I have horses.... and chickens..... and possums (along with everything else from bears, cougars, and coyotes on down). According to my veterinarian most horses in the US have been exposed to EPM and most never have a problem with it unless their immune system is compromised in some way (age, infirmity, parasites, etc.). Even if possums are just passing through they can (and will) urinate and defecate wherever they are when they need to go. Even if you have managed to completely eradicate every possum in your immediate locality (not likely) it's still possible for hay to be contaminated. There is nothing you can do to keep your horse completely unexposed unless it's in a bubble or you live somewhere where possums don't exist.
As far as them bothering the chickens, I've not had a problem. My coop is secure (unless a bear comes along) and my birds are locked in it at night. The possums that do frequent my property usually just join the barn cats for dinner. I can walk outside and talk to them without interrupting their dinner. I prefer to take a "live and let live" stand because wildlife is part of the reason I live in the country anyway.
As far as them bothering the chickens, I've not had a problem. My coop is secure (unless a bear comes along) and my birds are locked in it at night. The possums that do frequent my property usually just join the barn cats for dinner. I can walk outside and talk to them without interrupting their dinner. I prefer to take a "live and let live" stand because wildlife is part of the reason I live in the country anyway.