A good way to find out if the wormer you used could kill the lice is to go back to the feed store and just look at the box. If not just ask the people who work there if they have the powder.
My head is itching just thinking about it. I use to get lice alot growing up and the shampoos worked pretty well. I don't know if you can use them on horses, you can ask your vet though. Just be certain to wash any type of bedding and to clean out the bedding. Goodluck!