• Clinical science

Lice infestation


There are three species of lice that affect humans: Pediculus humanus capitis (head louse), Pediculus humanus corporis (body louse), and Pthirus pubis (pubic or crab louse). All are obligate, stationary ectoparasites that feed solely on human blood. Affected individuals most commonly present with pruritus; although early stages of infestation and mild cases may be asymptomatic. Detection of lice or nits (louse eggs) on the body or clothes usually confirms the diagnosis. Treatment includes topical pediculicides as well as nonpharmacologic measures (e.g., machine washing and drying contaminated clothing/sheets). Secondary skin infections from scratching may occur in all forms of pediculosis (lice infestation). The body louse acts as a vector for louse-borne diseases, including epidemic typhus, trench fever, and relapsing fever.

Pediculosis capitis (head lice infestation)

  • Parasite: Pediculus humanus capitis (∼ 3 mm in length)
  • Transmission: direct head-to-head contact or sharing hair accessories, bedding, or clothing
  • Epidemiology: most common in children (affects 1% of school-aged children in North America)
  • Clinical features
    • May be asymptomatic initially and in mild cases
    • Scalp/neck pruritus and excoriations
  • Diagnosis
    • Detection of nymphs and adult lice on the scalp or hair (often visible with the naked eye)
    • Confirmation with microscopy of a hair shaft or Wood's lamp examination (nits fluoresce and appear pale blue)
  • Treatment
    • 10-minute topical application of pediculicides followed by mechanical removal of lice and nits with a fine-toothed comb (repeat application after 10 days)
    • Clean combs, hairbrushes, and hair accessories of lice and nits (i.e., soak in hot water for 5–10 minutes)
    • Machine wash and dry bedding, clothes, etc. (≥ 55°C (≥ 131°F) for 30 minutes.); alternatively, dry clean or store in a sealed plastic bag for 2 weeks.
    • Children with head lice should receive prompt treatment but can continue attending school.
    • People in close contact with affected individuals should be screened for infestation.
  • Complications: secondary skin infections from scratching and skin break-down

The presence of nits alone does not indicate an active infestation.


Pediculosis corporis (body lice infestation)


Pediculosis pubis (pubic lice infestation)

  • Parasite: Pthirus pubis (also known as the crab louse; often referred to as “crabs” because of its crab-like appearance; ∼ 1.5 mm in length
  • Transmission: usually sexual contact (can also be transmitted via infested towels or bedding)
  • Epidemiology: most common in teenagers and young adults
  • Clinical features
    • Pruritus and excoriations in the pubic area; are common; can also occur in the perianal and axillary region.
    • Maculae ceruleae (pale blue-purple macules ) in patients with chronic infection
  • Diagnosis: identification of louse or nits in pubic hair with the naked eye, during microscopy, or Wood lamp examination
  • Treatment
    • 10-minute topical application of pediculicides (e.g., 1% permethrin cream or pyrethrin); reevaluate after 9–10 days, and repeat treatment if lice are found)
    • Machine wash and dry bedding, clothes, etc. (≥ 55°C (≥ 131°F) for 30 minutes); alternatively, dry clean or store in a sealed plastic bag for 2 weeks.
    • Screen for other STIs and treat sexual partners to prevent reinfection


Pediculosis ciliaris

  • Parasite: Pthirus pubis (same as in pediculosis pubis)
  • Transmission: direct contact with infested individuals, towels, or bedding
  • Epidemiology: most common in teenagers and young adults (may occur in children in close contact with infested adults)
  • Clinical features
  • Diagnosis: detection of lice or nits close to eyelash base (may also be seen in the eyebrows or hairline)
  • Treatment
    • Mechanical removal of lice and nits after applying petroleum jelly to eyelids
    • Treat associated conjunctivitis
    • Local pediculicide treatment for persistent cases if the above therapy fails


Louse-borne diseases

Trench fever (five-day fever) [5]

Epidemic typhus [6]

Louse-borne relapsing fever

Borrelia recurentis causes a recurrent (relapsing) fever.


  • General measures
    • Physical measures (e.g., mosquito nets long clothing, repellants )
    • Lethal measures (e.g., insecticide sprays , electric insect killers)
Overview of the most common ectoparasiticides
Drug Mechanism of action Clinical Use

Side effects

  • Topical application (additionally, ivermectin can be given orally)
  • Blocks the deactivation of Na+ channels → uncontrollable depolarization of neuronal membranes → seizures and ultimately paralysis and death of arthropod
  • Local application in head lice infestation. Dosage is dependent on the hair length.
  • Penetrates into the respiratory orifice of lice and blocks breathing
  • Carcinogenic