STD Home > Causes of Pubic Lice

Pubic lice is directly caused by an infestation with a small parasitic insect called Phthirus pubis. While not direct pubic lice "causes," certain risk factors can increase a person's chances of becoming infested. For example, having multiple sex partners or sharing clothing or bedding with an infested person puts one at a higher risk for infestation.

What Causes Pubic Lice?

The primary pubic lice cause is an infestation with a specific parasitic insect known as Phthirus pubis. This parasite is more commonly known as pubic lice (or crab lice or simply "crabs") -- so both the condition and the parasite that causes it go by the same name.
When a person is infested with lice -- whether it is pubic lice, head lice, or body lice -- the condition is known as pediculosis. Pediculosis pubis refers specifically to pubic lice.

Understanding Pubic Lice

A pubic louse (pubic lice is plural) is a small, wingless insect. It has three pairs of legs located directly behind the head. The legs end in sharp claws that are designed for feeding and allow the louse to hold on tightly to hair or clothing.
Pubic lice feed one or more times a day. To feed, they pierce the skin with their sharp claws, injecting irritating saliva and sucking blood. They do not become engorged like ticks, but they do become rust-colored from the ingestion of blood. This rust color is an identifying characteristic of lice.
Nits become firmly cemented at the base of the hair shaft and close to the skin so that they stay warm. They are hard to remove from the hair shaft. The life cycle of pubic lice from egg to adult is approximately one month.
Lice found on the hair and head are not pubic lice; they are head lice. Head lice are caused by an infestation with another parasite known as Pediculus humanus capitis.
(Click Transmission of Pubic Lice to understand how it is spread.)
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Last reviewed by: Arthur Schoenstadt, MD
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