Pthirus pubis the crab louse pubic louse

The crab louse or pubic louse, Pthirus pubis, is rounder, that is, shorter (1-2 mm) and broader than the two, more oval Pediculus species (Figure 23.4). In the crab louse the first pair of legs is much more slender than the second and third pairs. In the microscope the crab louse somewhat resembles a 'miniature crab'. Since they are so small, they are difficult to find even with a magnifying glass. In contrast to the clothing louse, the pubic louse including its eggs only occur on the hairy parts of the body. Pthirus pubis mainly occurs on hairs in the pubic

Figure 23.4 The public louse Pthirus pubis. Natural size 1-2 mm. (Photo: T. G.

Figure 23.4 The public louse Pthirus pubis. Natural size 1-2 mm. (Photo: T. G.

and perianal regions, and less frequently in the axillae, eyebrows, and beard. Since P. pubis prefers thick hairs, infestations of children are relatively rare, but if P. pubis are found on children it is usually on their eyebrows. However, lice on the eyebrows are often proved to be head lice. The pubic louse is a parasite of humans in all parts of the world. The prevalence of P. pubis appears to have been increasing in the human population in recent years (Kettle 1995). Pthiriasis or pediculosis pubis, as an infestation with P. pubis is denoted, is claimed to be the most contagious of the sexually transmitted diseases (Felman and Nikitas, cited in Kettle 1995). It is likely that most pubic louse infestations are not diagnosed by physicians. In other words, pthiriasis is most likely quite underdiagnosed and may be nearly as prevalent as head louse infestations.

A 'normal-sized' pubic louse population on a single infested person is about 10 lice. The pubic louse feeds more or less continuously during the day and night. Like the Pediculus species, the pubic louse is strongly dependent on its host and will die within 24 h if it is dislodged from the host. There is no indication that P. pubis, under natural conditions, transmits any human-pathogenic microorganism including viruses. The main physical symptom of pthiriasis is itching which, however, is relatively benign. The psychological suffering of pthiriasis may be more severe. Since the lice, during blood-feeding, injects an enzyme (which transforms haemoglobin to biliverdin) with the saliva, bluish spots can sometimes be seen in the skin. The faeces of pubic lice can be seen as a brownish powder on the skin or in the underclothes.

Like the other two species of human-parasitizing lice, the pubic louse is transmitted from one person to another by bodily contact, often by sexual activity or simply by sharing a bed with an infested person. It should be emphasized that transfer of pubic lice can occur by ordinary (non-sexual) bodily contact. However, transmission of P. pubis via toilet seats, towels, etc. is presumably very uncommon.

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