I do not give any "preventative" treatment to my birds for any reason. My prevention is good hygiene in the coop, clean it when it needs it. Fresh food and water daily, as in they only get enough food and water for one day, water cannot grow stagnant and bacteria filled, food cannot grow mold or rot. They have a warm, dry, draft free house. They have a safe and secure run. These things will all help reduce stress and susceptibility to disease. If there is a problem, a sick bird, a parasite issue, then I treat it. At that time I will also treat the entire flock even if they are not all showing symptoms, as they have all been exposed.