A louse is a small, wingless insect and an obligate parasite, meaning it depends on a host to live by feeding on its blood. A louse spends nearly its entire life on a single host, where it feeds and reproduces. While there are thousands of species of lice, only a few have adapted to live on humans.
Types of Human Lice
Three types of lice infest humans, each adapted to a specific part of the body. The most common is the head louse, Pediculus humanus capitis, which is found on the scalp and hair. These lice are particularly prevalent among children aged 3 to 11, with an estimated 6 to 12 million infestations occurring annually in this demographic.
A second type is the body louse, Pediculus humanus humanus, which lives and lays its eggs in the seams of clothing and bedding rather than directly on the body. These lice move onto the skin to feed on blood multiple times a day. Unlike other human lice, body lice are known vectors for diseases such as typhus. Infestations are often associated with crowded living conditions where regular changes of clothing are not possible.
The third type, the pubic louse, Pthirus pubis, is a distinct species with a crab-like appearance. These lice are found in pubic hair but can also inhabit other coarse body hair, such as eyebrows, eyelashes, and beards. Their unique shape is an adaptation for clinging to thicker hair strands.
The Louse Life Cycle and Detection
The louse life cycle has three stages: nit, nymph, and adult. The cycle begins with the egg, known as a nit, which the female louse cements to a hair shaft close to the scalp. Nits are tiny, oval-shaped, and can range in color from white to brown, hatching in approximately 6 to 9 days. Hatched nits appear pale or translucent, while unhatched, living eggs are darker.
Once the egg hatches, a baby louse, or nymph, emerges. A nymph resembles a miniature adult louse and is about the size of a pinhead. To survive and grow, it must feed on blood daily. The nymph stage involves three molts, or shedding of its skin, over about 9 to 12 days before it matures into an adult.
The final stage is the adult louse, which lives for approximately three to four weeks. Adult females, which are slightly larger than males, can begin to reproduce almost immediately after their final molt. A single female can lay between three and eight eggs per day for about 16 days, starting the cycle anew. Detection of an infestation involves finding nits attached to hair shafts, often within 6 millimeters of the scalp, or spotting crawling nymphs and adults.
Transmission and Prevention
Lice are most commonly spread through direct head-to-head or body-to-body contact. This method of transmission is particularly efficient for head lice among young children who play closely together. While they can’t fly or hop, they are adept at moving between hosts in close proximity.
Infestations can also occur through indirect contact with personal items, although this is less frequent. Sharing objects such as hats, combs, brushes, towels, or bedding can facilitate the transfer of lice from one person to another. Lice can survive for a limited time away from a human host, typically no more than 24 hours, as they are separated from their food source.
Preventing the spread of lice involves avoiding direct head-to-head contact, especially during school or daycare outbreaks. It is also advisable not to share personal grooming items, clothing, or headwear. Regularly washing linens, towels, and clothing in hot water reaching at least 130°F (54°C) can help prevent the spread within a household. For items that cannot be washed, dry cleaning or sealing them in a plastic bag for at least 72 hours will ensure any stray lice die.
Eradication Methods
For the person, over-the-counter medicated shampoos containing ingredients like pyrethrins are a common first step. Some lice populations have developed resistance to certain chemicals. If these treatments are not effective, a doctor may prescribe stronger medications such as malathion or ivermectin.
Manual removal is another component of treatment. Using a fine-toothed nit comb to meticulously go through the hair can help remove nits, nymphs, and adult lice. This process is often done in conjunction with medicated treatments. For children under two months old, medicated treatments are not recommended, and alternative methods should be discussed with a healthcare provider.
Cleaning the environment is necessary to prevent reinfestation.
- Wash all bedding, clothing, and towels that have been in recent contact with the infested person in hot water and dry on a high heat setting for at least 20 minutes.
- Place items that cannot be washed, like stuffed animals, in a dryer on high heat or seal them in a plastic bag for two weeks.
- Soak combs and brushes in hot water or rubbing alcohol.
- Vacuum carpets and furniture thoroughly.