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INTRODUCTION.
The common name comes from the intense itching caused by these mites burrowing in the skin
just below the surface, causing the skin condition commonly called scabies. Different
varieties of Sarcoptes scabiei (DeGeer) are considered biological races specific to
different mammal species and not easily transferable from one host to another. Scabies is
a medical condition and should be treated by a physician. It is covered here because its
symptoms may be confused with those caused by other mites and/or insects. These mites are
found worldwide, potentially wherever their human host is found.
RECOGNITION.
Adult small, female about 1/128-1/64" (0.33-0.45 mm) long, male about
1/128" (0.20-0.24 mm) long. Body broadly oval, almost round in shape, dorsoventrally
flattened (top to bottom) but dorsum somewhat hemispherical, not hard-shelled. Color
translucent, dirty white with more chitinized parts brownish. Dorsal and ventral surfaces
striate (covered with fine impressed lines/grooves), dorsal striae brokenlinterrupted by
strong, spinelike projections; posterior body hairs (setae) well developed, lanceolate
(spear-shaped). Dorsally with a pair of vertices hairs (setae) on anterior median margin
of propodosoma (plate just behind mouthparts and between lst pair of legs). Legs short,
posterior 2 pairs not visible from above; female with 1 st and 2nd pairs and male with
lst, 2nd, and 4th pairs ending in a long, stalked, suction pad, female with 3rd and 4th
and male with 3rd pair of legs ending in single, long, whiplike hair (seta).
SIMILAR GROUPS
(Females).
Manage mite (Notoedres cati) with dorsal striae
(fined impressed lines) not interrupted by spines but broken into a scalelike pattern,
anal opening distinctly dorsal, and not parasitic on humans but primarily on cats, dogs,
rabbits, rats, and other rodents.
Scab and manage mites (Psoroptidae: Psoroptes,
Chorioptes, Otodectes) with all legs relatively conspicuous, visible from above, and
parasitic primarily on domestic animals such as cattle, sheep, goats, horses, rabbits,
dogs, and cats, not humans.
Straw itch mite (Pyemotes ventricosus) with
elongate body, all legs long, distinctly visible from above, and posterior/rear enormously
distended in gravid females.
BIOLOGY.
Developmental stages include egg, larva, nymph, male adult, immature (2nd
nymph/deutonymph) and mature (adult) female. Larvae have only 3 pairs of legs. Mating
probably occurs on the skin surface and the change from immature to mature female occurs
after insemination. The inseminated female burrows into the horny epidermal layer of the
skin which requires about an hour and then never leaves her burrow. She lays eggs singly
at the rate of 1-3 per day over her reproductive period which is about 2 months, for a
total of about 40-50 eggs. She will extend her winding burrow 1/64-1/4" (0.5-5 mm)
per day, with it usually not exceeding 1/4-5/8" (5-15 mm) in length. Length of stages
in days for females is as follows: egg hatch 2.5-5, larva 1.5-3, lst nymph 1.5-4, 2nd
nymph 2-4, and from mating to I st egg laid 2. The life cycle (egg to egg) for females is
9.5-19 days, whereas males require 9-11 days; males have only 1 nymphal stage. Less than
10% of the eggs laid mature into adults; that's 90% mortality. Females die within their
burrow.
The human itch mites themselves are the problem.
Their burrowing causes severe itching. If the infestation occurs over a long period of
time or it is a reinfestation, an allergic reaction occurs consisting of intense itching
and redness or a rash over much of the body. The rash appears on many areas of the body
where the mites are not located. These mites are not vectors of any disease organisms.
HABITS.
Both sexes as well as larvae and nymphs tend to burrow into the skin. The larvae, nymphs,
and males usually make only shallow, temporary holes. Larvae and nymphs may burrow some
but they spend most of their time on the skin surface or in hair follicles. Males
sometimes burrow but spend most of their time on the skin surface looking for unfertilized
females. Egg-laying females make the largest and longest burrows, burrowing in the folds
of skin with a preference for the deeper furrows and cracks. Favorite sites include
wrists, hands (between the fingers, knuckles, and palms), elbows, buttocks, back of knee,
ankles, toes, penis and scrotum (males) and the underside of breasts (females); wrists and
hands account for about 60-70% of infested areas.
The first month of infestation is a period of
sensitization, during which people are often unaware they have a problem because there are
few or no symptoms. In about 45 days, the irritation becomes severe enough to cause some
loss of sleep. In about 100 days, the itching is practically continuous and almost
unbearable. Secondary infections from scratching can become a problem in themselves. In
cases of reinfestation, severe irritation usually develops within 24 hours.
Human itch mites are usually spread from person to person, such as
between people sharing the same bed. However, transmission is possible through dancing,
close personal contact, and normal intimate contact between members of a family.
Transmission via clothing and bedding used by an infested person almost never occurs.
Conditions conducive to transmission include close bodily contact under warm conditions;
these mites are essentially immobile at temperatures below 68'F (20'C).
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