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Ectoparasites are organisms that are found either on the skin or only in the superficial layers of the skin. Ecto is a prefix meaning “outer.” Virtually all ectoparasites are arthropods; that is, they are invertebrates with a chitinous exoskeleton.

The ectoparasites that cause human disease fall into two main categories: insects (six-legged arthropods) and arachnids (eight-legged arthropods). The ectoparasites discussed in this chapter include insects such as lice, flies, and bedbugs and arachnids such as mites, ticks, and spiders.

Many arthropods are vectors that transmit the organisms that cause important infectious diseases. A well-known example is the Ixodes tick that transmits Borrelia burgdorferi, the cause of Lyme disease. Table XII–3 describes the medically important vectors. However, in this chapter, the arthropods are discussed not as vectors, but as the cause of the disease itself. Table 69–1 summarizes the common features of diseases caused by the medically important ectoparasites that are described in this chapter. Ectoparasites of minor medical importance are briefly described at the end of the chapter.

TABLE 69–1Important Ectoparasites That Cause Human Disease


1. Lice


Pediculosis is caused by two species of lice: Pediculus humanus and Phthirus pubis. P. humanus has two subspecies: P. humanus capitus (head louse), which primarily affects the scalp, and P. humanus corporis (body louse), which primarily affects the trunk. P. pubis (pubic louse) primarily affects the genital area, but the axilla and eyebrows can be involved as well.

Note that the body louse is the vector for several human pathogens, notably Rickettsia prowazekii, the cause of epidemic typhus, whereas the head louse and the pubic louse are not vectors of human disease.

Important Properties

Lice are easily visible, being roughly 2 to 4 mm long. They have six legs armed with claws by which they attach to the hair and skin (Figure 69–1). Pediculus has an elongated body, whereas Phthirus has a short body and resembles a crab, and hence its nickname, the crab louse ...

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