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[Pthirus pubis]

Adult of P. humanus.

Adult of Pthirus pubis.

Causal Agent

Pthirus pubis, the pubic or crab louse, is an insect of the order Psocodea and is an ectoparasite whose only host are humans.

Life Cycle

Life cycle of pthiriasis.

Pubic lice (Pthirus pubis) have three stages: egg, nymph and adult. Eggs (nits) are laid on a hair shaft The number 1. Females will lay approximately 30 eggs during their 3-4 week life span. Eggs hatch after about a week and become nymphs, which look like smaller versions of the adults. The nymphs undergo three molts (The number 2, The number 3, The number 4) before becoming adults The number 5. Adults are 1.5-2.0 mm long and flattened. They are much broader in comparison to head and body lice. Adults are found only on the human host and require human blood to survive. If adults are forced off the host, they will die within 24-48 hours without a blood feeding. Pubic lice are transmitted from person to person most-commonly via sexual contact, although fomites (bedding, clothing) may play a minor role in their transmission.

Geographic Distribution

Pthiriais is common and is distributed worldwide.

Clinical Presentation

The majority of infestations are asymptomatic. When symptoms are noted they may include a tickling feeling of something moving in the hair, itching, caused by the an allergic reaction to louse saliva, and irritability. Pruritis caused by the bites of pubic lice may be very intense, and discoloration of the skin may occur if the infestation is left untreated for an extended length of time.

Modes of Transmission:

The main mode of transmission is person to person by sexual contact with someone who is already infested. Fomites (bedding, clothing) may play a minor role, although rare.

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  • Page last reviewed February 15, 2016
  • Page last updated February 17, 2016
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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