How can the duration of pediculosis and pthiriasis (lice infestation) be estimated?

Updated: Aug 16, 2019
  • Author: Lyn C C Guenther, MD, FRCPC, FAAD; Chief Editor: Michael Stuart Bronze, MD  more...
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Eggs depend on body warmth to incubate, so nits are attached to the hair shafts just above the level of the scalp. Since human scalp hair grows at a rate of approximately 10 mm/month (0.37 mm/day), the distance of nits from the scalp can be used to estimate the duration of infestation. Nits found several millimeters from the scalp are nonviable empty egg cases. They indicate chronic infestation.

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