Author: Dr Lindy P. Fox University of California San Francisco 2008-07-28
What is scabies?
Scabies is an intensely itchy infestation of the skin by the human mite Sarcoptes scabiei var hominis. It has a global prevalence of 300 million cases per year [Strong and Johnstone 2007; Chosidow 2006]. The
mite completes its entire life cycle on humans. Mites do not jump or
fly, but can crawl on the skin at a rate of 2.5cm per minute [Chosidow
2006]. The female scabies mite tunnels into the top layer of the skin
where she lays 10-25 eggs in burrows that are about 1 cm long [Leone
2007]. Two to three days later, larvae emerge from the burrows, dig new
burrows, mature, and mate. This cycle repeats every two weeks [Johnston
and Sladden 2005]. The total length of the life cycle is
30-60days [Leone 2007]. In most cases of scabies infestation, there are
only 5-15 (average of 12) mites on human skin at a time, except in
crusted scabies, where thousands to millions of mites can be present on
the skin.
WHAT ARE THE RISK FACTORS FOR DEVELOPING SCABIES?
Scabies
affects both sexes, all ages, all ethnic groups, and people of all
socioeconomic status. While anyone can get scabies, it is most prevalent
in areas of overcrowding, poor sanitation, and developing countries. Transmission
is by close personal contact (skin-to-skin) and, less commonly, through
fomites (clothing, bedding, etc.). The more mites that are present on
the skin (e.g., crusted versus classic scabies), the more likely that
the infestation will be transmitted to others through person-to-person
contact. Common routes of transmission are person-to-person contact with
infected family members or contacts at day care, schools, nursing
homes, hospitals, and long term care facilities. In adults, scabies is
often transmitted from person to person by sexual contact. There is no
evidence to date to suggest that scabies mites can transmit HIV
[Chosidow 2006]. Transmission of the mite through fomites is relatively
rare and usually results from contact with clothes and bedding
immediately after contamination by a highly infested person (e.g.,
crusted scabies).
What are the symptoms of scabies infestation?
Infestation
with the scabies mite causes a hypersensitivity reaction in the skin
which manifests as an intensely itchy rash. Patients infested with the
scabies mite typically complain of a sudden onset of severe pruritus
(itch). The statement that the itch is the worst itch a patient has ever
felt is common. The itch seems to affect the trunk and extremities,
with characteristic spreading to the face and neck. Although itching is
worse at night, this specific complaint does not help differentiate
scabies from other causes of an itchy rash, as most itchy dermatoses are
worse at night. The incubation period (time from infestation until
itching begins) is about 1 month, with a range of 2-6 weeks. In
patients who have been exposed to the mite before (prior episodes of
scabies), pruritus can begin within 48-72 hours after infestation.
There are four main clinical presentations of scabies infestation:
1. Classic scabies
2. Nodular scabies
3. Scabies in infants and the elderly
4. Norwegian or crusted scabies
Classic scabies
In
classic scabies, there is an average of 12 mites infesting the human
body at one time. Classic scabies presents with severe pruritus and
small (1-2mm) red bumps (papules), favoring the fingerwebs, wrists,
umbilicus (belly button), nipples, armpits, genitalia (penis, scrotum),
ankles, and toewebs . Close examination of the skin in these areas will
demonstrate burrows , tiny tortuous linear bumps on the skin, the tip of
which points to the mite. Sometimes, no burrows in the typical sites
are found, yet distribution of the itch and red papules suggest the
diagnosis. Itchy papules on the penis and scrotum in men and around the
nipples in women should be considered scabies until proven otherwise.
Other more rare presentations include hives, blisters, pustules, or no visible skin lesions (scabies incognito). An
eczema can accompany the lesions of scabies. In addition, if burrows
have been scratched or if the eczema is particularly severe, the burrows
might not be visible on examination of the skin. It is also important
to know that bacterial infection (impetigo) of scratched lesions or the
accompanying eczema can occur. Signs of bacterial infection include
weeping lesions with a honey-colored crust, often on a base of red skin.
Nodular scabies
Nodular
scabies presents with itchy, 4-5mm firm, red nodules . Areas that are
typically affected include the scrotum, penis, and armpits. These
lesions and their associated pruritus can persist for one year, even
after appropriate therapy.
