Pubic lice, commonly known as “crabs,” are not caused by a virus or a bacterium, which is a common misconception. This condition is instead a parasitic infestation caused by the specific insect Pthirus pubis. This tiny, six-legged creature requires a human host to survive and reproduce. Understanding this classification is essential for proper identification and treatment, as the required medical approach differs entirely from that used for viral or bacterial infections.
The Cause: A Parasitic Infestation
The organism Pthirus pubis is classified as an ectoparasite, meaning it lives on the exterior of the host. It belongs to the class Insecta, making it an arthropod. Unlike microscopic viruses or bacteria, the pubic louse is a visible insect that feeds exclusively on human blood to sustain its life cycle.
The life cycle, known as pediculosis pubis, involves three distinct stages: the egg (nit), the nymph, and the adult louse. A female louse lays nits firmly attached to the hair shaft near the skin, which typically hatch after six to eight days. Nymphs resemble smaller versions of the adult louse and must feed on blood to mature, reaching adulthood in about two to three weeks. Adult lice measure about 1.5 to 2.0 millimeters long and are characterized by their broad, flattened, crab-like appearance.
Recognizing Signs and Transmission
The presence of pubic lice is most commonly signaled by intense itching in the genital or perianal area. This itching is an allergic reaction to the saliva injected by the louse during a blood meal and often becomes more pronounced at night when the parasites are more active. Small, pale blue spots, known as maculae ceruleae, may also appear on the skin where the lice have been feeding continuously.
A visual inspection may reveal tiny, tan to grayish-white adult lice crawling on the skin or firm, oval-shaped nits glued to the base of the pubic hairs. Transmission of Pthirus pubis occurs overwhelmingly through close, prolonged physical contact between people. Sexual contact is the most common route for spread among adults, though lice can occasionally spread through contact with infested items like shared towels, bedding, or clothing.
Targeted Treatment and Eradication
Eradicating a pubic lice infestation requires a targeted approach using pediculicides, which are specialized insect-killing agents. Over-the-counter treatments containing permethrin 1% or pyrethrins combined with piperonyl butoxide are the first line of defense. These topical treatments should be applied precisely according to the package instructions to kill the adult lice. A second application is often recommended nine to ten days later to eliminate any newly hatched nymphs.
For cases that do not resolve with over-the-counter options, a healthcare provider may prescribe stronger treatments, such as malathion lotion or oral ivermectin. It is important to treat all affected areas of coarse hair, including the armpits, chest, thighs, or beard, as lice can migrate away from the treated pubic region. Shaving the hair is not an effective solution, as lice can still cling to the skin and hair stubble.
A crucial component of eradication involves cleaning the environment to prevent re-infestation. All clothing, towels, and bedding used in the two to three days before treatment must be washed in hot water (at least 130°F/54°C) and dried using the hot cycle. Items that cannot be washed should be dry-cleaned or sealed in a plastic bag for two weeks. To stop the cycle of transmission, all recent sexual partners must be informed and treated simultaneously. Sexual contact should be avoided until everyone has completed treatment and is confirmed lice-free.