Understanding the causes and
scabies
Scabies, known colloquially as
the seven-year itch, is a contagious skin infection that occurs among humans
and other animals. It has been classified by the WHO as a water-related
disease. It is caused by a tiny and usually not directly visible parasite, the
mite Sarcoptes scabiei, which burrows under the host's skin, causing intense
allergic itching. The infection in animals (caused by different but related
mite species) is called sarcoptic mange.
The disease may be transmitted
from objects but is most often transmitted by direct skin-to-skin contact, with
a higher risk with prolonged contact. Initial infections require four to six
weeks to become symptomatic. Reinfection, however, may manifest symptoms within
as little as 24 hours. Because the symptoms are allergic, their delay in onset
is often mirrored by a significant delay in relief after the parasites have
been eradicated. Crusted scabies, formerly known as Norwegian scabies, is a
more severe form of the infection often associated with immunosuppression.
The characteristic symptoms of a
scabies infection include intense itching and superficial burrows. The burrow
tracks are often linear, to the point that a neat "line" of four or
more closely placed and equally developed mosquito-like "bites" is
almost diagnostic of the disease.
In the classic scenario, the itch
is made worse by warmth and is usually experienced as being worse at night,
possibly because there are fewer distractions. As a symptom, it is less common
in the elderly.
The superficial burrows of
scabies usually occur in the area of the hands, feet, wrists, elbows, back, buttocks,
and external genitals. Except in infants and the immunosuppressed, infection
generally does not occur in the skin of the face or scalp. The burrows are
created by excavation of the adult mite in the epidermis.
In most people, the trails of the
burrowing mites show as linear or s-shaped tracks in the skin, often
accompanied by what appear as rows of small pimple-like mosquito or insect
bites. These signs are often found in crevices of the body, such as on the webs
of fingers and toes, around the genital area, and under the breasts of women.
Symptoms typically appear two to
six weeks after infestation for individuals never before exposed to scabies.
For those having been previously exposed, the symptoms can appear within
several days after infestation. However, it is not unknown for symptoms to
appear after several months or years. Acropustulosis, or blisters and pustules
on the palms and soles of the feet, are characteristic symptoms of scabies in
infants.
The elderly and people with an
impaired immune system, such as HIV, cancer, or those on immunosuppressive
medications, are susceptible to crusted scabies
On those with a weaker immune system, the host becomes a more fertile
breeding ground for the mites, which spread over the host's body, except the
face. Sufferers of crusted scabies exhibit scaly rashes, slight itching, and
thick crusts of skin that contain thousands of mites. Such areas make eradication
of mites particularly difficult, as the crusts protect the mites from topical
miticides, necessitating prolonged treatment of these areas.
In the 18th century, Italian
biologist Diacinto Cestoni (1637–1718) described the mite now called Sarcoptes
scabiei, variety hominis, as the cause of scabies. Sarcoptes is a genus of skin
parasites and part of the larger family of mites collectively known as scab
mites. These organisms have eight legs as adults, and are placed in the same
phylogenetic class (Arachnida) as spiders and ticks.
Sarcoptes scabiei are
microscopic, but sometimes are visible as pinpoints of white. Pregnant females
tunnel into the dead, outermost layer (stratum corneum) of a host's skin and
deposit eggs in the shallow burrows. The eggs hatch into larvae in three to ten
days. These young mites move about on the skin and molt into a
"nymphal" stage, before maturing as adults, which live three to four
weeks in the host's skin. Males roam on top of the skin, occasionally burrowing
into the skin. In general, there are usually few mites on a healthy hygienic
person infested with non-crusted scabies; approximately eleven females in
burrows can be found on such a person.
The movement of mites within and
on the skin produces an intense itch, which has the characteristics of a
delayed cell-mediated inflammatory response to allergens. IgE antibodies are
present in the serum and the site of infection, which react to multiple protein
allergens in the body of the mite. Some of these cross-react to allergens from
house-dust mites. Immediate antibody-mediated allergic reactions (wheals) have
been elicited in infected persons, but not in healthy persons; immediate
hypersensitivity of this type is thought to explain the observed far more rapid
allergic skin response to reinfection seen in persons having been previously
infected (especially having been infected within the previous year or two).
Because the host develops the symptoms as a reaction to the mites' presence
over time, there is usually a four– to six-week incubation period after the
onset of infestation. As noted, those previously infected with scabies and
cured may exhibit the symptoms of a new infection in a much shorter period, as
little as one to four days.
Scabies is contagious and can be
spread by scratching an infected area, thereby picking up the mites under the
fingernails, or through physical contact with a scabies-infected person for a
prolonged period of time. Scabies is usually transmitted by direct skin-to-skin
physical contact. It can also be spread through contact with other objects,
such as clothing, bedding, furniture, or surfaces with which a person infected
with scabies might have come in contact. Scabies mites can survive without a
human host for 24 to 36 hours. As with lice, scabies can be transmitted through
sexual intercourse even if a latex condom is used, because it is transmitted
from skin-to-skin at sites other than sex organs.
Scabies may be diagnosed
clinically in geographical areas where it is common when diffuse itching
presents along with either lesions in two typical spots or there is itchiness
of another household member. The classical sign of scabies is the burrows made
by the mites within the skin. To detect the burrow, the suspected area is
rubbed with ink from a fountain pen or a topical tetracycline solution, which
glows under a special light. The skin is then wiped with an alcohol pad. If the
person is infected with scabies, the characteristic zigzag or S pattern of the
burrow will appear across the skin; however, interpreting this test may be
difficult, as the burrows are scarce and may be obscured by scratch marks. A
definitive diagnosis is made by finding either the scabies mites or their eggs
and fecal pellets. Searches for these signs involve either scraping a suspected
area, mounting the sample in potassium hydroxide, and examining it under a
microscope, or using dermoscopy to examine the skin directly.
Mass treatment programs that use
topical permethrin or oral ivermectin have been effective in reducing the
prevalence of scabies in a number of populations. There is no vaccine available
for scabies. The simultaneous treatment of all close contacts is recommended, even
if they show no symptoms of infection (asymptomatic), to reduce rates of
recurrence. Asymptomatic infection is relatively common. Objects in the
environment pose little risk of transmission except in the case of crusted
scabies, thus cleaning is of little importance. Rooms used by those with
crusted scabies require thorough cleaning.