I too have had this problem... My solution was to purchase a couple of
mattress/box spring zip up covers.  They are like big plastic bags for
mattresses you can zip up.  Sold for allergy prone people, they work
great in keeping out ferrets.  Both my mattress and box spring have
been safe and sound since my purchase 2 years ago.
 
Take Care,
Moscha (with Skippy, BoBo, and Nosette happily dooking)
[Posted in FML issue 5020]