They have always been there.... they will always be there. It's called "nature". Even if you think you have gotten rid of all of them - you won't. It's not just mice. There is much more out there than that. If you are going to have livestock, poultry etc., you are going to have feed, straw, hay, shavings around, which is going to bring critters. You do what you can, but the only way to get rid of MOST of the mice or whatever else is designated as a nuisance is to get rid of your poultry, livestock etc. Remember, don't have dog or cat food, bird feeders out at night - they will be there to scarf that up too. Just a note, mice come back to nest where they were born and since they have huge litters and procreate profusely - good luck.