. It is caused by the mite Sarcoptes scabiei. The word scabies itself is derived from the Latin word for "scratch" (scabere).

Etiology

Sarcoptes scabiei var. canis (dog scabies mite)

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, and is almost impossible.

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 8 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, although 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 scabies burrow under magnification. The scaly patch at the left is due to scratching of the original papule. The mite traveled from there to the upper right, where it can be seen as a dark spot at the end of the burrow.

A tiny mite (0.3 to 0.9 mm) may sometimes be seen at the end of a burrow. Most burrows occur in the webs of fingers, flexing surfaces of the wrists 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 though. 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 advanced HIV, 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 color rather than red. Initially the itching may not exactly correlate to the location of the hole. Generally diagnosis is made by finding burrows - which often may be difficult because they are scarce, and because they are obscured by scratch marks. 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.

Domestic animals

Puppy with Scabies (Sarcoptic mange)

Many domestic animals have their own species of Sarcoptes mites. Though all can transiently affect humans,^[1][2] the mites that cause scabies in animals cannot reproduce on the human body and will die within a few days.[1] 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. Sarcoptes is a genus of skin parasites, and 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 “scabies leg”. The effects of Sarcoptes scabiei are the most well known, causing “scabies”, or “the itch”. The adult female mite, having been fertilised, burrows into the skin, usually the hands or wrists, however other parts of the body may also be affected, and lays its eggs.

Compromised immune systems

Norwegian scabies in homeless AIDS patient

People with compromised immune systems may not develop antibodies to the mites and may develop crusted Norwegian scabies. In this case, the scabies are taking over. 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.

Gallery of scabies infections

Evolution of infection

Treatment

Medications

Topical

• 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.^[4]
• Eurax (USP Crotamiton)^[5]
• Malathion Applied for 24 hours; effective in killing both adults and eggs.
• Lindane (Kwellada): For use with patients where permethrin has failed or is contraindicated.^[6]

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.^[7][8] Lindane is registered for use in 50 countries, with restricted-use status in 33 of these countries.^[8][9] The latter includes the U.S. and Canada, which support public health uses of pharmaceutical lindane but no longer allow agricultural applications.^[8][10] Lindane should be washed off with warm, and not hot, water to avoid absorption through the skin.^[11]

• There is some evidence that a 10% sulfur ointment in petroleum jelly applied topically is effective. It is cheap and readily available over-the-counter.^[12][13] It also has the advantange of being able to be used in pregnant women and infants under two months of age.
• Neem oil is deemed very effective in the treatment of scabies although only preliminary scientific proof exists which still has to be corroborated, and is recommended for those who are sensitive to permethrin, a known insecticide which might be an irritant. Also, the scabies mite has yet to become resistant to neem, so in persistent cases neem has been shown to be very effective.^[14]
• Tea tree oil in one study was more effective than commercial medications against the scabies mite in an in vitro situation. ^[15]

Oral

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.^[16]

Preventing reinfestation

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. Therefore it is recommended to wash and hot iron all material (such as clothes, bedding, and towels) that has been in contact with scabies infestation.

Cleaning the environment should include:

• Vacuuming floors, carpets, and rugs.
• Disinfecting floor and bathroom surfaces by mopping.
• Cleaning the shower/bath tub after each use.
• Daily washing of recently worn clothes, towels and bedding in hot water, drying in a hot dryer and steam ironing.

Itchiness during treatment

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

References

Notes

Resources

• 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
• American Social Health Association
• Chosidow O (2006). "Scabies". New Engl J Med 354 (16): 1718–1727. doi:10.1056/NEJMcp052784. PMID 16625010. 
• MedlinePlus Drug Information: Lindane
• Prevention and control of scabies in california long term care facilities

See also

• Lindane
• Crab louse
• Head louse

External links

• American Academy of Dermatology pamphlet on Scabies
• Scabies FAQ from the National Pediculosis Association