Lesley McDonald, of Sacred Heart School Dunedin, asks :-
Why is it that some children and teachers get nits constantly but others don't get them at all? Is it something to do with blood?
Trish Priest, an epidemiologist at the University of Otago, responded.
We don't have anyone who is doing research in this area, as far as I know. However, I've had a very quick look at PubMed, and I can't find any information about genetic or physiological host factors affecting the risk of infestation (presumably because this hasn't been widely studied, since the studies I found have not taken blood samples).
Reasonably consistent risk factors seem to be female gender (I expect probably because of hair length, which is also a risk factor where it's measured), age (children at higher risk, the exact age group at highest risk isn't completely consistent but possibly early to mid primary school age), coming from larger/more crowded households, going to schools with higher prevalence of infestation, and (in some studies) poor quality housing.
Measures of socioeconomic status are reported as risk factors in some studies and not others, but all except one of the studies in my brief search were done in developing countries so their socioeconomic status results may or may not be relevant to NZ.
So most of the risk factors could be conceptualised as relating to 'opportunity to be infested' - i.e. children whose hair is more likely to be in contact with the hair of other children, some of whom have headlice. I didn't see anything about teachers, but in principle I expect that teachers who teach children with headlice and/or whose own children have them, and who have long hair, are likely to be at increased risk.
It's a condition that causes a lot of angst, and which many parents of primary school aged children are concerned about. However I know, from writing an unsuccessful application for a grant to do a pilot study of prevalence in schools here, that it's quite hard to come up with a really convincing justification for funding research on head lice given all the other more serious health problems out there.