Asked February 03, 2018, 3:36 PM EST
Deschutes County Oregon
Worming with Ivomec is a good start. Next I would look at doing everything possible to remove their habitat. Ticks often climb grasses and brush so they can come into contact with people and animals passing by. Wherever possible I would recommend mechanically removing brush, leaf litter and any debris on the ground. If it is safe to burn and legally allowed in your area, burning dead grass and brush to kill the ticks and take away their habitat will help. Use of fencing to restrict access of wildlife (deer), stray dogs, raccoons, etc. to your horses is also helpful.
Finally, there are wipe-on and spray-on products containing cypermethrin or permethrin that can provide several hours of protection or more. Here is a link from University of Kentucky that might be of help: http://www.thehorse.com/articles/34096/protecting-horses-from-ticks. In addition, consulting your veterinarian may be a good idea as they may have a product they recommend.
I hope this is helpful and if you have any additional questions, please do not hesitate to contact me.
firstname.lastname@example.org or 541-480-3091.