**Head Lice** (*Pediculus humanus capitis*) are tiny, wingless, parasitic insects that live and feed exclusively on the human scalp and hair. Infestation, known as **pediculosis**, is a highly common public health concern, particularly among school-age children. Lice are not known to transmit systemic diseases but cause intense itching, secondary infections from scratching, and significant social and psychological distress. Infestation is primarily spread through direct head-to-head contact.
Taxonomy and Classification
Head Lice belong to the order Psocodea (formerly Phthiraptera), family Pediculidae. They undergo simple metamorphosis (egg, nymph, adult). Head Lice are specific to humans and are distinct from Body Lice and Pubic Lice (crabs), which infest different parts of the body. They rely entirely on human blood for survival and die quickly when separated from a host.
Physical Description
Adult Head Lice are very small, about the size of a sesame seed, measuring $2$ to $3$ mm long.
- **Appearance:** Translucent or grayish-white, often turning reddish-brown after feeding on blood. They have six legs, each ending in a specialized claw used to grasp hair shafts tightly.
- **Nits (Eggs):** The eggs are called **nits**. They are tiny (about $1$ mm), oval, and firmly cemented to the hair shaft close to the scalp. They are typically white or yellowish and can resemble dandruff, but nits cannot be easily flicked off the hair.
- **Nymphs:** Newly hatched lice are miniature versions of the adults, requiring blood meals to mature.
- **Damage Sign:** Intense itching (**pruritus**) of the scalp, small red bumps from scratching, and the visible presence of nits firmly attached to the hair shaft, especially behind the ears and at the nape of the neck.
Distribution and Habitat
Head Lice are found globally and in all socioeconomic groups. Their habitat is exclusively the hair and scalp of the human host. They require the warmth and consistent blood supply of the scalp to thrive. They are spread by direct contact, and much less commonly by sharing items like combs, brushes, hats, or scarves, as the lice cannot jump or fly.
Behavior and Conflict
Lice move by crawling and feed several times a day by piercing the scalp with their mouthparts.
- **Reproduction:** Females can lay up to $10$ eggs per day. Nits hatch in about 7–10 days, and the nymphs mature into adults in another 7–10 days, allowing the infestation to grow rapidly if untreated.
- **Itching:** The intense itching is an allergic reaction to the lice’s saliva injected during feeding. Persistent scratching can lead to bacterial skin infections (impetigo).
- **Stigma:** While scientifically harmless, head lice infestation carries a significant social stigma, leading to embarrassment and missed school days.
Management and Prevention
Control involves a combination of chemical treatment and mechanical removal.
- Use over-the-counter pediculicides (shampoos, rinses) containing ingredients like pyrethrin or permethrin. **Resistance** to these chemicals is increasingly common, requiring repeat treatments or the use of prescription alternatives.
- **Always follow directions** precisely, including the recommended waiting time and repeat treatment schedule ($7$ to $10$ days later) to kill newly hatched nits.
- Thoroughly comb the hair with a specialized **fine-toothed nit comb** daily for two weeks after treatment to remove all nits and newly hatched nymphs. This step is essential for success.
- Wash bedding, clothes, and towels used by the infested person in hot water ($130^\circ\text{F}$ or higher) and dry on high heat.
- Soak combs and brushes in hot water ($130^\circ\text{F}$) for at least 10 minutes or discard them.
- Items that cannot be washed (stuffed animals, helmets) can be placed in a sealed plastic bag for two weeks.
Conservation and Research
Head Lice are managed as a public health nuisance. Research focuses on understanding the genetics of insecticide resistance (super-lice) and developing non-chemical treatments, such as suffocants (lice suffocate due to ingredients like benzyl alcohol) or specialized heat treatments that dehydrate and kill the lice and nits.