How to tell if your neighbor is poisoning my horses?

Well whenever one of my horses is sick I blame the neighbor or some person 10 years ago in my past. I want to find out if they are actually poisoning my horses. I've set up cameras, and I haven't been able to catch them, so I now believe that the vet i'm paying money to fix my horses, is really hurting my horses.

Answer: 
That is terrible of you to blame other people when you have no proof. Is it just stupid to blame people from 10 years ago. Have you ever considered the fact that YOU may be the one poisoning the horses? Have you ever even bothered to ASK the vet what's wrong with them? What are his opinions? No vet would poison an animal. It takes a lot of work to be a vet and no one who hates animals is going to toil through years of training. I think you are the one making your horses sick. Stop feeding them crap and remove poisonous plants. Change their water and take care of them. Your horses aren't going to get consistently sick if you treat them right. People like you shouldn't own horses.