Those Horrible, Yucky Warts On Horses
They are contagious so the horse acquires them from other horses, halters, grooming brushes, fences, things like that. They usually only affect young horses. They are self limiting, meaning they resolve on their own. It may take up to six months for them to disappear.
If it looks like they are getting to many on the lips which will keep the horse from eating enough or the nostrils which can interfere with breathing, then you can purchase wart removal medication from your local Vet. He will give you the how to and how not to treat the warts with whatever preparation he gives you. All horses going to competitions, sales or shows should be cleared up first. Not only because they are Contagious, but also because they are unsightly and frowned upon by other fussy humans.