Here is a Cornell Research Paper on ivermectin. I had a pile sit on a concrete pad for a year even after it was composted for a couple years on the farm. Found live and fat earth worms in the pile. My hot humid environment does tend to degrade the chemicals a lot faster.
I followed up with a bean sprout test and found the beans did great. no deformation.
Solid reply! Appreciated it since the closest horse ranch's manure I can grab with my tractor does include dewormers. I'm checking the brands but it's tricky since the horsewoman took great offense at my initial question. Her knee-jerk reply was her girlfriend sells produce at a local organic foods market, "No questions asked." The latter comment begs a dissertation but I need to move on.
I regularly use horse manure from a very old pile. This past winter I built a new hot frame and wanted to put a foot of fresh manure in the bottom to serve as the heat source. So I went to my usual barn and instead of taking manure off the bottom of the old pile I took the manure off the top of the pile being used. I wanted the fresh manure to get more heat. I was surprised to find that there were red wigglers in the freshest manure available.
I offer this as a suggested test. You might also dig into the pile in various spots to get a better reading over a period of time in the life of the pile.
Location: Huntsville Alabama (North Alabama), Zone 7B
I recently went back for more manure and asked the local stable owner how often they used de-wormers and she said "every three months". I mentioned to her that I had found red wigglers in the horse manure I got from her before and she indicated that they always find worms in the piles before she moves them to the older compost piles. So some chemicals do break down like they should but having an active environment helps.
Lessons in life will be repeated until they are learned.