If you feed peanut butter to your dog, as many people do - check very carefully that it doesn't contain xylitol - this has, apparently been added to certain brands of peanut butter recently. It's highly toxic to dogs.
It seems that a lot of people use peanut butter to feed and attract wildlife, badgers spring to mind, this started me thinking that it may be toxic to other animals, fox's being canine would seem highly likely.
Thanks for the warning, I'l check my jar.