August 07 2022

SCABIES

What is scabies?     

Scabies is a very contagious disease that is produced by an immunological reaction of hypersensitivity to the infestation of the skin by the mite Sarcoptes scabiei var. Hominis , which produces as an outstanding result an intense itch.

This mite is a forced parasite of man, being also different from other mites that produce scabies in animals. Pet animals like the cat and the dog suffer from human-like scabies, produced by a variety other than humans (Sarcoptes scabiei var canis, in the case of the dog), that some can be transmitted to people, especially when caressing or sustaining an infested animal.

Scabies is most commonly spread in places and occasions where close or intimate contact occurs between people or the use of diverse objects for personal use is shared (home, school, nursing home, etc.).As a risk factor for contagion, overcrowding is a much more important factor than lack of hygiene.

The hospitalization, children day care centers, nursing homes and other health care centers are places where epidemic outbreaks occur among users or inmates, with no relation to the degree of personal hygiene, but rather with overcrowding. Scabies is not considered to be a sexually transmitted disease, but there is no doubt that intimate contact is a determinant of contagion between persons.

Mites have been identified in the domestic dust, soil, furniture, and bedding, where they can survive for more than 10 days, so transmission can not be ruled out from these sources.

Who affects?     

The annual incidence of scabies worldwide has been estimated at 300 million people and is a major public health problem in areas of scarce economic or health resources. Epidemiological studies in different parts of the world place the prevalence of the disease in values ranging from 2.7 cases per thousand inhabitants to extreme cases, such as almost half the population (46%). In some parts of the world, the infestation covers more than 60% of the population. Scabies is a problem of particular importance in developing countries, since it affects a large number of people, being endemic in tropical countries.

Some studies indicate that the disease is more frequent in the age group of 20-28 years. As has been said above, the prevalence of the disease is higher in conditions of overcrowding, that is, of accumulation of people in relatively small spaces; This is also true in urban areas, during the winter months, in women, and children.

Some population groups are at high risk for severe or encrusted scabies, a form of the disease that is particularly severe and highly contagious, including patients who have been treated with cortisone derivatives both locally and oral or parenterally, those transplanted of organs and/or under anti-rejection therapy, mentally retarded patients, physically handicapped, HIV-infected patients or by other viruses, in addition to patients with malignant diseases.

How is it transmitted?     

The mite is transmitted by direct contact with the skin of an infected person, usually requiring prolonged contact between both. The newly infected patient usually hosts an average of 5-10 mites, although in the case of Norwegian scabies this number can amount to millions of parasites.

The agent responsible for the disease completes its life cycle entirely in humans. The infestation begins when a fertilized female reaches the skin. After an hour, the female digs a tunnel in the thickness of the stratum corneum, the so-called groove. Mite females lay 10-25 eggs throughout their life cycle, that are accumulated in the furrow along with the parasites' debris. These are irritants and are partly responsible for the production of itching. In the tunnels that they excavate in the thickness of the skin, that can have until 1 cm in length. In 3 or 4 days, the eggs hatch, pierce the stratum corneum and the larvae finish their ripening process on the surface of the skin over the next two weeks. The life cycle of the parasite lasts from 30 to 60 days.

At first, the invasion is limited to a number of the mites that rarely exceed 15-20 parasites, with itching being very limited and not very intense. As the infestation progresses, so does the itching, which can become unbearable. After the infection occurs, the symptoms may take several weeks to develop completely.

A newly infected subject has an average of 10-16 female mites on the skin, although this number can reach millions in the case of forms with crusted scabies, the so-called Norwegian scab. Male mites and young mites live on the skin, but they can also dig the skin and take refuge in its thickness to proceed to the molting.

In people without problems of their defenses, scabies is usually located on the hands, fingers and wrists. It is also common for scabies to affect the flexion surfaces of the elbows, armpits, and ankles, as well as the submammary fold, buttocks and male genitalia.

If the patient suffers from an immune disorder or has diminished defenses (advanced age, infants, certain medications, HIV infection), scabies affects in a more general way, appearing in places other than the usual ones and with a more diffuse character, which can affect virtually any site of the skin, including the thorax, head and palms of the hands and soles of the feet.

What are the symptoms of scabies?     

The main symptom of scabies is an intense itch, which worsens during the night, although in some rare cases the patients do not suffer any symptoms.

The disease develops little by little. The patient perceives a slight itching, which is usually attributed to a rubbing, an insect bite or to have dry or irritated skin. Scraping destroys part of the furrows, producing momentary relief. Itching may decrease during the day to worsen when the patient lies down.

The scraping serves, in addition to producing an immediate relief, to spread the mites to other areas of the body. After a few weeks, the infestation becomes more and more intense and worse tolerated.

The lesions on the skin are preferably located in the interdigital spaces of the hands and flexion surfaces of the wrist, armpits, feet, ankles and also on the waist. Other possible locations are the nipples and the fold under the breast in the woman and the penis and scrotum in the man.

