No pour-on wormers, horse skin is much thinner than cattle skin! You burn them fast!! I asked about it after doing our new cattle last year, worming was so EASY!
The person I know who tried pour-on for her horse, lost the use of horse for 3 months of summer while his back was scalded and bald during that time. Also had a lot of soreness to go with the other stuff.
Use a louse powder for the equines. Remove any bird nests above areas where they equines hang out. Birds often carry lice and it shakes down from nests of barn swallows if they nest above. We have dealt with that problem over the years.
You do have to work the powder down deep to reach skin on mules and donkey's. Hair is very dense in their winter coats. Don't forget roots of mane and tail.