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Scabies - Seven year itch - Sarcoptes scabiei

by Michael Todd Sapko MD, PhD

What is Scabies? Rather than being an infection, scabies is a skin infestation, like lice rather than a bacterial infection. Scabies is a skin disease caused by small mites named Sarcoptes scabiei. From Greek and Latin these words mean “to cut the flesh” and “to scratch.” The mites burrow into the skin, make tunnels or tracks, lay eggs, and repeat. The scabies mites are difficult to detect and even harder to treat, which earned them the reputation as the original seven year itch.

Scabies Symptoms and signs

1.. Scabies causes extreme itchiness.

The itchiness is caused by the body’s immune reaction to the Scabies mites, their droppings, and their eggs. The itchiness can be so extreme that patients have had to be sedated to prevent them from lacerating and destroying their own skin as they scratch. In fact, if the disease is not diagnosed early, the skin can be scratched so raw (excoriated) that only the scratches can be seen and not the evidence of the scabies is obscured.

2.. If the infestation is detected early enough, there are several tell tale signs and symptoms of scabies.

One characteristic sign of scabies is the burrow that a mite makes just under the surface of the skin. After the female mite is impregnated, she burrows down and across the skin leaving feces and then depositing eggs. The trek leaves a visible line in the skin. This line can be very subtle. It may look like a welt or the remnants of a scratch. The mite itself is too small to be seen with the naked eye but a collection of feces plus the eggs plus the mite itself may appear as a black dot at one end of the burrow. The entire track can be up to a centimeter long, but even then it can be very difficult to detect.

3.. Another important sign and symptom of traditional scabies is that they only seem to invade specific regions of the body.

These areas include the wrists, the back of the hand at the base of the fingers, the antecubital fossa (the soft part where the arm bends; i.e. the other side of the elbow), the anus and genital region, a strip around the waist (like a belt), and the chest and armpits. It is not clear why the mites prefer these areas, but they generally spare the head and neck.

The other feature/characteristic of scabies is the appearance of red, raised bumps over the infested areas.

These bumps may be solid (papules) or fluid filled (vesicles). They do not contain the scabies mites but are the skin and immune system’s reaction to the presence of the mites in the skin. The papules and vesicles are usually between one to three millimeters across and cover the affected region. There are spaces of healthy skin in between the bumps.

Scabies in newborns / infants / babies

When scabies occurs in newborn babies who cannot scratch themselves, the papules and vesicles may coalesce (group together) to form large, 1-2 centimeters wide, hardened bumps. Like papules and vesicles, these nodules do not contain mites.

Norwegian scabies symptoms and signs

There is another form of scabies that behaves completely differently than typical scabies, but is becoming more prevalent in the developed world. Crusted scabies, previously referred to as Norwegian scabies, is a mite infestation by the same Sarcoptes scabiei except that the skin reaction is different. In crusted scabies the skin becomes thick, scaly, and flaky. In fact, crusted scabies can look a lot like psoriaisis. Instead of discrete lesions, crusted scabies covers large surfaces of the body. Also, crusted scabies will often extend to areas that typical scabies does not, namely the face, neck, and torso. Moreover, the skin lesions may not be itchy at all.

Scabies Transmission

Typical scabies is passed from one person to another through direct physical contact. While the mites are incredibly contagious, they do not jump or fly, they simply crawl. Scabies mites also live on inanimate objects (fomites - such as physical objects like towels and lounge chairs) for three to seven days. Most cases of scabies occur as outbreaks; scabies are passed between people or from people to clothing, linen, furniture, etc. The outbreaks occur most often in households, hospitals, nursing homes, and homeless shelters.

What is a Scabies infestation?

In typical scabies there may only need to be ten to fifteen mites at any one time to be considered an infestation. In crusted scabies, however, the standard human infestation includes millions of scabies in the skin at any one time. While the environment (clothes, furniture) is only a minor way to transmit the mite in typical scabies, it is a real problem in people with crusted scabies, mostly because of the sheer number of mites involved.

The number of cases of crusted scabies is increasing; there are about 300 million cases of scabies, typical and crusted, reported each year worldwide. Crusted scabies occurs mostly in immunocompromised individuals, such as people with HIV, with long-standing cancers of the blood, and those undergoing immunosuppressant therapy. The number of crusted scabies cases increases as the number of immunocompromised individuals increases. In these patients the immune system cannot fight off the scabies mites very effectively. This explains, at least in part, the large number of mites in this form of scabies.

Scabies Diagnosis

Making the diagnosis of scabies is notoriously difficult. While syphilis is called The Great Imitator, scabies can certainly imitate a number of skin diseases, too. Diagnosis depends on a keen eye and a good history and physical examination. Doctors must be specifically looking for the mite burrows, which are quite small. The rash also tends to look like the rash associated with a number of other skin disorders. Therefore distribution on the skin is an important clue. For crusted scabies, though, this distribution does not hold up, therefore other clues are important like a history of HIV/AIDS, homelessness, institutionalized individuals, etc.

Once the scabies diagnosis is suspected, the physician may try to obtain a living sample of the mites or the eggs of the Sarcoptes scabiei. This can be done by taking a scraping of the suspected skin lesion. If crusted scabies is likely, a drop or two of a strong base (potassium hydroxide) can be used to dissolve some of the thickened skin while preserving the sample.

One of the more interesting scabies diagnostic tests is the “burrow test,” that is, a test that attempts to detect the mite’s burrow. Since the Sarcoptes scabiei mite creates a burrow through the skin, the physician can try to use a dye to fill in this burrow. The tip of a fountain pen can be placed at the entrance to the burrow and the ink will flow into the track. Alternatively a solution of tetracycline (an antibiotic) can be infused into the suspected burrow. Once the hole is flooded, a special light called a Wood lamp can be used to examine for the presence of a burrow.

If the diagnosis of scabies is still in question, a skin biopsy can be taken as a more definitive test. The skin smaple will be examined under a microscope for mites, eggs, and the characteristic skin reaction that Sarcoptes scabiei causes.

Scabies Treatment

Generally the first goal is to get the patient to stop scratching. Again, typical scabies and often crusted scabies can be exceedingly itchy. The first line therapy is to administer topical and/or oral or intravenous antihistamines. In severe cases, the patient may need to be sedated with IV sedatives like benzodiazepines.

The next step is to destroy the scabies mites in the skin. This is usually accomplished by a topical anti-scabies medicine (scabicidal agent). There are several effective topical treatments available (see Table). If topical treatments do not work, oral ivermectin has been shown to be effective.

Patients with crusted scabies may need additional support for topical treatments to work. As the name implies, crusted scabies creates thick, crusty scales on the skin. This scaly skin may prevent the topical treatment from penetrating into the skin and the burrows. Therefore, in crusted scabies, the skin may be prepared in some way first. Warm water baths can be used to soak the skin and soften it. Also, salicylic acid or Lac-Hydrin can be used to dissolve the upper layers of crusted skin. Once soft, the skin can be carefully removed through gentle scraping (not cutting!).

Everyone and everything in contact with the person should be screened for scabies

Scabies rarely occurs in isolation and is very contagious. Family members, housemates, staff, healthcare workers, and any close contacts should be screened for scabies soon after the initial diagnosis is made. Any intimate contact should be treated whether there are signs or symptoms of scabies or not. Since the disease is an infestation rather than an infection, the immune system is only partially helpful and may not be at all helpful in severely immunocompromised patients. Treatment should be given to everyone that has the disease simultaneously. Otherwise the mites can be passed among people and objects indefinitely, making treatment ineffective.

It is important to remove any fomites from the patient’s environment, that is, physical objects that could be harboring Sarcoptes scabiei mites. This could be anything and everything. All clothes and linens (bed, bath, kitchen, etc.) should be washed with very hot water. All carpets and rugs should be thoroughly vacuumed. If the vacuum collects into a bag, discard it immediately and if it is a canister vacuum, the canister should be emptied and washed promptly. If anything in the house cannot be laundered, it should be covered and left undisturbed for one week (the amount of time a mite can live away from a human).

People doing the cleaning for potential fomites should wear gloves and disposable covering over their clothes. They should also wash their entire body thoroughly afterwards. If the patient diagnosed with scabies will be cleaning, the first topical treatment should be applied before cleaning begins. For outbreaks in hospitals or institutions, the cleanup is obviously much more extensive and difficult. It may require specialized professional cleaning or for the infested area within the institution to be abandoned for one week.

Pets do not need to be treated for human scabies infestations.

This is because the Sarcoptes scabiei that affects humans is the Sarcoptes scabiei var hominis variety. This means that it cannot affect any other animal accept a human. If the animal is a carrier of the Sarcoptes scabiei var hominis, the mites will die out after seven days. To be thorough, the animal could be given a bath.

It should be mentioned that this difference among species works both ways. Occasionally a human may be infested with a Sarcoptes scabiei variety that is specific to that animal. The pet will need treatment for scabies, but the human owner will not. The Sarcoptes scabiei will die within a week of being away from the animal’s skin.

Additional risks associated with Scabies

In some cases, the itchiness from scabies can be so severe that patients cause serious damage to their skin. The skin itself may break down and become infected with bacteria, like staph and strep. In these cases a cellulitis or superinfection may occur that requires antibiotics in addition to the scabicidal agent. The itchiness may last for some time after the mites have been eradicated by the topical medicine. Oral antihistamines should be enough to control the itchiness at this stage, though.

It is very important for anyone diagnosed with scabies to follow all directions given by their physician.

This includes treatment, cleaning, and follow up appointments. The physician will need to make sure that the disease has been controlled and that all sources of scabies have been eliminated from the environment. Incomplete treatment (and continually passing the mites round and round) is one of the few ways that scabies treatment is rendered ineffective. Therefore it is critical to follow all instructions for scabies treatment.

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