The common name “crabs” refers to an infestation by the parasitic insect Pthirus pubis, formally known as pediculosis pubis. This ectoparasite feeds on human blood. While often associated with sexually transmitted infections, it is fundamentally a parasitic infestation that requires direct human-to-human contact for transmission.
Identification and Lifecycle of Pubic Lice
Pthirus pubis is a flat, gray-white insect, measuring 1.5 to 2.0 millimeters in length. The insect’s appearance is distinctively crab-like, featuring three pairs of legs, with the second and third pairs possessing large, pincer-like claws adapted to grasping coarse hair shafts. This unique morphology is what gives the parasite its widespread nickname.
These lice prefer areas of the body with coarse, widely spaced hair, making the pubic region their primary habitat. They can also infest other areas with similar hair characteristics, such as the armpits, chest, abdomen, thighs, beard, or even the eyebrows and eyelashes, but they rarely inhabit the fine hair of the scalp. The lifecycle of the pubic louse consists of three stages: the nit, the nymph, and the adult louse.
Nits, the eggs, are laid and cemented firmly to the base of the hair shaft close to the skin. These eggs typically hatch into nymphs after six to eight days. The nymph is an immature louse that must feed on blood to develop through three molting stages. The entire process from nit to a sexually mature adult takes about 22 to 27 days, and an adult louse will generally live for three to four weeks on the human host.
Transmission Routes and Risk Factors
The primary method of acquiring pubic lice is through close, prolonged skin-to-skin contact with an infested individual. Since the lice cannot jump or fly, this direct physical proximity is necessary for the parasites to transfer from one host’s hair to another. Consequently, transmission most frequently occurs during sexual activity, which involves extended periods of intimate contact between the pubic regions.
Any close physical contact that allows for the transfer of hair from one person to another creates a transmission risk, even in the absence of sexual intercourse. The risk of non-sexual transmission is low because adult lice cannot survive for more than 24 to 48 hours when separated from a human host and its blood meal. Contracting the lice from inanimate objects like shared bedding, towels, or clothing is possible but uncommon.
Risk factors for infestation include having sexual contact with an infected person or having multiple sexual partners. The presence of pubic lice is not an indication of poor hygiene, as the parasites are attracted to hair, not unwashed skin. Condoms offer protection against many sexually transmitted infections, but they do not prevent the skin-to-skin contact necessary for the transfer of pubic lice.
Recognizing the Signs and Confirmation
The most common symptom is intense, persistent itching, also known as pruritus, in the affected hairy areas. This itching sensation is an allergic reaction to the saliva the louse injects into the skin while feeding on blood. The discomfort often becomes more noticeable at night when the parasites tend to be more active.
Visual examination can reveal signs of an infestation, though the lice themselves are difficult to spot due to their small size and translucent color. Finding rust-colored specks on undergarments, which are the louse droppings, is a common indicator. The nits, or eggs, appear as specks glued firmly to the hair shafts near the skin.
In some cases, persistent feeding can result in the appearance of pale bluish-gray spots on the skin of the lower abdomen or thighs. Diagnosis is typically confirmed by a healthcare provider who uses a magnifying device to identify the characteristic adult lice, nymphs, or firmly attached nits.
Eradication and Follow-Up Care
Treatment involves using medicated products designed to kill the parasites and their eggs. Over-the-counter options, such as lotions containing 1% permethrin or mousse formulations with pyrethrins and piperonyl butoxide, are the standard first line of defense. These topical treatments must be applied according to the package directions to the entire affected area and left on for the recommended amount of time.
Since the initial treatment may not kill all the nits, a second application is recommended nine to ten days later to eliminate any newly hatched nymphs before they can mature and lay more eggs. If the infestation is resistant or if lice are found in the eyelashes, a healthcare provider may prescribe stronger medications, such as malathion lotion or oral ivermectin. For louse infestations of the eyelashes, petroleum jelly or a specific prescription ointment is often used, as standard lice treatments should never be applied near the eyes.
Proper environmental cleaning is necessary to prevent re-infestation from any lice that may have fallen off the host. All clothing, bedding, and towels used during the two to three days before treatment must be machine-washed in hot water at a temperature of at least 130 degrees Fahrenheit and then dried on a high heat setting. Items that cannot be washed can be dry-cleaned or sealed in a plastic bag for two weeks, which is long enough for any remaining lice to die without a blood source.
Eradication requires informing all sexual partners from the previous month that they are at risk and need simultaneous examination and treatment. Treating partners at the same time is necessary to break the cycle of transmission and prevent re-infestation. Avoiding sexual contact until both the person and all partners have completed treatment helps ensure the infestation is resolved.