Once infected, the patient takes between three and six weeks to have symptoms, which appear as he develops an immune response to the mite and its excrement. In contrast, reinfection of the patient results in a response in the form of symptoms within 24 hours, since he had previously developed those defenses.

The most characteristic lesion of scabies is the furrow, formed by tunnels on the skin on which the parasite lives. These grooves are usually raised, generally thin and curved, measuring 1-10 mm in length. In addition to these lesions, there are papules (thickened areas), blisters, eczema, the formation of nodules (bumps), and lesions produced by scratching, in addition to the infections referred previously.

In patients with crusted scabies, the lesions are similar to psoriasis or wart-like lesions and may be accompanied by nail (hyperkeratosis) lesions. Occasionally, the disease is accompanied by increased count of eosinophils leukocytes in the blood and swollen lymph nodes.

Complications     

Scabies is a skin disease that usually results in mild to moderate discomfort. However, extensive forms with scabs or blisters can be debilitating, with pain to movement, cracking or wounds on the skin. Scabies is often complicated by secondary infections of the skin, which can lead to serious infections caused by germs such as staphylococci or streptococci including sepsis. These infections can also be complicated by other processes such as vasculitis (inflammation of blood vessels) and glomerulonephritis (inflammation of the kidney).

Diagnosis     

The doctor may have a firm suspicion that a patient suffers from scabies based on itching (which may be absent in some rare cases) associated with typical furrows. These can be identified by the tests with mineral oil, ink or by the fluorescence tests with tetracycline.

The confirmation of the diagnosis requires the identification of the parasite, its eggs or its dejections in the furrows or the nails. For this purpose, it is needed to obtain samples of the lesions, the skin or under the nails.

In some cases, it may be necessary to obtain samples for a skin biopsy or other techniques, such as videodermatoscopy and the epiluminescence microscope, which are not within the reach of most clinical dermatologists and do not appear to offer obvious superiority over routine methods.

Treatment     

It is advisable to treat all persons with a close relationship with patients, including the partner (s).

Bedding, towels, and personal clothing should be washed in warm or hot water. Those items that can not be washed should be kept out of the contact of anyone for a minimum of 3 days, which is the time it takes for most mites to die outside their natural host, the human being.

Permethrin

This medicine is the first choice drug to treat both adults and children. A single topical application of permethrin produces the cure of almost 98% of the usual cases of scabies. No other antiparasitic achieves similar or superior efficacy. Although the cure rate is very high, at the present time it is considered advisable to reapply the medication one week after the first application.

Permethrin 5% is applied throughout the body, from the neck down, maintaining contact with the cream between 8 and 14 hours. It is the most used treatment in the world, although it is not exempt from side effects.

1% Permethrin, available over the counter, is ineffective for the treatment of scabies.

Lindane

This is a similar compound, though not identical to lindane used as a pesticide for agricultural uses. It is sold in the form of 1% cream, shampoo, and lotion. It is applied similarly to permethrin. Approximately 10% of the administered dose is absorbed through the skin, which can become toxic.

Lindane may have a comparable efficacy to ivermectin if it is given in two separate doses over a two-week interval, although that substance is less toxic. As the product accumulates in the liver and attaches itself to the brain tissues, it is important not to administer more doses of this substance.

Este compuesto se utiliza cuando existen razones para no emplear la permetrina u otros antiparasitarios externos, ya que tiene una notable toxicidad. Se aplica tópicamente y se deja actuar durante 8 horas.

Both lindane and permethrin should be applied throughout the body except for the head. In the elderly and patients in whom are produced relapses, it is also convenient to apply the medication to the skull. Nails should be brushed vigorously with a nail brush or, if not, a toothbrush. Avoid applying the medication to the damp or freshly bathed skin, as there is a danger of more lindane being absorbed. It is preferable that the skin is dry.

Ivermectin

Ivermectin, another antiparasitic substance, produces a cure rate after a single application of 70%, far from the cure rate of permethrin. To achieve the same healing rate as this, two separate applications are needed for 2 weeks.

Oral ivermectin is an alternative to local treatment. Two 12-mg doses are given within one-week interval. Although not a treatment available in all countries, it may be an option in severe forms of scabies, alone or associated with permethrin.

Sulfur

This substance has constituted the classic treatment of scabies for over a century and a half, applying it to 5-10% throughout the body for three or four days. Although sulfur is effective in solving parasitization, the patients accept it poorly due to the unpleasant odor, staining, and causing dryness.

Crotamiton

The standard application includes two doses on consecutive days leaving the drug in place for 24 hours.

Other antiparasitic substances

Other therapeutic resources include malathion and benzyl benzoate

Post-treatment evolution

The rash and itching may persist for up to two weeks after the treatment finish. If the symptoms persist for more than 3 months, there may have been a failure due to treatment resistance, a reinfection from a person in the circle closest to the patient (partner, relative, friend) or an infected object. Other possible causes of treatment failure include drug allergy, poor drug application, or poor drug penetration (crusty scabies).

Sometimes, although the treatment has taken effect, the patient may feel worse by developing contact dermatitis to the drug or, more rarely, cross-sensitivity to other domestic parasites