A louse (plural: lice) is a small, wingless ectoparasite that feeds exclusively on human blood several times a day. An infestation is medically known as pediculosis. Louse infestations are common globally, affecting people of all ages, and are not reflective of poor hygiene or socioeconomic status.
Identifying the Different Types
Humans can be infested by three distinct types of louse, each characterized by its preferred habitat on the body. The head louse (Pediculus humanus capitis) is the most commonly encountered, residing in the hair and on the scalp. Adults are 2 to 3 millimeters long, roughly the size of a sesame seed, and are grayish-white or tan.
The body louse (Pediculus humanus humanus) is slightly larger, measuring 2.3 to 3.6 millimeters in length. This type primarily inhabits clothing, particularly along the seams, only moving onto the skin to feed. Body lice are known to transmit diseases such as typhus, which head lice do not.
The pubic louse (Pthirus pubis), informally called “crabs,” is morphologically distinct. It has a short, round body and appears crab-like under magnification. Pubic lice are the smallest of the three and prefer the coarse hair found in the pubic region, though they can also be found in armpit hair or eyelashes.
Understanding the Life Stages
The louse life cycle consists of three phases: the egg (nit), the nymph, and the adult. The entire cycle takes approximately 30 to 35 days, and understanding this progression is important because effective treatments must interrupt it.
The nit is the louse egg, laid by the female and cemented firmly to a hair shaft near the scalp. Nits are small, oval, yellowish or whitish specks that do not easily flake off like dandruff. They require the warmth of the scalp to survive and hatch into nymphs within 7 to 10 days.
Nymphs are the immature form of the louse, resembling a smaller adult. They begin feeding on blood immediately after hatching to support rapid growth. Over 7 to 12 days, the nymph molts three times, gradually increasing in size.
The final stage is the adult louse, which can live for up to 30 days on a host. Adult females are larger than males and can lay several eggs per day, ensuring the continuation of the infestation. Treatment protocols are designed to eliminate adults and newly hatched nymphs before they mature and reproduce.
Effective Eradication Strategies
A systematic approach to elimination involves combining chemical treatments, mechanical removal, and environmental cleaning.
Chemical Treatments
Many over-the-counter pediculicides contain insecticides, such as permethrin, that target adult lice and nymphs. Due to documented insecticide resistance, it is important to follow the product’s instructions exactly. Most chemical treatments do not reliably kill nits, requiring a second application after 7 to 10 days. This second treatment targets nymphs that have hatched since the first application, preventing them from maturing and laying new eggs. If live lice are still present 24 to 48 hours after the second treatment, a different product or a prescription option may be necessary.
Mechanical Removal
Mechanical removal, often called wet combing, is a non-chemical method using a specialized fine-toothed louse comb. Combing the hair thoroughly, often with conditioner, helps physically remove mobile lice and attached nits. Regular combing every few days for two weeks can effectively break the reproductive cycle.
Environmental Cleaning
Environmental cleaning focuses on items in close contact with the infested person to prevent re-infestation. Bedding, clothing, and towels used within 48 hours prior to treatment should be washed in hot water (at least 130°F) and dried on a high-heat cycle for at least 20 minutes. Combs and brushes should also be soaked in hot water for about 10 minutes to kill any remaining lice or nits.