the only way to deal rationally with the dangerously irrational is to delete it....it can be a very sad thing to contemplate, but if a vicious dog which cannot be cured bites you and endangers others, it makes little difference WHY it is vicious....It doesn't understand, and both it & us will be better off.

Yes, you DO have to catch it first...but if you know who harbored that dog, there are measures that can be taken to dissuade them from their bad habits