In order to assuage your concerns about the safety of horse manure, and to get this post off the top of the list, let me offer the solution for safely using it to assure no chemicals in it will be harmful to your wormies.

When you get some horsie-poop from someone, just let it lay out in the sun for about 30 days. All the chemo-worm killers will dissipate.

That said, this post will lose its eternal position alone and atop the threads.

You're welcome,

Chuckiebtoo

I had never used or heard of feed-through fly control until your post. I try to get the manure in the worm beds before the flies hatch.... but I do have some flies.

I searched some common non-toxic varieties and found diatomaceous earth as the only ingredient I would worry about. That small amount should not harm the worms.
I have never heard of anyone having any problems vermicomposting with aged horse manure, except for overheating. Worms love it!

Your comment about the differences in horse dung is so true! Quite the variety is available out there.