Pediculosis pubis is caused by the crab louse, Pthirus pubis. The lice are 0.8 to 1.2 mm long and difficult to see. The egg form, the nit, adheres to hair and clothing. Transmission occurs both by sexual contact and less frequently by fomites, such as clothing and towels. Pruritis due to hypersensitivity is typically the primary clinical manifestation. Small blue macules (maculae ceruleae) may also develop secondary to louse anticoagulant saliva injection during feeding. Patients may also notice small blood stains on the undergarments secondary to bleeding from louse bites.
Management and Disposition
Diagnosis is made by demonstration of lice or nits. This can be done either visually or by microscopic examination if the diagnosis is uncertain. Treat with topical permethrin 1% cream or pyrethrins 0.33% with piperonyl butoxide 4%. Either agent should be washed off after 10 minutes. Nits should be removed with a comb or tweezers. Repeat treatment in 1 week should be considered but is not uniformly recommended. Alternative regimens include topical malathion 0.5% lotion washed off after 8 to 12 hours or oral ivermectin 250 µg/kg once weekly for 2 weeks. Advise patients to wash all clothing and linens used in the preceding 24 hours and to avoid sexual contact until cured. All sexual partners should be screened.
Pthirus pubis—On Pubic Hair. Pthirus pubis, or the crab louse, in the pubic hair (pediculosis pubis) of a patient complaining of itching. Note the nits attached to the hair. (Reproduced with permission from Morse SA, Holmes KK, Ballard RC, Moreland AA, eds. Atlas of Sexually Transmitted Diseases and AIDS. 4th ed. Saunders Elsevier; 2010. Copyright © 2010 Elsevier, Ltd.)
Nits are easier to find on examination than are mature lice; the average number of lice in an infestation is only 10.
Although an effective treatment, lindane is no longer recommended as first-line therapy due to concerns regarding toxicity.
Evaluate patients with pediculosis pubis for other STIs.
Pthirus pubis—On Eyelashes. Pthirus pubis lice noted in the eyelashes. (Reproduced with permission from Spalton DJ, Hitchings RA, Hunter PA, eds. Atlas of Clinical Ophthalmology, 2nd ed. London, United Kingdom: Mosby-Wolfe Ltd; 1994. Copyright © Elsevier.)