1-10. MITES
Mites are minute animals. Most species are barely visible to the naked eye.
They are found virtually worldwide. There are over 29,000 species of mites, but only a
few of them attack humans. They are medically important because certain species
transmit scrub typhus (southwest Pacific and the Orient). In attacking a person, various
species of mites cause conditions ranging from mi Id irritation to severe forms of
dermatitis.
a. The Human Itch Mite, Sarcoptes scabiei. This mite (see figure 1-25)
causes a disease of the skin known as scabies or the "seven year itch." The Iife-cycle
consists of egg, larva, nymph, and adult, with the female undergoing two nymphal
stages. The mature female burrows into a person's skin, particularly at the hands and
wrists, and other areas of the body where the skin creases or folds. As she burrows,
she deposits eggs at a rate of about two per day. Egg laying continues for about 4 to 5
weeks, and a single female lays an average of 40 to 50 eggs. Eggs generally hatch in 3
to 4 days, and the larvae migrate from the maternal burrow to form new burrows nearby.
After about 3 days, larvae molt in the burrows to become nymphs. Total development
time for females averages 14 to 17 days. After reaching maturity, mating occurs either
in the burrows of virgin females or on the surface of the skin after which the fertiIized
female repeats the process of burrowing and egg laying.
Figure 1-25. Adult human itch mite.
b. Chiggers (Trombiculid Mites). This is the common chigger or "red bug." Its
bite causes an intense itching and infection can result from scratching the bites. In the
Asiatic-Pacific area, certain species of this mite are vectors of scrub typhus. The Iife
cycle of the trombiculid mite is very complex. Females lay eggs singly in the soiI. In
about 2 weeks, the six-legged larva hatches and crawls into nearby vegetation or along
the ground in search of a host. Many species feed on a wide variety of vertebrate hosts
including reptiles, rodents, large mammals, and humans. The larva feeds at the surface
of the host skin, usually requiring 1 to 3 or more days for engorgement. Then the fully
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