No idea on that. I just took my chance one year, everything was fine, and have been doing it since. I free range all my foul except during summer when I am gardening, so I figure they all hang out together 9 months of the year, what's 3 more months? LOLYes, I've heard something about that also. I've heard that it could be present in the soil and that turkeys are susceptible to it but not chickens. However, chickens can catch it from turkeys, just not from the soil. I think there is suppose to be a way to test the soil for it, but I don't know what that method is.