The case of your child is not of thread worm ,but of Cutaneous larvae migrans .Here what happened is the larvae of the worms lost its normal route and wander here and there.The reason may be;

1)Immunity of the person which prevents the normal progression of the life cycle,

2)Infection from non human type of species which usually infect dogs,cats etc.When they accidently enter in to the human system ,they cannot progress to its normal course and there by wander here and there.

Threadworm (pinworms) will not cause this.Usually it is caused by
Non human species of hookworm(Ancylostoma brazilience),Strongyloids stercoralis,Ganthostomiasis,Loa loa,Dirofilaria,Fasciola etc etc.

So most probably it will go by itself,as the worm cannot complete its life cycle it retur to the intestine and go by stool or perish.

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