Lice: Identification, Life Cycle and Spread

What lice look like

Empty nit shells are white and further from the scalp. Since hair grows about 1 cm a month, a nit more than 1 cm from the scalp is almost always an old, empty shell and does not mean active infestation.

The lice life cycle

1. Eggs (nits): laid by the female, about 6 to 10 a day for head lice. They hatch in 7 to 10 days.
2. Nymphs: baby lice, which must feed on blood immediately. They molt three times and become adults in about 7 to 12 days.
3. Adults: live up to about 30 days on a person, feeding several times a day. Females start laying eggs 1 to 2 days after mating.

This timeline explains treatment. Many lice products do not kill every egg, so lice can hatch after the first treatment. A second treatment 7 to 10 days later (day 9 is common) kills newly hatched nymphs before they are old enough to lay eggs. Full details are in head lice treatment.

How lice spread

• Head lice: mainly by direct head-to-head contact, during play, sports, sleepovers and selfies. Sharing hats, brushes or pillows is a much less common route. They are common in children aged 3 to 11, regardless of cleanliness.
• Body lice: live and lay eggs in clothing, moving to the skin only to feed. They spread through shared clothing and bedding and are linked to crowding and lack of access to laundry, such as homelessness or refugee camps.
• Pubic lice: almost always through sexual contact. Rarely, through shared towels or bedding.

Pets do not carry human lice, and human lice cannot live on animals.

Lice myths

• "Lice jump or fly." They crawl only.
• "Lice mean poor hygiene." Head lice infest clean hair as readily as dirty hair.
• "They survive for weeks on furniture." A head louse off the scalp usually dies within 1 to 2 days.
• "Everything in the house needs bagging for weeks." Washing recently used bedding and hats in hot water is enough for head lice.
• "Nits mean you are still infested." Only live lice, or nits very close to the scalp, mean an active infestation.

Lice or something else?

Symptoms

• Itching, from an allergic reaction to louse saliva, which can take 4 to 6 weeks to start after a first infestation
• A tickling or crawling feeling
• Red bumps on the scalp, neck or shoulders
• Trouble sleeping, since head lice are more active in the dark
• With body lice: intense itching and thickened, darkened skin in long-term cases
• With pubic lice: itching in the groin and bluish spots where lice have fed

Scratching can break the skin and lead to bacterial infection such as impetigo.

Confirming an infestation

The most reliable method is wet combing: wet the hair, add conditioner, and comb section by section from the scalp with a fine-toothed lice comb, wiping the comb on a white tissue after each stroke. Finding a live louse confirms active infestation.

Treatment overview

• Head lice: over-the-counter permethrin 1% or pyrethrins, ivermectin 0.5% lotion (now sold without a prescription), or prescription products such as spinosad or malathion, plus combing, as described above.
• Body lice: usually cured by bathing and washing all clothing and bedding in hot water (at least 130 °F, about 54 °C) and drying on high heat.
• Pubic lice: permethrin or pyrethrin products, treatment of sexual partners, and STI testing. See pediculosis for all three types.

"Super lice" are head lice resistant to permethrin and pyrethrins, now common in much of the US. If over-the-counter products fail after correct use, a doctor can prescribe an alternative.

Frequently asked questions

What do lice look like?

Adult lice are about the size of a sesame seed, tan to grayish, with six legs. Nits are tiny oval eggs, yellowish-white, glued firmly to hairs near the scalp. Pubic lice are shorter and wider, resembling tiny crabs.

How long can lice live off the body?

Head lice usually die within 1 to 2 days away from the scalp, because they need regular blood meals and warmth. Nits generally cannot hatch away from the head. Body lice can survive about a week in clothing without a person.

How do you get lice?

Head lice spread mostly by direct head-to-head contact. Pubic lice spread mainly through sexual contact. Body lice spread through shared clothing and bedding. Lice cannot jump or fly, and you cannot catch human lice from pets.

Can lice live on pillows?

Only briefly. A head louse that falls onto a pillow usually dies within 1 to 2 days. Washing pillowcases and recently used bedding in hot water and drying on high heat is enough. There is no need for pesticide sprays.

What is the difference between lice and nits?

Lice are the live insects. Nits are their eggs, glued to the hair shaft. Finding nits close to the scalp suggests an infestation, but only live lice confirm it is active. Empty nit shells further down the hair can remain after successful treatment.