I had bought a mini stallion that had lice so bad they has eaten quarter size holes in his neck. I soaped him down in lice shampoo for humans and then for the next 2 months I soaked him and my other mare in bronco fly spray, and I mean soaked! I never had them come back. Since you can not bathe, stick with the powder for lice, I like using python dust if I buy a horse with lice and can not bathe them. As far as other horses, I would powder them too just in case they have gotten them already. I would powder the one who has them and the rest regardless of them having them or not. I brought the new one home and within 3 days my mare had a few on her already. In the spring I would scrub them all down with delicing shampoo and make sure to do a hefty fly/lice spray everyday for a month or two. Spray all the stalls and any equipment used on the horses with lice spray. My vet also told me to put a full tube of ivermectin down him once a week for a month as it would help to kill off the lice from the inside as well as my treatments for the outside. I would check with you vet before doing this though. I know they are a bugger to get rid of and as long as you keep up with a treatment plan that is working until they are gone from everything, then they will be gone for good! That little yearling stallion is now a happy healthy 5 year old with NO LICE! Poor guy, had quarter sized holes that were 1/2inch to 1 inch deep in both sides of his neck and was skin and bones from the lice. He has a severe infestation!