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[Microbiology](https://durgical.com/en/microbiology/) · [Ectoparasites](https://durgical.com/en/microbiology/groups/ectoparasites/)

# 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

## Key points

- Only the body louse transmits disease.
- Nits within 6 mm of the scalp = active infestation.
- Head lice do not mean poor hygiene.
- Crab louse = STI; on a child's eyelashes, an abuse warning.
- Contact precautions until treatment is complete.

## 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) →](https://durgical.com/en/conditions/pediculosis/)
- 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:

- [Ivermectin](https://durgical.com/en/pharmacology/ivermectin/)

## 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.

## In Colombia

Head lice are very common in Colombian schoolchildren; body lice are seen in people living on the street and in crowded settings.

## Memory aid

The head louse annoys, the body louse infects: only the louse that lives in clothing carries typhus.

## 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.

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## Related

- [Sarcoptes scabiei](https://durgical.com/en/microbiology/sarcoptes-scabiei/)

## Related conditions

- [Pediculosis (head, body and pubic lice)](https://durgical.com/en/conditions/pediculosis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

**Contact**
Gown and gloves; dedicated equipment. For C. difficile, soap-and-water hand washing.

**Droplet**
Surgical mask within about a metre; single room.

**Airborne**
N95 respirator and a negative-pressure room.

[← Back to the group: Ectoparasites](https://durgical.com/en/microbiology/groups/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](https://www.cdc.gov/dpdx/pediculosis/index.html) (CDC, 2024); [About Lice | Lice | CDC](https://www.cdc.gov/lice/about/index.html) (CDC, 2024); [Head lice: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000840.htm) (MedlinePlus (NLM), 2026).

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Representative model, for educational purposes only.

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