Science Questions

Sun, 27th Mar 2005

Part of the show Avian Flu, Viruses, Bed Bugs and Murder

Question

Maggie asked:

I found head lice on an elderly lady I was looking after and used a special shampoo that the doctor told me to use. But it didn't kill them. I know this because I could still see black moving dots. How long does it take for eggs to hatch out, and how long does it take to lay them again?

Answer

The life cycle of the louse is very distinct and discreet in that the eggs take seven days to hatch, and that's almost invariable. Up until that time, you're not going to get rid of all the little ones that are going to hatch out. But it takes them nearly ten days to become adults, and so therefore they can't lay any eggs until they've reached that point. In between those two points, you have a stage where the lice won't move into anybody else. While they're half-grown, they stay put. It's only when they get to about 6 or seven days old that they start getting interested in moving house. From then on, they'll start shifting onto somebody else. Your lady wouldn't have been infectious if she only had baby ones on her.

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