The pitbulls that attack are not always the unloved, poorly trained, poorly behaved ones. Over the years I've known of a number of attacks involving pits that were beloved family pets, considered safe with children, considered safe in general. They weren't necessarily farm dogs, but were, up until the instant of attack, gentle, friendly dogs that were normally obedient. Then they're let off leash, or in some cases not--I've known of a couple attacks that occurred while the dog was still on leash, just allowed close enough to the horse to jump at it and get hold of it. The dog, usually one that is unfamiliar with horses, sees the horse being ridden past and suddenly something in his little doggy brain snaps and he goes for the horse--most often grabbing onto the nose and then won't let go. Given their determination and strength, it can be very difficult to get the dog off of the horse. After hearing about one such attack on a horse I vowed to always carry a knife, a hunting knife, that if necessary I could use to protect myself and my horse while out riding (or driving)--and yes, if at all possible I would use that knife to slit the throat of any dog that grabbed onto my horse & wouldn't come off! Thankfully it has never been necessary.
I pray that no one in my neighborhood ever gets a pitbull. We do not need one of those running loose around here.