I had a hen with Gapeworm and it seemed so hopeless when I read all the info on gapeworm.
I did make the mistake of putting her in another pen and realized after that as she pooped she was contaminating the soil in the new pen.
My hen was in the pen for over a month before I knew what it was so she really contaminated the pen and it did not result in an outbreak at all. The pen is big and I made two other pens that they can take turns going into, but knowing earthworms move around...it would seem impossible to eliminate the problem.
It has been years and I have not had another hen that has the symptoms of gapeworm. They all seem healthy and plump.
I have relaxed since.
Gapeworm is in earthworms, the hen that got gapeworm just loved earthworms. She was always first when I was digging in the garden.
I do not worm my chickens. I think if they are healthy that they can handle worms. I always have a dirt bath with DE in it. It is supposed to help if they eat it to help with parasites.
I live in BC. We get a lot of rain, not sure if it helps get rid of things in the ground?