I got mites too .. and trying to find the source I was stumped. I used the barck for about a week before switching to aspen, and a couple weeks after he started sitting in his water bowl. I don't know where he got them, but I was super pissed and sad. I was and still am A new BP owner so I was scrambling as to what I had to do to get rid of them .. it sucked so bad.
Maybe the bark? Maybe the Vet? MAYBE ME!??!?! Whatever .. they are gone now *crosses fingers*