Ectoparasites are parasitic organisms that live on the outer skin surfaces of humans and other animals (ecto means "outer"). Two relatively common STIs are caused by ectoparasites: pubic lice and scabies.

Pubic Lice

Pubic lice, more commonly called crabs, belong to a group of parasitic insects called bit­ing lice. They are known technically as Phthirus pubis. Although tiny, adult lice are visible to the naked eye. They are yellowish-gray and under magnification resemble crabs, as I Figure 15.6 shows. A pubic louse (the singular of lice) generally grips a pubic hair with its claws and sticks its head into the skin, where it feeds on blood from tiny blood vessels.

Incidence and Transmission

Pubic lice are quite common and are seen frequently in public health clinics and by pri­vate physicians. Pubic lice are especially prevalent among young (15- to 25-year-old) single people and are frequently associated with the presence of other sexually transmitted infec­tions. Pubic lice are often transmitted during sexual contact when two people bring their pubic areas together (Centers for Disease Control, 2009b). The lice can live away from the body for 1 to 2 days, particularly if their stomachs are full of blood. They may drop off onto underclothes, bedsheets, sleeping bags, and so forth. Eggs deposited by the female louse on clothing or bedsheets can survive for several days. Thus it is possible to get pubic lice by sleeping in someone else’s bed or by wearing his or her clothes. Furthermore, a successfully treated person can be reinfected by being exposed to her or his own unwashed sheets or underclothes. Pubic lice do not necessarily limit themselves to the genital areas. They can be transmitted, usually by the fingers, to the armpits or scalp.


Most people begin to suspect something is amiss when they start itching. Suspi­cions become stronger when scratching brings no relief. However, a few people seem to have great tolerance for the bite of a louse, experiencing little if any dis­comfort. Self-diagnosis is possible simply by locating a louse on a pubic hair.


Both prescription and over-the-counter medications (lotions and creams) are available for treatment of pubic lice (Centers for Disease Control, 2009b). These

medications should be applied to all affected areas and washed off after a few minutes. It is advisable to apply the lotion or cream to all areas where there are concentrations of body hair—the genitals, armpits, scalp, and even eyebrows. These treatments should be repeated 7 to 10 days later if lice are still present. Be sure to wash all clothes and sheets that were used before treatment.