Pediculosis and Pthiriasis (Lice Infestation) Differential Diagnoses

Updated: Aug 16, 2019
  • Author: Lyn C C Guenther, MD, FRCPC, FAAD; Chief Editor: Michael Stuart Bronze, MD  more...
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DDx

Diagnostic Considerations

True nit infestation must be distinguished from hair casts (pseudonits). Hair casts are ringlike remnants of the inner root sheath of the hair follicle. They are amorphous and freely moveable along the hair fiber.

Many scalp conditions can cause pruritus. Seborrheic dermatitis presents with erythema and scaling. It affects the scalp, eyebrows, nasolabial folds, and central chest. Acne necrotica presents with inflamed follicular papules and pustules, black crusts, and scarring. It is extremely pruritic, and patients usually pick at the lesions.

Free-living primitive psocid lice feed on decaying matter in leaves, old books, and animal habitats. They may cause human scalp infestation in children when they visit a library or doghouse that is infested. Psocids have large heads with massive jaws, large hind legs, and long antennae and are distinguished easily from Anoplura lice. [6]

Other problems to be considered in the differential diagnoses of head louse infestation include the following:

  • Dandruff

  • Fibers

  • Scabs

  • Desquamated cells

  • Dirt

  • Dried hairspray/gel

  • Dermatophyte infection

  • Piedra (black piedra from Piedraia hortae, white piedra from Trichosporon asahii and other species of Trichosporon)

  • Hair shaft abnormalities (ie, monilethrix, trichorrhexis nodosa)

  • Delusions

Other problems to be considered in the differential diagnoses of body louse infestation include the following:

Other problems to be considered in the differential diagnoses of pubic louse infestation include the following: