The patient described in the vignette has symptoms and signs indicative of
infestation with Phthirus pubis,
the crab louse. The organism is transmitted almost exclusively by sexual
contact, although infestation may result from other forms of close personal
contact or the sharing of fomites (eg, clothing). Pubic lice feed on human blood
and although generally restricted to the genitalia, occasionally infest other
hair-bearing areas. Most patients who have pediculosis pubis report genital
pruritus or a sensation of something crawling on their skin. In addition to lice
or nits, examination reveals erythematous, crusted papules. Occasionally, small
blue-gray spots, known as maculae ceruleae, are observed on the lower abdomen
and thighs. These lesions, which may mimic bruises, are believed to represent
heme pigment that has been altered by the saliva of feeding lice.
Genital herpes simplex virus infection can be differentiated from pubic lice infestation by the presence of clustered vesicles or pustules on an erythematous base. Human papillomavirus infection appears as flat-topped or rough (verrucous) flesh-colored papules. In moist or occluded areas, papules may coalesce to form large cauliflower-like plaques. The lesions of molluscum contagiosum are translucent or white papules that, at first glance, may resemble vesicles or pustules. However, careful inspection and palpation reveals that they are solid. A central dimple or umbilication is the hallmark of molluscum lesions, although it may be absent in small papules. Scabies infestation produces erythematous papules and occasionally pustules located in flexural areas such as the interdigital webs, wrists, axillae, or at the waist. In males, the penis and scrotum may be affected; in this area, lesions may be nodular rather than papular. At times, S-shaped burrows can be seen at the site of mite invasion.
Clayton BD, Krowchuk DP. Skin findings and STDs. Contemp Pediatr. 1997;14:119-137
Weston WL, Lane AT, Morelli JG. Color Textbook of Pediatric Dermatology. 2nd ed. St Louis, Mo: Mosby, Inc; 1996:84-87, 90-92,126-127