I was told pinworms are species specific.Oh no not again!
I had a crazy thought, it couldn't be your chickens spreading the worms/larvea about when they feed? And then when the horses graze they pick them up?
I am sorry that you are going through this. I never knew pinworms could be so resistant.
I am not worried about Dapper Dan's health; he is plump and shiney and feisty. But I know they are uncomfortable for him. Someone in an earlier post said that older horses have a harder time with parasites. Maybe that is part of the problem.