Scabies in infants and the elderly
Scabies
is in infants and the elderly, especially nursing home patients, may
present differently from classical scabies. Lesions may be more
widespread and the head, scalp, face, palms, and soles may be involved.
Lesions may be larger than in classic scabies and not necessarily
accentuated in the typical locations. In infants and young children,
small blisters and pustules are the most common lesions seen (as
compared to pink papules as seen in adults). In young babies, pinkish
brown nodules are commonly seen [Johnston and Sladden 2005]. In
addition, the symptom of itch, which is one of the hallmark features of
classic scabies, may be hard to determine in these populations.
Norwegian or crusted scabies
Crusted
scabies (the name given to describe the condition in which millions of
scabies mites infest the skin) most commonly affects patients in
institutions (nursing homes), patients with immunosuppression (systemic
steroids, organ transplant recipients, AIDS, HTLV-1 infection,
hematologic malignancy), and patients with severe neurologic disorders
or developmental delay (e.g., Down’s syndrome). Lesions are very
different than those of classic scabies. Lesions are marked by
thickening of the skin topped with thick, white scale. The scale of
crusted scabies resembles fine white sand stuck on the skin with egg
white . Typical locations include the trunk, elbows, knees, penis, and
soles. Patients may or may not experience itch as a symptom. As
compared to classic scabies where an average of only 12 mites are
present on the human body at one time, in crusted scabies, thousands to
millions of mites are present in the scaly lesions. Because there are so
many mites on the skin, crusted scabies should be considered highly
infectious.
HOW IS THE diagnosis OF scabies made?
The
diagnosis of scabies is made by history (time of onset of symptoms,
location of itch, affected close contacts), physical examination
(typical skin lesions and areas of involvement), and confirmation by a
skin scraping. Although similar symptoms beginning the same time in
family members strongly suggests the diagnosis, lack of itching in close
contacts does not exclude the diagnosis of scabies. Because the Sarcoptes scabiei mite
is too small to be seen by the unaided human eye, doctors can perform a
skin scraping (called a “scabies preparation” by dermatologists) and
look for the mite, its eggs, or its feces (called scybala) under the
microscope. The scabies mite is very easy to find in
crusted scabies due to the number of mites causing the infestation, but
can be hard to find in cases of classic scabies where there is an
average of only 12 mites on the body at one time. Therefore, while a
positive “scabies prep” definitively makes the diagnosis of scabies, a
negative prep does not necessarily rule it out, especially if the
symptoms and location of lesions highly suggest the diagnosis. It
is also important to note that the recurrence of symptoms may not mean
that the diagnosis of scabies was incorrect, as treatment might have
been incomplete or reinfestation through contact with an infested family
member may have occurred [Johnston and Sladden 2005].
HOW IS SCABIES TREATED?
The
treatment of scabies requires both eliminating the mite from the skin
and treating the symptom of itch. In addition, close contacts of a
confirmed case (family members, roommates, sexual partners, etc.)
require treatment at the same time, whether or not they have symptoms in
order to prevent reinfestation or spread to others. Since the scabies
mite can live without human contact for up to 3 days, the morning after a
topical or oral treatment, treated persons are advised to wash all
bedding, towels, and clothes used or worn in the last 48 to 72 hours, in
hot water (60°F) and dry on high heat. Items that cannot be washed
and/or dried can be dry cleaned or quarantined in a tied-off plastic bag
for at least 72 hours. Many doctors recommend repeated topical or oral
therapy as outlined below. The same regimen regarding washing/drying of
clothing, towels, and bedding is recommended the morning following each
treatment. Of note, the treatments outlined below are for
classic scabies. The treatment of crusted scabies requires repeated
topical and oral therapy, as well as topical agents that remove the
excess scale.
Treatments commonly used in the United States to eliminate the scabies mite:
Permethrin 5% cream
is approved by the US Food and Drug Administration (FDA) for the
treatment of scabies. It is 95% effective after one dose and is more
effective than lindane (Strong and Johnstone 2007). In adults and
children over the age of 2 years, it is applied from the neck down and
left on the skin for 8-14 hours (usually overnight). In
infants and the elderly, permethrin should also be applied to the scalp,
face, and neck. Permethrin is then washed off in the morning. Patients
should make sure to apply the medication all over the body, including
the armpits, navel, underneath nails, between fingers and toes, in the
groin, and in the buttocks, as failure to treat these areas can lead to
treatment failure and recurrence of symptoms. In the
morning, the cream is washed off and treated persons are advised to wash
all bedding, towels, and clothes as described above. It
is recommended that this treatment be repeated in one week. Patients and
their close contacts may require multiple treatments, especially if
reinfestation is suspected.
