Need help with mites

[Albeon]

My boa constrictor has a recurring problem with mites, and I simply can't get rid of them. While I'm managing the problem and there are never more than a few mites on him at any given point, I can't get rid of the things.

I've already treated him and his terrarium with several mite sprays and taken him to the vet and had him treated with ivermectin. I've moved him to a new terrarium with new substrate and a different hide, water dish, and decorations and placed that new terrarium in an entirely different room. I did all that while he was in a tub being treated with mite spray. I also treated this new terrarium with mite spray before putting him back in it, and they STILL didn't go away.

I live in Massachusetts and it's the dead of winter, so they're not coming in from outside. He's the only reptile in the house, so it's not cross-contamination. I'm honestly at my wit's end with this. I've done everything I can think to do and have even gone beyond what most people and sites say to do. Is there anything else I can do to get rid of these things? My poor snake practically lives in his water bowl now. Thanks in advance for your help.