Pediculus humanus and Pthirus pubis !
- Parasite
- Contact
- Exam priority: medium
Head, body and pubic lice: wingless blood-sucking insects. Only the body louse spreads diseases such as typhus.
Updated 2026-11-01
Also called: head louse, body louse, crab louse, nits
At a glance
- Shape
- Pediculus elongated; Pthirus broad and crab-shaped
- Family
- Pediculidae (Pediculus) and Pthiridae (Pthirus)
- Type
- Wingless blood-sucking insect (Anoplura), obligate ectoparasite
- Size
- Adult 2–3 mm, about the size of a sesame seed
Morphology
- Nits (eggs) cemented to the base of the hair shaft, hard to remove.
- Three pairs of legs with claws to grip hair.
- Pthirus has a body wider than long and large claws.
Transmission
- Head louse: head-to-head contact; less often combs, caps or pillows.
- Body louse: clothing and blankets in crowding, without changing or washing clothes.
- Crab louse: close contact, mainly sexual.
- Lice crawl; they do not jump or fly, and pets do not spread them.
Reservoir: Humans only
Isolation precautions
What the care team adds to standard precautions.
- Contact
- Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.
Life cycle
- The female glues nits to the base of the hair; the body louse lays them in clothing seams.
- Nits hatch in about a week (6–9 days).
- The nymph looks like a small adult, feeds on blood and moults three times.
- About 7 days later it is adult; the female lays several nits a day and lives up to a month.
- Without blood it dies within 1–2 days off the host, so spread needs close contact.
Virulence factors
- Louse saliva causes itch and scratch lesions.
- Scratching promotes impetigo and neck lymph nodes.
- The body louse transmits Rickettsia prowazekii, Bartonella quintana and Borrelia recurrentis.
Diseases it causes
- Head, body and pubic pediculosis Pediculosis (head, body and pubic lice) →
- Epidemic typhus, trench fever and relapsing fever: spread only by the body louse
- Crab lice on a child's eyelashes: a warning sign of possible sexual abuse
Clinical picture
- Itch on the scalp, nape and behind the ears.
- Body louse: excoriations on the trunk in homeless or crowded people.
- Crab louse: pubic itch; sometimes axillae, beard or eyelashes.
Diagnosis
- Finding a live nymph or adult louse confirms infestation.
- Nits within 6 mm of the scalp suggest active infestation; farther out, past infestation.
- Wet combing with a fine metal comb.
Treatment
Topical pediculicides (permethrin, malathion and others) and mechanical nit removal; topical or oral ivermectin is used in resistant cases, and permethrin resistance is growing. Body lice are controlled by bathing and hot-washing clothes.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Avoid sharing combs, caps and towels.
- Check and treat affected household members at the same time.
- Hot-wash clothing and bedding.
- Pthirus: treat sexual partners and screen for other STIs.
Test yourself
Think of the answer, then tap the question to see it.
Which type of louse transmits disease?
Only the body louse (Pediculus humanus corporis): epidemic typhus, trench fever and relapsing fever.
Which finding suggests active head lice infestation?
Live lice or many nits within 6 mm of the scalp.
Where does the body louse lay its eggs?
In clothing seams; it moves to the skin only to feed.
What does finding crab lice on a child's eyelashes mean?
It is a warning sign of possible sexual abuse that must be assessed.
Related
Related conditions
← Back to the group: Ectoparasites
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: CDC - DPDx - Pediculosis (CDC, 2024); About Lice | Lice | CDC (CDC, 2024); Head lice: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2026).
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Representative model, for educational purposes only.