Lindane 1% cream is approved by the FDA for the treatment of scabies. It is applied similarly to permethrin. This
agent, although effective, is considered second- or third-line therapy
as there is the potential risk of toxicities to the bone marrow
(aplastic anemia) and the nervous system (seizures) [Chosidow 2006;
Leone 2007]. Lindane is not recommended for pregnant or
breastfeeding women or children younger than 2 years. Its use should be
reserved for patients in whom all other therapy has failed.
Precipitated sulfur,
although not approved by the FDA for the treatment of scabies, is used
commonly for children under the age of 2 and pregnant or breast feeding
women. It is compounded as a 3-6% lotion or 5%, 10%, or 40% in
petrolatum. It is applied every 24 hours (with bathing in between
applications) for 3 days.
Ivermectin
is an oral medication that has been used extensively for the treatment
of tropical parasitic infections, but is not FDA approved for the
treatment of scabies. Ivermectin has been shown to be 70-74% effective
after one dose and 95% effective after two doses for the treatment of
scabies (Chosidow 2006). It is often used with topical therapy such as
permethrin, but on occasion it is used alone. Ivermectin is especially
useful when the disease is more widespread (involves a great deal of the
body), a patient is not able to tolerate or comply with topical
treatment, a patient has generalized eczema in addition to or due to the
scabies infection, or the patient is in an institution (nursing home,
hospital) where there is high risk for transmission to other close
contacts. The typical dose of ivermectin is 200µg/kg orally given twice,
two weeks apart. The two doses are recommended based on the facts that
the life cycle of a mite is about two weeks, ivermectin does not kill
eggs, and because ivermectin for scabies has been shown to be more
effective after two doses, as noted above [Chosidow 2006]. Ivermectin
should not be used in children weighing less than 15kg or in women who
are pregnant or breastfeeding [Chosidow 2006].
Other, less commonly used topical therapies include crotamiton, benzyl benzoate, and malathion [Leone 2007].
Scabies treatment in special cases:
Infants:
· 6% precipitated sulfur in petrolatum x 24 hours x 3d
· Permethrin 5% cream if age > 2 months
Children:
· Permethrin 5% cream
Pregnant and breastfeeding women:
· 6% precipitated sulfur in petrolatum x 24 hours x 3d
· Ivermectin, permethrin, and lindane contraindicated
Nodular scabies:
· Antiscabetics followed by steroid injections (given by a licensed physician)
Crusted scabies:
· Oral ivermectin
· Keratolytic agent (5%–10% salicylic acid in petrolatum)
· Nails are cut short and brushed with a scabicidal agent
· Average treatment is three weeks
Treatments addressing the symptom of itch:
Most
patients with scabies require treatment for the itch that accompanies
the infestation. The most common agents used are moisturizers, topical
corticosteroids, and oral antihistamines. These should be used liberally
to provide relief from the itch.
WHAT CAN I EXPECT AFTER TREATMENT?
More
often than not, scabies is cured by the treatments outlined above.
Itching usually begins to improve soon after treatment. Of note, even
though the mite has been eliminated and is no longer living on human
skin, itching may persist for three to six weeks, with gradual
improvement noted by three weeks. Nodules on genitals and
in the armpits may persist for up to six months, but respond to strong
steroid ointments and steroid injections into the lesions themselves.
The latter should only be performed by a physician, preferably a
dermatologist, in order to avoid unwanted side effects.
LINKS TO SELECTED REFERENCES
Strong M and Johnstone PW. Interventions for treating scabies. Cochrane Database of Systematic Reviews 2007 Jul 18;(3):CD0003203
(Nothing in this Knol should be construed as individual medical advice. Patients should consult with their own physician regarding the diagnosis and treatment of scabies. Not
all of the medications discussed are FDA-approved for the treatment of
scabies, and some side-effects and contraindications have not been
listed.)