Issue No. 211
Howard V. Weems, Jr.
February, 1980
Introduction
Sucking lice are small wingless external parasites that feed on blood. Three types of sucking lice infest humans: the body louse (Pediculus humanus humanus Linnaeus, (fig. 1) also known as Pediculus humanus corporis), the head louse (Pediculus humanus capitis De Geer), and the crab louse or pubic louse (Pthirus pubis (Linnaeus)) (fig. 2). The head louse and the body louse are morphologically indistinguishable but are easily distinguished from the crab louse. The crab louse usually infests the hairs of the pubic and perineal regions but may move to the armpits, beard, or mustache. It occurs rarely on the eyelids and in a few instances has been found in all stages on the scalp of unusually hairy individuals. It is relatively immobile when on the host, remaining attached and feeding for hours or days on one spot without removing its mouth parts from the skin. Although they are irritating pests, crab lice are not known to be vectors of human diseases, whereas body lice and head lice are known to be vectors of at least 3 human diseases: epidemic or louseborne typhus, caused by Rickettsia prowazeki de Rocha-Lima; trench fever, caused by Rochalimaea quintana (Schmincke) Krieg (long known as Rickettsia quintana); and louse-borne relapsing fever, caused by Borrellia recurrentis (Lebert) Bergy et al. (PAHO, 1973). Crab lice most commonly inhabit adults and are not found on children prior to puberty. Infestation with crab lice is said to result most often from contact during coitus. As with body lice and head lice, but less so with crab lice, transmission may occur from crowding of infested clothing with uninfested clothing in locker rooms and gymnasiums, by sleeping in infested beds, or from contact with badly infested persons in a crowd. Pubic lice tend to remain on their hosts throughout their lives unless dislodged, taken off with clothing, or controlled.
Little is known about the incidence of infestation with Pthirus in a human community, but generally it seems to be much lower than with Pediculus. Humans differ in their sensitivity to the bite of Pthirus. To most it causes less irritation than that of Pediculus humanus, but some experience severe pruritus. The consequent scratching produces a localized eczematous condition of the pubic or axillary regions. “Blue spots” which may result from the bite of the crab louse are 0.2-3.0 cm in diameter, with an irregular outline, are painless, do not disappear on pressure, and appear to be in the deeper tissues. They appear some hours after the crab louse has bitten and last for several days (Buxton, 1947). This bluish-gray discoloration of the skin is due to a poisonous saliva injected by the crab louse, similar to the melanoderma caused by the body louse (Riley and Johannsen, 1938).