DD has them again 
Older DD and DS both also had them when they were using the early years toilets. Never caught them again once they moved to KS2 toilets.
Nobody else in the house has them or has caught them. If it was a case of her catching them repeatedly from home someone else in the family would have them surely?
She only ever catches them during school terms. This is the 6 time in 18 mths 
So now the whole house needs bleaching again, bedding, teddies everything needs to go through a hot wash. I know the drill off by heart.
The school flatly refused to send a letter out. I've had kids at the school for over 8 years and there has never been one letter about threadworms. Over a thousands about bloody nits though.
Receptionist looked disgusted when I went in to ask them again to send a letter out. I explained what advice I had been given, that in schools things like playdough are massive harbingers of eggs etc, toilets need a deep clean with bleach, surfaces need deep cleaning. Again it's been ignored. Apparently nobody else has said their children have them.