Scabies is caused by a mite (a tiny insect-like organism) that's hardly visible without a microscope. The mite is disc-shaped, pearly-white in colour, with 4 pairs of brown legs, and reaches an adult size that is less then 0.5 mm.
Scabies mites can infest warm and moist folds of skin, such as armpits, buttocks, or groin. They can be found behind the knees, on the inside of elbows and wrists, or between fingers. Scabies usually isn't seen on the scalp or on the palms of hands or the soles of feet. It is possible, however, for young children or people who harbour very large numbers of mites to be susceptible in those areas. Seniors might also find mites at the hairline, neck, temple, and forehead regions.
Scabies is quite a common condition, affecting 200 million people worldwide.
Scabies is a highly contagious condition. If you have close contact with a person infested with the scabies mite, your chances of catching it are fairly high. Crowded living conditions, close body contact (e.g., sleeping in the same bed), even holding hands for a while can easily allow the mite to spread from one person to another.
Since the scabies mite won't live away from a human body for more than a few days, direct contact is a much more likely source of transmission than clothing, bedding, or towels. The mites that cause scabies live specifically on humans - they can't be transmitted to or caught from animals, such as dogs.
A female mite lays 3 to 4 eggs per day, just under the surface of the skin. It takes about 2 weeks for these eggs to develop into larvae and finally adults, after which the adults emerge to the surface of the skin to mate. Once mating is complete, the adults reinvade the skin of their host or another person. The presence of the burrowing adult mite, eggs, and larvae cause a terrible itch. The number of infesting mites averages 5 to 10 but varies depending on the person's hygiene.
There is a severe variant of scabies called Norwegian scabies (crusted scabies). It is usually seen in people with weakened immune systems (such as people with AIDS), or can occur in outbreaks in nursing homes or hospitals. In these cases, the number of infesting mites may be in the millions.
Another form of scabies, called scabies incognito, causes an extensive infestation due to corticosteroids (e.g., betamethasone*, hydrocortisone, desonide) applied to the skin.
Symptoms and Complications
Scabies causes an intense itching that's worse at night or after bathing. The itching results from an allergic reaction the body has to the mites' feces or excrement. From 2 to 6 weeks after the initial contact, a person will develop a rash, even on parts of the body that aren't infested. Scratching gives no relief from the itch, but can cause bleeding and open sores that are then susceptible to bacterial infections.
The burrow where the mite lives appears as a slightly raised, greyish-white thread on the skin. The female mites lay their eggs at the closed end of these burrows.
In Norwegian scabies, thick crusts form on the skin. Patches of these crusted areas can be found on the palms, soles, buttocks, and ears. Even the nail beds can be infested and appear crusted and thickened. The number of mites associated with this is particularly high, but the itching isn't as intense as in other infestations.
Making the Diagnosis
Mite burrows can often be detected. A doctor might put a drop of mineral oil onto a burrow and take a light scraping of the skin in that area. Mites, or their eggs and feces, can then be seen under a microscope.
When it's hard to see the burrows, a special blue or black ink can be applied to the skin. Most of the ink can then be blotted away from the surface, and only the burrows will retain the colour.
Wood's light is another diagnostic tool – when a specific antibiotic solution is applied to the skin and the surface is then wiped clean, this particular wavelength of light allows the burrows to be viewed by a doctor.
Because the scabies rash has a similar appearance to eczema, psoriasis, or rashes caused by insect bites, it's sometimes misdiagnosed. Scabies might only be identified when a person hasn't responded to creams to treat these other conditions. Another factor that characterizes scabies is extreme itchiness, even when there's very little visible rash.
Treatment and Prevention
To prevent getting scabies in the first place, try to avoid direct contact with somebody who's infested. Be wary of using public areas such as tanning booths unless you're sure that they've been disinfected.
A one-time application of permethrin cream or lotion to the skin is usually effective in curing scabies, but a second application is recommended after a week to ensure all mites are killed. The whole body has to be cleaned (with warm water, not hot) and covered with the cream. Clean clothes should be worn during treatment, which lasts 8 to 14 hours, and then again after the cream has been washed off.
Clothes worn during the 3 days before treatment and any used bed sheets or towels should be washed in hot soapy water and then placed in the dryer on the hot cycle to kill both the mites and their eggs.
Get special instructions from your doctor or pharmacist about how much cream infants or young children need. A small amount of permethrin can be absorbed through the skin, and might come out in breast milk. If you're pregnant or breast-feeding, talk to your doctor about an alternative treatment.
After treatment, the itching won't go away immediately and might last for several weeks. This can be relieved with antihistamines, mild soaps, or prescribed corticosteroid lotions. But if you still feel intense itching after a month, you should see your doctor again as you may need to be retreated.
It's a good idea for everyone living under the same roof to be treated at the same time. This will lessen the chances of reinfection with the scabies mite. Disinfect your home, and wash all clothing and linen in hot water and then dry on a hot cycle. You could also put clothes, linens, toys, or household articles in a sealed plastic bag for a week. The pest will die off and the clothes can be worn again.